algal 'greening' and the conservation of stone heritage structures

Science of the Total Environment 442 (2013) 152–164

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Algal ‘greening’ and the conservation of stone heritage structures

Nick A. Cutler a,b,⁎, Heather A. Viles a, Samin Ahmad a, Stephen McCabe c, Bernard J. Smith c

a School of Geography and the Environment, Oxford University Centre for the Environment, South Parks Road, Oxford, OX1 3QY, UKb Department of Geography, University of Cambridge, Downing Place, Cambridge, CB2 3EN, UKc School of Geography, Archaeology and Palaeoecology, Queen's University Belfast, Elmwood Avenue, Belfast BT7 1NN, UK

H I G H L I G H T S

► We investigated green algal biofilms on four sandstone heritage structures in Belfast.► The relationships among greening and environmental variables were complex.► Green biofilms had little or no impact on the physical integrity of building stone.► The influence of green biofilms on moisture levels in stone may be bioprotective.

⁎ Corresponding author at: Churchill College, Cambridg336202; fax: +44 1223 336180.

E-mail address: [email protected] (N.A. Cutler).

0048-9697/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.scitotenv.2012.10.050

a b s t r a c t
a r t i c l e i n f o
Article history:Received 10 August 2012Received in revised form 10 October 2012Accepted 11 October 2012Available online xxxx

Keywords:Green algaeBiodeteriorationWeatheringAlgal soilingBuilding stoneClimate change

In humid, temperate climates, green algae can make a significant contribution to the deterioration of buildingstone, both through unsightly staining (‘greening’) and, possibly, physical and chemical transformations.However, very little is known about the factors that influence the deteriorative impact and spatial distribu-tion of green algal biofilms, hindering attempts to model the influence of climate change on building conser-vation. To address this problem, we surveyed four sandstone heritage structures in Belfast, UK. Our researchhad two aims: 1) to investigate the relationships between greening and the deterioration of stone structuresand 2) to assess the impacts of environmental factors on the distribution of green biofilms. We applied anarray of analytical techniques to measure stone properties indicative of deterioration status (hardness, colourand permeability) and environmental conditions related to algal growth (surface and sub-surface moisture,temperature and surface texture). Our results indicated that stone hardness was highly variable but onlyweakly related to levels of greening. Stone that had been exposed for many years was, on average, darkerand greener than new stone of the same type, but there was no correlation between greening and darkening.Stone permeability was higher on ‘old’, weathered stone but not consistently related to the incidence ofgreening. However, there was evidence to suggest that thick algal biofilms were capable of reducing the in-gress of moisture. Greening was negatively correlated with point measurements of surface temperature, butnot moisture or surface texture. Our findings suggested that greening had little impact on the physical integ-rity of stone; indeed the influence of algae on moisture regimes in stone may have a broadly bioprotectiveaction. Furthermore, the relationship between moisture levels and greening is not straightforward and islikely to be heavily dependent upon temporal patterns in moisture regimes and other, unmeasured, factorssuch as nutrient supply.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

A substantial proportion of the world's tangible heritage isconstructed from stone (Scheerer et al., 2009): it is essential to under-stand the biogeochemical processes that occur on and in stone struc-tures if this heritage is to be effectively conserved. Lithobiotic(stone-dwelling) microorganisms are key players in many of the bio-geochemical processes that occur in stone. Bacteria and fungi are

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known to contribute to the degradation of stone heritage structures(referred to as ‘stone’ hereafter) through both biophysical and bio-chemical means (Gaylarde et al., 2003; May et al., 1993; McNamaraand Mitchell, 2005; Warscheid and Braams, 2000). However, in cer-tain areas (notably those with cool, humid climates) sub-aerialgreen algal species may also make a major contribution to the deteri-oration of stone (Gaylarde and Gaylarde, 2005). Green staining(‘greening’) resulting from the chlorophyll in green algal cells is acommon feature on stone structures in temperate latitudes (in theTropics, cyanobacteriamay alsomake amajor contribution to greening:see Gaylarde and Gaylarde, 2005) and there is also evidence thatsub-aerial green algae are connected with the physical disintegration

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153N.A. Cutler et al. / Science of the Total Environment 442 (2013) 152–164

of stone. However, little is known about the overall contribution thatgreen algae make to the biodeterioration of stone and the ecologicalfactors that influence their growth on stone surfaces (Cutler andViles, 2010). This undermines efforts to model the role of algal biode-terioration under altered climatic regimes. Based on this, the overallobjectives of this research were to 1) investigate the relationshipsamong green algal growth and the deterioration of building stone,including impacts on the movement of moisture and 2) to investi-gate the relationship among environmental factors and the distribu-tion of green biofilms on stone buildings (Table 1). To achieve theseaims, we surveyed four stone heritage structures in Belfast, UK, andcompared the properties of stone surfaces with and without algalcover.

The biodeterioration of stone has both aesthetic and physical com-ponents. Changes in surface appearance related to organic activity aredescribed as biofouling; biological activity that directly contributes tothe physical breakdown of the stone is termed bioweathering (Cutlerand Viles, 2010). The aesthetic impact of green algal biofilms is obvi-ous and well understood. In addition to staining due to pigmentspresent in algal cells, the formation of green biofilms also enhancesthe entrapment of particulates which darken the stone surface overtime (Viles and Gorbushina, 2003; Viles et al., 2002). Both the partic-ulates and carbon fixed by the algae provide a source of nutrition forheterotrophic microorganisms which may further degrade the stonesurface (Gorbushina et al., 1993; Saiz-Jimenez, 1997).

In contrast to the readily apparent impact of green biofilms on theappearance of stone, the physical and chemical impact of green algaeon stone is largely unknown. There have been reports linking greenalgae to biophysical deterioration of stone. Wakefield et al. (1996),for example, observed granular disintegration of a sandstone struc-ture associated with growth of fine, filamentary structures of algaefrom the genus Trentepohlia. The expansion and contraction of algalcells associated with wetting/drying or freeze/thaw cycles may alsocontribute to the weathering of stone substrates (Hall and Otte,1990). Many microorganisms, including bacteria and fungi (bothfree-living and as lichen symbionts) contribute to stone weatheringby biochemical means, e.g. the production of organic acids (Bockand Sand, 1993; McNamara and Mitchell, 2005). Green algae areknown to produce small quantities of organic acids (John, 1988)and may contribute to biochemical degradation of stone by increasingconcentrations of dissolved CO2 through respiration (Welton et al.,2003). As well as direct impacts on structure, green algal biofilms mayalso have indirect effects on the stone substrate, most notably on the in-gress and egress of moisture. Sub-aerial green algae are highly effectiveat sequestering moisture; algal cells expand when hydrated and turgid

Table 1The objectives of the study, along with allied hypotheses and analytical approaches.

Hypotheses

Objective 1: greening and biodeteriorationPhysical integrity: Algal greening is likely to promote the physicaldisintegration of stone, through both biophysical andbiochemical processes.

H1: The degree of gwith variability in st

Appearance: Green biofilms are likely to promote the darkening ofstone surfaces, by trapping particulates and facilitating thedevelopment of other microbial communities.

H2: Surface greennecorrelated.

Movement of moisture: Green algal cells swell when wet, blockingpore spaces, lowering surface permeability and trappingsub-surface moisture.

H3: Surface permeagreen biofilms.

H4: Sub-surface mogreen algal patches.

Objective 2: greening and environmental factorsEnvironmental factors: Green biofilms are moisture-limited andlikely to be most dense in areas that trap and retain moisturee.g. rough, uneven and shaded surfaces or in regions with areliable supply of moisture e.g. close to ground level

H5: Green biofilmslocations on the surground level) and a

algal cells could block pore spaces, forming a surface layer of low per-meability. If this process occurs, a green algal biofilmwould retainmois-ture at the stone surface and prevent the egress of deep-seatedmoisture, meaning that both the surface and sub-surface of the stonewould remain wetter for longer periods. Water is key component instone weathering: it is therefore possible that green algal biofilmshave negative repercussions for both the appearance and long-term sta-bility of stone heritage structures.

In order to model the impacts of green biofilms, it is necessary tounderstand the ecology of green algae living on stone surfaces, atboth macro- and microscales. On a large scale, sub-aerial greenalgae have been demonstrated to respond to fluctuations in climate.Environmental change during the twenty first century is thereforelikely to alter the relative importance of green algae as agents of bio-deterioration (Viles and Cutler, 2012). Changes in moisture supplyappear to be particularly significant (Bellinzoni et al., 2003). Algal bio-deterioration is likely to become more prevalent in temperate areasreceiving increased rainfall, particularly if a greater proportion ofthe rainfall occurs in extreme events. Under this scenario, stone struc-tures are likely to be subject to prolonged wetting and, presumably,dramatic increases in the density and extent of green algal biofilms.Anecdotal evidence suggests that this process may be already under-way in parts of the NW UK (Smith et al., 2010).

Algal greening is also sensitive tomicroenvironmental variation. Sur-face texture has been linked to biofilm formation in previous studies(Barberousse et al., 2006; Tomaselli et al., 2000) and is likely to influencebioreceptivity (the suitability of a surface for biological colonisation).Surface irregularities influence surface drainage and may providemicrosites for algal colonisation i.e. rough surfaces are likely to havehigher bioreceptivity than smooth substrates (Guillitte, 1995). Similarly,anecdotal evidence suggests that green biofilms are more prevalentclose to ground level, where moisture is available from rain-splash andrising dampness.

1.1. Study area

We conducted our study in central Belfast, UK. Belfast has a largestock of sandstone heritage structures, mostly dating from the mid-to late-nineteenth century (Curran et al., 2010). Sandstone (Sst) isthe dominant building stone in NW UK and is globally significant asa material used in the construction of heritage structures (UNESCO,2011). Belfast has a temperate, maritime climate with mild wintersand cool summers. Mean annual minimum and maximum tempera-tures range between 5.8 and 12.5 °C; average annual precipitation isaround 860 mm and there are ~155 days with precipitation each

Analysis

reening will be positively correlatedone surface hardness.

Measured greenness (δa*) and hardness forgreen and non-green areas

ss and darkness will be positively Measured greenness (δa*) and darkness (δL*)on stone surfaces with patchy algal cover

bility will be lower in areas with Measured surface permeability for green (G)and non-green (NG) areas.

isture levels will be higher beneath Inferred sub-surface moisture levels for wallswith G and NG areas, before and afterexperimental wetting

will be most prevalent in cool, moistface of the stone (probably close toreas with a roughened texture.

Measured greenness (δa*) and possibleexplanatory variables, including surfacemoisture, temperature and texture.

154 N.A. Cutler et al. / Science of the Total Environment 442 (2013) 152–164

year (Met Office, 2011), creating favourable conditions for the growthof green algae. Models of future climate change predict increased sea-sonality in the first part of the twenty first century, with wetter win-ters and drier summers (Smith et al., 2010). The city is therefore theideal location to study the interaction of algal communities, thestone surfaces on which they live and the environment.

We surveyed four sandstone heritage structures in central Belfast,namely All Souls Church (AS), Crescent Church (CC), Ewart's Building(EB) and Fitzroy Church (FC) (Fig. 1). All of the buildings were locatedclose to busy roads. The structures were similar in age (115–142 years old) and constructed from a variety of Carboniferous andPermo-Triassic sandstones. The sandstones varied in terms of grainsize, texture and colour. Scrabo and Dugannon sandstones are light,and range in colour from off-white to buff or pinkish in colour. Theyare fine-grained and often contain laminations and lenses of silt andclay, leading to a heterogeneous substrate on small spatial scales. In

Fig. 1. Details from the study structures, showing a) the ashlar walls of Ewart's building; b)areas; the unevenness of the surface has been caused by the detachment of a flake of stone ~course; d) detail of a rock-faced, Scrabbo block from All Souls Church; note the uneven surdotted lines and labelled ‘G’. The algal patches on All Souls Church are smaller and more di

contrast, Giffnock and Locharbriggs sandstones are biscuit-colouredand red-brown, respectively. These sandstones are fine- to coarse-grained and are usually well sorted (Curran et al., 2010). The widerange of sandstone colours and textures present means that Belfast'sbuilding stock may provide analogues for sandstone weathering inmany different locations.

2. Materials and methods

2.1. Sampling strategy

The greening of buildings is inherently patchy. Consequently, thestudy was conducted at two scales, focusing on the properties of indi-vidual blocks (centimetre scale) and larger sections of wall (metrescale). In both cases, stone surfaces with obvious algal staining wereselected for study. Unpublished research by the authors suggests

detail of an ashlar block in Giffnock stone from the same building, showing G and NG1.5 mm thick; c) the rock-faced walls of All Souls Church; note the inclined ashlar stringface texture of the and patchy blackening. Green algal patches on are demarcated withspersed than those on Ewart's Building.


that the algal biofilms on the study structures have similar speciescompositions. For the block surveys, three to five stone blocks werepurposefully selected on each of the four buildings (Table 2). Blockswith patchy algal cover were selected so that green (G) andnon-green (NG) areas could be compared (Fig. 1b, d) and stone hard-ness, permeability and surface colour were recorded. For the transectsurveys, vertical survey lines extending from the base of the wall to~2 m above ground level were established on each building(Table 3). The transect lines were located on north-facing sectionsof wall with obvious algal staining. Surface moisture levels, surfacetemperature and colour were recorded at 4 cm intervals along eachtransect line (50 measurements/transect). Sub-surface resistivitywas measured via electrode arrangement at the same 4 cm intervalsalong the transect lines. Stone hardness was also measured for eachblock crossed by the transect lines.

2.2. Greening and biodeterioration

2.2.1. Surface hardnessMeasurements of surface hardness were taken using an Equotip

type D portable hardness tester (Viles et al., 2011), which measureshardness in Leeb units (Proceq SA, Zurich, Switzerland). For theblock surveys, ten measurements, concentrated in an area of ~1 cm2

in the centre of the G and NG patches, were taken. The measurementswere then used to calculate mean hardness and the coefficient of var-iation of surface hardness (CVSH), a normalised metric of variabilityderived by dividing the sample standard deviation by the samplemean. The assumption was made that the CVSH would act as aproxy for the degree of weathering i.e. this metric would be higherin weathered stone, due to the presence of softer, weathered patchesalongside less altered material. It is possible that a stone surface couldbe highly weathered and uniformly soft (resulting in a low CVSH) butnone of the stone surfaces we surveyed exhibited this characteristic.For the transect surveys, surface hardness was recorded in the centreof each stone block on the transect line, again using 10 measurementsconcentrated in an area of ~1 cm2.

2.2.2. AppearanceThe discolouration of stone surfaces was measured relative to an

unweathered surface of the same stone type. Colour measurementsbased on the L*a*b* system were made using a hand-held KonicaMinolta CM-600d spectrophotometer (Konica Minolta Sensing Inc,Osaka, Japan). Reference values were established on an unweatheredblock of the same stone type, using the average of 10 measurements.The study focussed on deviations on the red-green scale (δa*) and onthe light-dark scale (δL*), referred to hereafter as ‘greening’ and‘darkening’. On these scales, negative values indicate that the

Table 2Details of block survey (Sst = sandstone; RF = rock-faced, ASH = Ashlar).

Location Block no. Sst type

All Souls Church (constructed 1896) 1 Scrabo2 Scrabo3 Scrabo4 Scrabo5 Scrabo

Crescent Church (constructed 1887) 1 Scrabo2 Scrabo3 Scrabo4 Locharbriggs5 Locharbriggs

Ewart's Building (constructed 1869) 1 Giffnock2 Giffnock3 Giffnock

Fitzroy Church (constructed 1874) 1 Locharbriggs2 Locharbriggs

readings are greener and darker than the reference, respectively. InL*a*b* colour space, the distance between two points (essentially avector length) is described as δE. In principle, the untrained humaneye cannot detect a δE valueb1 (although this does vary somewhataccording to hue, saturation and the sensitivity of the observer)(Wyszecki and Stiles, 2000). Therefore, for an area of stone to be per-ceptibly greener or darker than an unweathered reference sample, itmust have a δa* or δL*valueb−1. For the stone blocks, deviationsfrom the reference values were calculated for the G and NG areas,again using the average of 10 measurements. For the transect surveys,single colour measurements were made at 4 cm intervals along thetransect line (i.e. 50 measurements/transect).

2.2.3. Movement of moistureOn the stone blocks, Karsten tube water penetration tests were

used to compare the permeability of G and NG areas. The Karstentubes comprised a 30 mm diameter hemispherical bulb with a10 cm graduated tube attached. The open side of the bulb was ad-hered to stone surface with a continuous compressible seal(Blu-Tack) and 10 ml of water was added. The volume of remainingin tube was recorded at 30 s intervals over a period of 15 min inorder to calculate infiltration rate. Permeability tests were onlyconducted on blocks with comparatively smooth, ashlar surfacesthat were suitable for the attachment of Karsten tubes. Infiltrationrates for G and NG areas were compared with each other, and witha reference rate established on a new wall of a similar stone type.

On the transect lines, sub-surface moisture conditions were in-ferred for all five buildings using 2D electrical resistivity tomography(ERT). The ERT surveys were carried out with a GeoTom device(Geolog 2000, Ausburg, Germany), following the methodology ofSass and Viles (2006). Geoelectrical measurements were carried outby applying a constant electrical current to the stone surface via elec-trodes and measuring the resulting voltage difference at two poten-tial electrodes (refer to Kneisel, 2003; Sass, 2003 for further details).Resistivity values were then calculated from the known voltage andcurrent values. The device was connected via multicore cables to aline of 50 self-adhesive ECG pads spaced at 4 cm intervals along thetransect lines (giving a total transect length of 1.96 m). We utiliseda Wenner electrode array, a configuration that has been used success-fully in ERT on other buildings, as it provides good resolution of struc-tures lying parallel to the stone surface and a high signal-to-noiseratio (Sass and Viles, 2006, 2010). The resistivity measurementswere converted into geometrically accurate representations ofsub-surface resistivity using the Res2DINV inversion software withrobust inversion settings (Loke, 1999). The electrical resistivity ofbuilding stone is largely determined by its moisture content (Loke,1999); it was therefore assumed that low resistivity equated with

Surface Aspect Property studied

Hardness Colour Permeability

RF W Y Y NRF W Y Y NRF W Y N NRF NW Y N NRF SW Y N YRF N Y N NRF N Y N NRF N Y N NASH N Y Y YASH N Y Y NASH N Y Y YASH N Y Y YASH N Y N NASH N Y Y YASH N Y Y Y

Fig. 2. The relationship between mean hardness and CVSH for G (filled circles) and NG(unfilled circles) areas. The arrow indicates an outlying value from the G dataset.

Table 3Details of transect surveys.

Location Aspect Wall construction Notes

All Souls Church N Coursed, rock-faced Scrabo Sst blocks with Doulting Lst string coursetowards top of transect line;

Base of transect in shallow (c. 60 deep) trench; stone noticablydarkened with bright green and orange algal patches

Crescent Church N Squared and pitched, rock-faced Scrabo Sst blocks with dressedLocharbriggs string course at top of plinth

Patchy blackening and green algal cover

Ewart's Building N Large, dressed Giffnock Sst blocks with projecting string course andmoulded plinth top

Thick algal and bryophyte cover at base of plinth

Fitzroy Church N Squared and pitched, rock-faced Scrabo Sst blocks with two dressedScrabo Sst string courses

Greening apparent across whole surface; stone crumbling in places;building cleaned in 1980s


high moisture availability and vice versa, although it is possible thatlow resistivity values may also be due to the presence of salts, mate-rial type and characteristics such as porosity.

After carrying out the initial ERT surveys, the movement of mois-ture was assessed by means of a wetting experiment. Water was ap-plied to the area in either side of the transect lines to simulate drivingrain, broadly following the methods of Sass and Viles (2010). First, 2 Lof water was applied in a fine mist spray over a period of approxi-mately 5 min. The wall was wetted evenly in a swath extending ap-proximately 10 cm either side of the transect line. ERT surveys werethen carried out 15 and 30 min after wetting to assess the ingress ofwater into the wall. Resistivity-depth plots were produced by calcu-lating the average resistivity values in 5 cm depth bins. The impactof algal cover was assessed by comparing mean resistivity values inzones below algal patches with those falling outside these zones.The zone boundaries were defined by edges of the algal patches andprojected into the stone normal to the transect line.

2.3. Greening and environmental conditions

In addition to measuring surface hardness, colour and factors re-lating to the movement of moisture, we also recorded environmentalparameters likely to be important in determining the distribution ofalgal patches, including surface moisture, temperature and texture.We restricted our analysis to the transect surveys. An analysis of co-variance (ANCOVA) was used to model the relationship betweengreening (δa*) and the potential explanatory variables (moisture,temperature, CVSH and surface texture).

2.3.1. Surface moisture and temperatureSurface moisture levels along the transect lines were measured

using a hand-held Protimeter moisture meter (GE, Fairfield, CT,USA). The device records surface moisture levels in % wood moistureequivalent (w.m.e.); it is easy to use and routinely applied in assess-ments of moisture in the construction industry and can be used onstone to provide a picture of relative moisture trends (although, aswith ERT, readings may be influenced by the presence of salts:Eklund et al., 2012). Single Protimeter measurements were made at4 cm intervals (in the same locations where the colour readingswere taken) and averaged for each block crossed by the transectline (the number of measurements per block, n, varied according toblock size). Surface temperature measurements were made at thesame intervals, using a Fluke 62 mini infra-red thermometer (Fluke,Everett, WA, USA). In the case of both moisture and temperature,the focus in this study was on relative differences along the transectline, rather than absolute values.

2.3.2. Surface textureThe blocks crossed by the transect lines varied in terms of surface

texture. For each sample point along the transect lines, surface tex-ture was recorded as either ashlar (ASH, where the stone was workedto a smooth, even surface) or rock-faced (RF, where the surface of thestone was roughly hewn) (Fig. 1b, d).

3. Results

3.1. Greening and biodeterioration

3.1.1. Physical integrityIn the block surveys, mean stone hardness varied widely from

308±91 to 546±62 Leeb units. There were no significant differencesin hardness among the three stone types (ANOVA: F2,16=1.17, p=0.33). CVSH ranged between 6 and 32% and was related to meanhardness: harder surfaces were less variable. When data from allthe blocks were analysed, there was no significant difference betweengreen (G) and non-green (NG) sections within blocks in terms of theCVSH. However, the spread of the G data was heavily influenced by anoutlying value (indicated by an arrow in Fig. 2). When this value wasomitted, G areas were significantly less variable than NG patches(t-test: t=2.2, p=0.045) as well as being significantly harder(t-test: t=2.2, p=0.04). The range in mean stone hardness was sim-ilar in the transect surveys (315.1±27.5 to 391.5±16.8 Leebs). Stonehardness exhibited considerable variability along the transect lines,particularly on Fitzroy Church (Fig. 3d) where CVSH ranged from 7to 58% (Table 4). There was no overall correlation between CVSHand the degree of greening on the transect surveys (Table 5).

3.1.2. AppearanceColour measurements made during the block survey revealed the

pronounced impact of algal staining: when G and NG areas were com-pared, the mean difference in δa* was −5.8. However, there was nosignificant difference in the darkening of G and NG areas on theblocks (t-test: t=0.71, p=0.49). The results of the transect surveysindicated that all of the surfaces were, on average, greener and darkerthan unweathered stone of the same type (Table 4). Each of walls hadclearly defined green patches; All Souls Church also had patches ofpositive δa* values associated with a reddish-orange algal covering

Fig. 3. A summary of the data collected from the transect surveys a) All Souls Church; note the positive peaks on the right hand side of the δa* plot (which indicate patches of orange-redalgae), the relatively highmoisture values close to ground level and the increase in temperature with increasing distance from the ground; b) Crescent Church; c) Ewart's building; notethe high variability in greening andmoisture levels; d) Fitzroy Church; note the consistent greening and high variability in surfacemoisture and CVSH. The transect profiles are shown onthe left hand margin of the plot (solid stone shaded grey). The vertical dotted lines on the δa* and δL* plots represent a departure of −1 units from the reference value derived fromunweathered stone; points to the left of these lines would be perceptibly greener or darker than unweathered stone. Error bars indicate 1 SE.


(Fig. 3a). The patches varied in size: those on the uneven, rock-facedScrabowalls (All Souls Church and Crescent Church)were small. In con-trast, the smooth, ashlarwalls of Ewart's Buildingwere characterised by

large, continuous areas of green (Fig. 1a). The average level of greeningwas less than the threshold value of −1 units on All Souls Church(probably due to the presence of red-orange algal cover), but more

Table 4Summary of transect data (w.m.e. = wood moisture equivalent; SH = surface hardness; CVSH = coefficient of variation of surface hardness).

δa* Mean moisture(% w.m.e.)

Moisture range(% w.m.e.)

Mean temp.(°C)

Temp. range(°C)

δL* Mean SH(Leeb units)

Mean CVSH(%)

All Souls Church −0.7±0.4 14±0.6 7.6–32.5 11.5±0.2 9.6–13.4 −19.5±0.6 391.5±16.8 24±2Crescent Church −1.5±0.2 15±0.5 7.2–29.8 12.5±0.1 10.6–13.6 −12.3±0.5 366.4±24.6 19±3Ewart's Building −4.4±0.7 20±1.4 8.5–64.5 9.5±0.04 8.8–10.0 −8.4±1.4 361.1±16.8 20±3Fitzroy Church −3.2±0.1 21±1.6 7.6–32.5 10.5±0.1 9.4–11.4 −11.5±1.6 315.1±27.5 27±4


pronounced on the remaining buildings. Fitzroy Church was noticeablygreen along the whole transect (Fig. 3d). The wall base was greenerthan the upper parts of the transect lines on the Crescent Church andEwart's Building, but there was no discernible structure to the greeningon All Souls Church or Fitzroy Church. The darkening of the stone wasmarked on all four structures, particularly All Souls Church (Fig. 3a).The δL* values recorded ranged from −2.3 to −29.6 but, as withgreening, there was little overall structure to the colour change:the only gradient was apparent on Ewart's Building, which wasdarker at the base (Fig. 3c). There was no significant correlation be-tween greening and darkening when all the blocks surveyed wereconsidered (Spearman-rank: Rs=−0.13, p=0.42).

3.1.3. Movement of moistureThe results of the block analyses indicated that infiltration rates

for weathered stone were higher than those for unweathered stonefor three of the four buildings (All Souls Church, Crescent Churchand Ewart's Building: detailed results not shown). Infiltration rateson the Fitzroy Church blocks were higher than unweathered stonein NG areas and similar in G areas. There were no perceptible differ-ences in infiltration rate for G and NG areas for All Souls Church andCrescent Church. For Ewart's Building and Fitzroy Church, infiltrationrates were lower in G areas.

The ERT surveys conducted along the transect lines had a maxi-mum penetration depth of ~32 cm. With the exception of Ewart'sBuilding, resistivity was broadly distributed according to a) heightabove ground level and b) depth below stone surface. In terms ofheight, the lower parts of the walls were dominated by low resistivityvalues (‘cool’ colours i.e. blues and greens on the ERT plots: Fig. 4). Interms of depth, three distinct zones were apparent: 1) a thin (~5 cm)zone near the wall surface characterised by high resistivity values(‘hot’ colours i.e. yellows and reds: Fig. 4); 2) a zone at intermediatedepth (~5–10 cm) characterised by low resistivity values and 3) thedeepest part of the wall, characterised by high resistivity values.This pattern may be seen in the resistivity/depth plots (Fig. 5). ForAll Souls Church, Crescent Church and Fitzroy Church (Fig. 5a, b andd), mean resistivity increased with depth and the lowest resistivityvalues were found at a depth of 5–10 cm. The opposite pattern wasobserved for Ewart's Building: surface resistivity was variable butgenerally high and mean resistivity decreased with increasingdepth. Across all the walls, the highest values (assumed driest)tended to be clustered in high, deep locations; the lowest valueswere generally in low locations of intermediate depth.

In addition to these broad patterns, it was also possible to discernsmall-scale features, such as individual stone blocks and abruptchanges in material (designated by Roman numeral in Fig. 4). For

Table 5Correlations between the variables recorded on the transect surveys. Significancecodes: * 0.05; ** 0.01; *** 0.001.

δa* Surface moisture Surface temperature δL*

Surface moisture 0.23 –

Surface temperature 0.37* −0.57*** –

δL* −0.13 0.43** −0.28 –

CVSH 0.12 −0.08 −0.02 −0.15

example, stone blocks and mortar joints are clearly visible in the AllSouls plot (Fig. 4a(i)). Similar structures were much harder to discernon the Ewart's Building plot, where the stone blocks were larger andthe intervening joints were thinner (Fig. 4c). Certain surface featureswere associated with abrupt changes in resistivity. For example, thelower string course (a stonework feature taking the form of a hori-zontal band) on Ewart's Building had comparatively low resistivityvalues (Fig. 4c(ii)). Inclined string courses with low resistivity werealso observed on the Crescent Church plot (Fig. 4b(ii)). However,the upper string course on the Fitzroy Church (Fig. 4d(iii)) and thestring course on the All Souls plot (constructed of coarse, porousDoulting limestone) had high resistivity (Fig. 4a(iii)).

Visual inspection suggested that green patches on the surfacewere associated with relatively high resistivity values (i.e. inferreddrier conditions) in the intermediate subsurface. This was particularlyevident on Ewart's Building transect, where resistivity values in theplinth (which had a thick algal coating) were higher than those inthe non-green block above (Fig. 4c(iv)). Similar patterns were ob-served on the Crescent Church (Fig. 4b(iv)). Mean resistivity valuesfor the stone beneath G and NG zones provided some evidence tosupport this observation. For example, mean resistivity valuesbelow green patches were higher than those of adjacent non-greenareas for both All Souls Church and Ewart's Building (Fig. 6). The op-posite was observed for the Crescent Church; however, high meanvalues for NG areas were driven by exceptionally high resistivity ina patch towards the top of the transect line. The association betweenalgal cover and high resistivity values was largely restricted to wallbases. In the upper parts of the walls, where algal biofilms were gen-erally scarcer, there appeared to be no consistent relationship be-tween greening and subsurface moisture levels.

The overall distributions of resistivity values did not change dur-ing the wetting experiments. For example, wetting had almost no ef-fect on the resistivity profile of the Ewart's Building transect, largelybecause the applied water drained readily from the smooth surfaceof the stone (the hydrophobic properties of extensive algal covermay have promoted this process). Not surprisingly, the applicationof water tended to lower resistivity values (Fig. 5a, c–d). The impactof wetting was apparent at all depths 15 min after the applicationof water. The observed patterns were typified by the Fitzroy Churchsection (Fig. 7). Prior to wetting (t=0), the section was characterisedby high resistivity regions on the surface (Fig. 7(i)) and at depth,mostly in the upper parts of the wall (Fig. 7(ii)). Individual blockswere visible, including some with higher resistivity than their sur-roundings (Fig. 7(iii)) and some with lower resistivity values(Fig. 7(iv)). Fifteen minutes after wetting (t=15), resistivity readingsat depth were generally lower (Figs. 5d, 7(v)). Surface resistivity wasalso reduced in areas of high initial resistivity (Fig. 7(vi)). Drying atthe surface (Fig. 7(vii)) and at depth (Fig. 7(viii)) was evident30 min after the application of water; toward the base of the wall, re-sistivity was lower (Fig. 7(ix)). Overall, resistivity readings did not re-turn to their original level in this period. Resistivity increased at depthin some locations (for example, the deepest layers of the Ewart'sBuilding transect: Fig. 4c); anomalies of this type have been observedin previous studies and have been interpreted as an artefact of the in-version routine, rather than drying of the stone (refer to Sass andViles, 2010, for further details).

Fig. 4. ERT surveys of four structures, showing the inferred 2D ERT section (LHS of each plot), a bar representing algal patches (shaded grey) and a plot of δa* values at each electrodeposition (electrodes spaced at 4 cm intervals along the transect). On the ERT sections, ‘hot’ colours indicate high resistivity values (assumed dry conditions) and ‘cool’ colours indicatelow resistivity values (assumed damp conditions). The surface of the wall is to the right of the section; the section starts at a depth of ~2 cm below the wall surface. Green patches aredemarcated where δa* values are b−1. The figures on the vertical axis of the ERT section indicate measured distances (in metres) along the transect. Key to areas indicated by romannumerals: (i) individual stone blocks; (ii) string courses with comparatively low resistivity values; (iii) string courses with comparatively high resistivity; (iv) sub-surface areas withanomalously high resistivity, relative to surroundings.


3.2. Greening and environmental conditions

3.2.1. Surface moisture and temperatureMean surface moisture levels varied within a narrow range (14±

0.6% to 21±1.6% w.m.e.), with considerable small- (centimetre-)scale variability (Table 4; Fig. 3). Variation was greatest on Ewart'sbuilding and Fitzroy Church (Fig. 3c and d). There was some evidencethat the wall bases were damper than higher sections (e.g. on AllSouls Church and Crescent Church: Fig. 3a and b). Surface tempera-ture broadly followed this trend, increasing with height above groundlevel in a linear fashion. On average, surface temperature varied by2 °C over the length of the transect lines (a vertical distance of~2 m). On Ewart's Building, the temperature range was only 0.7 °C;variation was higher on the rock-faced walls, reaching 3.2 °C on theAll Souls transect (Fig. 3a and d).

3.2.2. Correlation structureThere were some significant correlations between the variables

recorded on the transect surveys (Table 5). There was a significantpositive correlation between greening and surface temperature, indi-cating that cooler surfaces were greener (although the relationshipwas not significant when the points with the two lowest temperaturevalues were removed: Fig. 8a). In addition, there was a highly signif-icant negative correlation between surface moisture and surface tem-perature (moist surfaces were cooler); moisture levels were alsorelated to darkening (drier surfaces were darker) (Fig. 8b and c). Asimple linear regression of greening on temperature was significant(p=0.02) and indicated that temperature accounted for 14% of thevariance in δa*. None of the other potential explanatory variableswas significant in the ANCOVA. There was no overall correlation

between greening and surface moisture; these variables followedeach other closely on Ewart's Building (Fig. 3c), but the correlationwas not significant. Greening and darkening were correlated onEwart's Building and the Crescent Church (green areas were darker),but not on the other walls. There was no significant difference betweenthe degree of greening on rock-faced and ashlar blocks (Mann–WhitneyU-test, p=0.36). None of the correlations with CVSH were significant.

4. Discussion

The surveys demonstrated that exposed stone surfaces that hadbeen exposed in Belfast for around 100 years were, on average,greener and darker than recently-exposed stone of the same type.The link between greening and the physical condition of the stonewas less clear and it is likely that the physical disintegration ofstone in Belfast is mainly driven by abiotic factors. However, therewas evidence that hardness was less variable in green areas. Thinbiofilms on the upper parts of walls (sampled in the block survey)had little impact on permeability. However, the ERT surveyssuggested that thick algal biofilms on the bases of the walls were as-sociated with relatively dry subsurface conditions. Greening waspoorly correlated with the environmental variables measured in thisstudy, although there was a significant negative correlation betweentemperature and greening, suggesting a link between evaporationrates and biological activity.

4.1. Physical structure [H1]

Harder surfaces exhibited less variability in hardness than softersurfaces (Fig. 2). There was no compelling evidence to link green

Fig. 5. Resistivity depth plots for a) All Souls Church; b) Ewart's Building and c) Fitzroy Church. The mean figures are derived from resistivity values binned at 5 cm depth intervals.The values plot for t=0 indicates the values under ambient conditions; the t=15 and t=30 plots indicate the impact of wetting. Note the differences in the scaling of the y-axis.Error bars indicate 1 SE.


biofilms to high levels of weathering. Conversely, there was evidencefrom the block studies that algal patches were associated with lessweathered surfaces (i.e. harder patches with lower CVSH). Thismight indicate that green algal cover has a broadly bioprotectiverole. Of course, green algal patches can be transient and the biofilmssurveyed in this study may not have had sufficient time to contributeto weathering processes. Detailed examination by scanning electronmicroscope (SEM) would be required to definitively establish wheth-er or not microbes were associated with weathering (De Los Ríos etal., 2004). However, it seems likely that the main factors driving theweathering of stone in the study locations were abiotic processes.

Fig. 6. Comparison of mean resistivity values in stone lying beneath green algal patches(G) and areas without algal cover (NG). Error bars indicate 1 SE.

Buildings in Belfast city centre are exposed to a range of abiotic dete-riorative processes, notably salt weathering (Turkington and Smith,2000). Although concentrations of atmospheric SO2 and NOx are indecline (DOE, 2005), air quality in the city can be poor due to a com-bination traffic congestion, meteorological and topographic factors(Smith et al., 2010). The only way to definitively establish the relativecontribution of greening to the physical deterioration of stone wouldbe the use of long-term, high resolution studies on suitably config-ured test walls.

4.2. Appearance: greening and darkening [H2]

The study identified changes in the appearance of the stone sur-faces over time. On average, the stone surfaces studied were greenerand darker than fresh stone of the same type. Darkening of stone canoccur for several reasons, including the gradual accumulation of dustparticles and/or the formation of pigmented microbial biofilms (in-cluding crusts of lichenized fungi e.g. Verrucaria spp.) Microbial actionmight also induce the oxidation of manganese in the stone (de laTorre and Gomez-Alarcon, 1994), which would also lead to darken-ing, although we did not investigate this. Greening and darkeningappeared to be closely associated in plinth areas. However, thelower parts of a wall are prone to soiling (e.g. by particles of soilentrained in rain splash) so the darkening of the stone in these loca-tions is not necessarily related to biological processes. The relation-ship between greening and darkening was weaker on the upperparts of the wall: in the block surveys, for example, there was no cor-relation between algal cover and darkening, despite stark differencesin greenness between G and NG areas. We did not distinguish be-tween biotic and abiotic darkening. However, it seems likely thatthe darkening of the stone was driven by processes that have

Fig. 7. The results of the wetting experiment carried out on the Fitzroy Church transect. Time after wetting is indicated in minutes (i.e. t=0, 15 and 30 min). Key to areas indicatedby roman numerals: (i) high surface resistivity; (ii) high resistivity at depth; (iii) individual block with high resistivity; (iv) individual blocks with lower resistivity values than theirsurroundings; (v) lowered resistivity at depth (at t=15); (vi) lower surface resistivity (at t=15); (vii) evidence of surface drying (increased resistivity, at t=30); (viii) increasesin resistivity at depth towards top of wall (at t=30); (ix) decreased resistivity at depth towards base of wall (at t=30). Note the resistivity scale differs from Fig. 4. Error barsindicate 1 SE.


operated independently from biofilm formation. In Belfast, as in manyother urban areas, blackened stone is often associated with the for-mation of surface crusts that form when minerals in the stone reactwith atmospheric pollutants such as SO2 (Turkington and Smith,2000). Anecdotal evidence suggests that green biofilms are associatedwith black crusts (Smith et al., 2010), possibly because changes instone properties associated with crust formation increase thebioreceptivity of the substrate. This was observed on All Souls Church(Fig. 1 d). However, we could not find a consistent relationshipwhen allof the buildings were considered. Whilst greening and darkening maycoincide, it seems likely that this is due to a confounding variable(such as moisture supply) rather than a direct causal correlation.

4.3. Moisture movement [H3]

The block studies indicated that the permeability of the old stonewas higher than that of new stone, suggesting an opening-up of thestone structure by weathering processes. However, when a largernumber of individual blocks were studied, permeability was not con-sistently related to algal cover. The lack of significant variation may berelated to the thinness of the biofilms studied; in each case the algalcovering on the blocks was bright green but thin enough so that thecolour and texture of the underlying stone could be seen. Muchthicker algal coverings were observed on some of the transect studies(e.g. at the base of the Ewart's Building transect) and these seem to

Fig. 8. Scatterplots illustrating significant correlations in the dataset, based on block averages from the transect lines: a) relative greening (δa*) and temperature; b) surface mois-ture and surface temperature and c) relative darkening (δL*) and surface moisture.


have had more related to the properties of the stone (surface temper-ature and moisture, in particular). It may be that there were insuffi-cient algal cells present on the surface of the individual stone blocksstudied to affect permeability or initiate significant bioweathering.

The ERT analyses revealed details of the 2D distribution of mois-ture in the survey walls. On average, the lowest mean resistivityvalues were recorded for Fitzroy Church; this was also the greenestbuilding, suggesting a connection between moisture and greening.There was also evidence from the ERT surveys that the lower partsof the walls studied were both damper and greener than the uppersections. This result is not surprising, as the lower parts of walls aresubject to the capillary rise of ground water and rain-splash.

There was evidence that algal cover was associated with higher re-sistivity values (and inferred drier conditions) at depth (see, for exam-ple, Fig. 6). This pattern was apparent in the ERT data from All SoulsChurch and Ewart's Building. It was also apparent on the ERT plot forCrescent Church, although a patch of very high resistivity valuesmeant it was not observed in averaged values. It is therefore possiblethat algal biofilms impede the ingress of moisture, consistent with ear-lier studies (Smith et al., 2011). However, it will require systematic,long-term monitoring of subsurface moisture levels to determinewhether or not algal biofilms are responsible for this effect and what(if any) impact this has on the long-term stability of stone structures.

In the wetting experiments, mean resistivity values did not returnto their original values within 30 min and it was difficult to assess theimpact of algal cover on moisture levels at depth. However, there wasevidence of rapid surface drying and it is likely that, outside of periodsof prolonged rainfall, the greater part of the wall surface is ‘dry’ formuch of the time. Under these conditions, metabolic activity in algalcells is likely to be restricted to comparatively short spells followingwetting events. If this is the case, subsequent studies will have touse continuous measurements of surface moisture and integrate theresults over time, to derive a time-of-wetness metric. These futurestudies would also have to establish the minimummoisture thresholdabove which lithobiotic algae can function.

4.4. Environmental parameters

If moisture was the main determinant of greening, then the patchydistribution of the algae, coupled with the sharp patch boundaries,would suggest significant and abrupt variability in surface andnear-surface moisture levels. This was not observed: surface moisturevalues were highly variable, but largely unrelated to the incidence ofalgal patches. This result was surprising, as other studies have indicat-ed that moisture is a key limiting factor for sub-aerial algae(Bellinzoni et al., 2003).

The weak correlation between surface moisture levels and the in-cidence of algal patches may have been due to the survey strategyadopted. Surface moisture levels on exposed stone surfaces are ex-tremely dynamic, varying on a variety of timescales from minutes tomonths (Sass and Viles, 2006). The measurements taken in thisstudy were point values and cannot reveal long-term patterns. How-ever, moisture measurements performed during comparatively dryspells can reveal areas in which residual moisture persists, and it isthese areas that are likely to be most favourable for algal growth. Sim-ilarly, point measurements of temperature can reveal trends in tem-perature that could drive differences in evaporation rates, andhence available moisture. It is possible that the moisture dynamicsthat influence algal distribution cannot be captured by point studies.Furthermore, it may be that the Protimeter, whilst suitable for rapidsurveys, is a poor guide to the moisture conditions experienced bythe algae. The presence of dissolved salts in pore water, for example,can produce anomalously low resistivity readings in geoelectrical de-vices (Sass, 2003). Even though the walls we studied showed no ob-vious signs of efflorescence, it could be that the presence ofsub-surface salts confounded our results.

In contrast tomoisture readings, surface temperaturewas correlatedwith greening. Areas with lower temperatures were, on average,greener than warmer areas. The relationship was strongest on wallswhere the plinth was very green; temperature performed less well asa predictor of greening on the upper parts of walls, where temperatures


were generally higher. Abrupt changes in temperature associatedwith discrete algal patches were not observed. The overall range oftemperature variation was comparatively small (~2 °C) and proba-bly insufficient to make a significant impact on algal metabolism.The relationship between greening and temperature was thereforeprobably due to lower rates of evaporation in cooler areas at thebase of walls, although other effects in these areas, e.g. an increasedsupply of moisture and nutrients through rain-splash, may also havebeen important.

4.5. Unmeasured environmental variables

The greening observed on the transect lines exhibited high spatialheterogeneity, often on sub-block scales, but was unrelated to most ofthe environmental variables recorded. The weak relationship be-tween greening and the explanatory variables meant that it was notpossible to satisfactorily model the greening of sandstone buildingsin Belfast. Of the environmental variables studied, only surface tem-perature was related to greening, and this factor only accounted foraround 14% of the observed variability in greening. This indicatesthat there were other, unmeasured, factors driving the incidence ofalgal patches. It is likely that a reconfigured metric of moisture, e.g. an-nual time-of-wetness, would improve the explanatory power of themodel. Other likely candidates for explanatory variables include fine-(sub-block-) scale surface morphology and macronutrient status.

Surface morphology encompasses centimetre- and sub-centimetresurface roughness and inclination (measured relative to a verticalplane). On the rock-faced Scrabo walls, algal patches appeared to be as-sociatedwith centimetre-scale variations in surface topography. Certainmicrotopographic configurations e.g. hollows or shallow slopes, couldbe more suited to algal colonisation because they collect moistureand/or dry more slowly (Miller et al., 2012). At a larger scale, the incli-nation of architectural featuresmay influence hydrological regimes. Ob-servations of buildings in Belfast suggest that inclined stone surfaces(e.g. the tops of window sills, buttresses and projecting string courses)are preferentially colonised by algae, perhaps because they dry moreslowly than vertical faces. The underside of projecting features may at-tract algae because they are often in shade. The simple categorisation ofsurface texture used in this study did not capture differences in green-ing related to surfacemorphology. However, it is possible that high res-olution studies that focus on fine-scale surface morphology (perhapssystematically contrasting inclined and vertical features) will be moresuccessful. Exposure trials utilising stone blocks with a variety of archi-tectural features would be particularly useful.

Macronutrient supply is a key factor regulating biological activityin all ecosystems and spatial heterogeneity in this factor is often asso-ciated with patchiness at various scales. Building stone is an oligotro-phic habitat and nutrients such and nitrogen (N) are likely to belimiting or co-limiting (with moisture) to algal productivity (John,1988; Round, 1981). Nitrogen is unlikely to be derived directly fromstone substrates; N will therefore be supplied predominantly throughwet and dry deposition. Changes in atmospheric concentrations ofNOx are therefore likely to influence overall levels of greening andmay be more influential than changing precipitation patterns in thiscontext. However, small-scale hydrological factors are also likely toinfluence the distribution of deposited macronutrients across theface of buildings, and hence the distribution of algal patches. Thisphenomenon has not been studied, to our knowledge, but it couldcertainly be a contributory factor to fine-scale distribution of algalgreening.

In addition to environmental conditions, biological processese.g. dispersal or competition for limiting resources, can drive spatial pat-terning in biological communities (Fortin and Dale, 2005). Species-leveldifferences in the dispersal capability, or the distribution of parentpopulations, could lead to local differences in the composition of algalcommunities. It is difficult to generalise about this factor without

knowing the detailed species composition of the biofilms studied. Theairborne dispersal of green algae is poorly understood (Sharma et al.,2006, 2007) and the complex wind environments of urban settingsmake it difficult to estimate how dispersal might impact on the distribu-tion of algal communities. However, the microbes found on buildingstone tend to be generalists and green algae are found in a host ofterricolous, corticolous, saxicolous and terricolous habitats, includingother buildings (Rindi, 2007; Rindi and Guiry, 2003, 2004). Sourceareas for colonisation are therefore likely to be numerous. Given thegeographical proximity of the study structures, and similarities in theirsurroundings, there was nothing to suggest that dispersal differencescontributed to the observed variations in greening.

In addition to dispersal, other biological processes might also havestructured the biofilms in this study. Community interactions are poorlyunderstood for microbes in the environment and exceptionally difficultto monitor. It is not clear whether macroscale patterns (such as largealgal patches) can emerge from microscale competitive interactions.Contagious spread following a random colonisation event could leadto patchiness that would not necessarily be related to environmentalvariables, providing these factorswere not limiting. Fortuitous colonisa-tion events might be favoured by surface irregularities, as discussedabove. Finally, there is likely to be a substantial random component tothe incidence of algal biofilms, e.g. cleaning or abrasion of the stone sur-face, overshadowing by trees or buildings and transient moisturesources (e.g. leaks), to name a few.

Variations in community composition could impact on studies of thistype, as different communities (with different rates and types of meta-bolic activity) would reduce the comparability of different structures.Unpublished research by the authors suggests that the algal communi-ties on Belfast sandstone are very similar in composition. However,other studies of this typewould have to establish algal community com-position (including an assessment ofwhether the community included asignificant component of cyanobacteria) before comparing rates andpatterns of weathering between buildings.

5. Conclusions

Green biofilms can have an aesthetic impact on heritage struc-tures, but in our study this appeared to be limited to green staining(i.e. the algae were not consistently associated with the darkeningof the stone). Whilst the growth of green algal biofilms may be con-sidered visually detrimental, we could find no evidence that algalgreening is directly involved in the weathering of sandstone (othertechniques, e.g. use of scanning electron microscopy, would be re-quired to establish definitively whether or not green algae contribut-ed to the weathering of the stone). ERT data suggest that algal patchesare associated with higher resistivity values (and, by implication,lower moisture levels) in the immediate subsurface. Coupled withdata on the relationship between stone hardness and algal cover,there is therefore evidence that green algae might have a broadlybioprotective role. Further study, ideally based on test structures,will be required to establish whether or not this is the case. Our at-tempts to model greening suggest that surface temperature is corre-lated with the distribution of algae across the surface of a stonebuilding, but this factor is likely to be confounded with other vari-ables. It is likely that moisture is a key factor in determining the over-all distribution of green algal biofilms. However, the relationshipbetween moisture levels and greening is not straightforward and islikely to be heavily dependent upon the temporal distribution ofmoisture and, possibly, transient wetting events. Overall, it seemsthat algal greening of sandstones is more related to environmentalconditions (particularly climate and atmospheric chemistry) thanstone type. It may therefore be possible to model algal greening ofsandstones from the scaled-down outputs of regional climatemodels.


Acknowledgements

We thank three anonymous reviewers for their insightful com-ments. This work was funded by EPSRC grant no. EP/G011338/1

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