spirulina synchronization.pdf
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Synchronization of fluid-dynamics related and physiological time scalesand algal biomass production in thin flat-plate bioreactors
Alemayehu Kasahun Gebremariam and Yair Zarmia)
Jacob Blaustein Institutes for Desert Research Ben-Gurion University of the Negev Midreshet Ben-Gurion,84990, Israel
(Received 22 October 2011; accepted 7 December 2011; published online 6 February 2012)
Experiments on ultrahigh density unicellular algae cultures in thin flat-plate bioreactors (thickness
2 cm) indicate that: i) Optimal areal biomass production rates are significantly higher than intraditional ponds or raceways, ii) productivity grows for radiation levels substantially higher than
one sun; saturation emerging, possibly, at intensities of about four suns, and iii) optimal volumetric
and areal production rates as well as culture densities increase as reactor thickness is reduced.
The observations are reproduced within the framework of a simple model, which takes into account
the random motion of cells across the reactor thickness, and the competing effects of two
physiologically significant time scales. These are TR, the time that elapses from the moment a
reaction center has collected the number of photons required for one photosynthetic cycle until it is
available again for exploiting impinging photons (110 ms), and TW, an average of the decay time
characteristic of photon loss processes (several ms to several tens of ms). VC 2012 American
Institute of Physics. [doi:10.1063/1.3678009]
I. INTRODUCTION
A. Algal mass production
Micro-algal mass-production has attracted great interest
since the middle of the twentieth century due to the potential
for the production of valuable materials for the aquaculture,
cosmetic, food and pharmaceutical industries, as a potential
source of proteins, as a photosynthetic gas exchanger for
space travel, as a means for waste-water quality improve-
ment, carbon dioxide fixation, and biomass conversion, and
as a renewable energy source through hydrogen and bio-diesel production.113 A constraining factor has been the
lack of efficient large-scale cultivation techniques. Open and
closed systems have been the two major classes of
bioreactors.
Open bioreactors (outdoor ponds or raceways) suffer
from imprecise control over process parameters, and little or
no control over temperature and incident light intensity. Fur-
thermore, CO2 utilization efficiency is low due to lack of tur-
bulent flow and escape of gases from the culture
medium.2,5,6 Contamination by other micro-organisms
causes a reduction in culture growth rate and product quality.
The low output rate per reactor surface area and high produc-
tion costs make this type of bioreactors uneconomical for
most products, except for high-value compounds. No less
important is the observation, known already for many years,
that the optimal areal productivity in open ponds does not
depend on pond depth.14 These limitations have led to a rise
in interest in enclosed bioreactors, which offer better control
over process variables, greater CO2 utilization efficiency,
and reduced contamination.113,1532 This paper focuses on
flat-plate bioreactors.
B. Thin flat-plate bioreactorsReview of experimentalresults
Figure 1 shows a simplified side view of a thin flat-plate
reactor. The height and width of the reactor are dictated by
the needs of the production plant, but it is just a few cm (typ-
ically, 12 cm) thick. Light, either natural or artificial, hits
one or both flat sides of the reactor, its intensity falling off as
it propagates through the culture. Air bubbles, fed at the bot-
tom, rise by buoyancy to the top.
The characteristics of biomass-production of Spirulina
platensis and Nannochloropsis in thin flat-plate reactors
have been studied in Refs. 2131. The qualitative features of
the experimental observations were the concurrent rise of the
optimal volumetric and areal production rates and of culture
density at optimum, as reactor thickness was reduced from
20 to about 1 cm. For example, in experiments on Spirulina
platensis, the optimal dry-weight density grew from about 1
to 2030 kg m3
, the optimal volumetric production rate
grew from about 10 to about 600 dry weight gr m3 h1, and
the optimal production rate per unit reactor surface area
grew from 2 to about 6 dry weight gr m2 h1. The low val-
ues, obtained for a 20 cm thick reactor, are comparable to
those obtained in open bioreactors. Finally, when the totalphoton flux density on both flat sides of a reactor of 0.75 cm
thickness, was varied from 270 to 8000 lmol m2 s1 (about
4 times the photosynthetically active part of the solar flux at
midday), the optimal density rose from about 6 to about 30
dry weight kg m3, and the optimal volumetric production
rate rose from 100 to 1200 dry weight gr m3 h1. Results of
the same characteristics were obtained in the cultivation of
Chlorococcum littorale.32 Most important, the optimal pro-
duction rate grew steadily as the photon flux was increased
up to extremely high values. Saturation seemed to begin set-
ting in only at the highest photon flux.26,27,30,32 Photo-a)Electronic address: [email protected].
0021-8979/2012/111(3)/034904/11/$30.00 VC 2012 American Institute of Physics111, 034904-1
JOURNAL OF APPLIED PHYSICS 111, 034904 (2012)
http://dx.doi.org/10.1063/1.3678009http://dx.doi.org/10.1063/1.3678009http://-/?-http://dx.doi.org/10.1063/1.3678009http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://-/?-http://dx.doi.org/10.1063/1.3678009http://-/?-http://dx.doi.org/10.1063/1.3678009http://dx.doi.org/10.1063/1.3678009 -
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inhibition and photo-damage did not seem to have significant
effects even at the highest radiation levels.
C. Modeling biomass production in bioreactors
There is ample literature on modeling algal biomass pro-
duction. Common to many works is the assumption that the
algae cells are exposed to continuous radiation under steady
state conditions. This assumption applies, for example, to a
leaf under sunshine, and is a valid approximation when the
optical depth of the reactor (or shallow pond) is large, e.g.,
in the case of plankton in the ocean (the depth of the
bioreactor is, probably, tens of meters, if not more), or to
algae a low-density shallow pond of depth of about 10 cm or
more. First, the time it takes a cell to cross the full length
of the optical depth is then much longer than any physio-
logically significant time scale. Second, for the low culture
densities involved, the attenuation of radiation intensity is
slow. During the time a reaction center completes one
photosynthetic cycle, the cell has moved only a very short
distance. Hence, to a very good approximation, it is
exposed to constant radiation intensity during a cycle.
Such models yield what has been observed experimentally,
namely, that the optimal areal production rate is independ-
ent of the optical depth. This was found already yearsago,14 and, in recent years, in the detailed and thorough
analysis presented in Refs. 3339 for optical depths in the
excess of about 10 cm.
However, the predictions based on the steady state,
continuous-radiation assumption do not reproduce the exper-
imental observations2132 of biomass production in thin flat-
plate reactors at ultrahigh culture densities reviewed in Sec. I
B. Reactor thicknesses of less than 10 cm were not modeled
in older works, such as Ref. 14. They have been analyzed,
for example, in Ref. 39. The predictions do not conform to
observations. The volumetric and areal production rates are
found to decrease as the optical depth is reduced from around
10 down to 1 cm. Moreover, the range of culture densities,
over which productivity is significant, is the same as for opti-
cal depths in the excess of 10 cm. The use of light intensities
higher than one sun was not considered.
D. Modeling biomass production in thin flat-platereactors
The discrepancy between predictions based on theassumption of continuous radiation and experimental obser-
vations in thin bioreactors is a consequence of the fact that
the cells are not exposed to continuous radiation, but to short
light flashes. This is a consequence of the combined effect of
the small optical depth (a few cm), the turbulence induced in
the fluid and the high culture density.
The importance of matching the frequency and duration
of light pulses to the physiological time scales that character-
ize the biomass production has been pointed out in Refs.
19, 1632, 41, 42, 4553. Fluid-dynamical turbulence is
one possible means for generating exposure of cells to inter-
mittent light flashes. Its role in channel flow was modeled
years ago.54,55 Its role in high-density cultures in tubular col-lectors was analyzed through detailed numerical models in
Refs. 5661.
This paper presents a simple model for large-scale
biomass production in ultrahigh density cultures cultivated
in thin flat-plate bioreactors. The focus is on the effect of
turbulence induced in the fluid. By construction, the model
is not meant to reproduce the details of cell physiology.
The need to account for such details is avoided by identi-
fying the major factors that control large-scale biomass
production. The model reproduces the qualitative features
of observed biomass production in such bioreactors. One
message of this work is that bioreactors of small optical
depth (be they flat-plate or tubular ones) seem to probe
time scales of significance in the physiology of micro-
organisms.
Turbulence induced by rising air bubbles triggers a mac-
roscopic diffusion process, random motion of the algae cells,
owing to which cells cross the optical depth of the reactor in
a few tens of milliseconds. In addition, at the ultrahigh cul-
ture densities employed, the attenuation of radiation intensity
is rapid, so that the thickness of the layer near the reactor
wall, across which light intensity is high (the photic zone),
is 1 mm. Throughout the rest of the reactor, radiation inten-sities are very low, mostly even under the compensation
point (radiation intensity at which, under steady state condi-tions, the rate of photosynthesis is equal to the respiration
rate). Consequently, the cells are exposed to intermittent
light intensity, which may be viewed approximately as short-
duration light flashes.
The combined effect of the random motion and the
high density leads to an increase in productivity in two
ways. First, as cells are exposed to short duration light
flashes, saturation of the photosynthetic process (express-
ing the inability of cells to exploit photons, when they
arrive at too high a rate) is deferred from the commonly
observed low r adiation levels of a fraction of one
sun18,17,30,44,45,61 to several suns.26,27,30,32 In addition, as
cells, which have just collected enough photons for one
FIG. 1. (Color online) Side view of thin flat plate micro algae photo bio-
reactor, illuminated on both sides.
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photosynthetic cycle, are going through the cycle while
wondering through the reactor, others are collecting pho-
tons in or around the photic zone.
The parameters affecting biomass productivity are pre-
sented in Sec. II. The fluid-related parameters control the
(approximately) exponential extinction of light as it propa-
gates through the dense culture, and the macroscopic diffu-
sion of cells throughout the culture. The latter ischaracterized by a diffusion coefficient, D (1050 cm2/s).
The photosynthetic cycle is treated in a black box
approach. It is characterized by two physiological time
scales, which are proffered as the main factors controlling
the production process. They are: TR, the reaction time
(110 ms), and TW, the maximum charging waiting time
(several ms to several tens of ms). TR is the time span,
which elapses from the moment a reaction center has col-
lected enough photons for one photosynthetic cycle,
undergoes the cycle, until it becomes available again to
exploiting the continuously impinging photons. TW is an
average of the longest time span, over which photons col-
lected for the next photosynthetic cycle are not lost tocompeting processes. In the absence of detailed knowledge
regarding the physiology of cells, which have been accli-
mated to intermittent, short, light flashes, values for TRand TW are estimated using information obtained in
experiments on dilute cultures exposed to continuous radi-
ation, and on results concerning loss rates of photons by
the photo-system. The model is described in Sec. III.
Results of its predictions, obtained through a numerical
simulation, are presented in Sec. IV A discussion is pre-
sented in Sec. V.
II. SYSTEM PARAMETERS
A. Light attenuation
Once nutritional requirements are satisfied and environ-
mental conditions are controlled, light is the major limiting
factor in biomass productivity. Efficient utilization of high
light intensities increases the biomass yield;2132 unless the
intensity is so high that photo-inhibition becomes
important.5,42
For dense cultures, light intensity decreases as a func-
tion of the distance from the irradiated reactor surface into
the culture. This decrease may depend on culture density and
on algae type. (Different species may have different attenua-
tion profiles for different spectra.8,17,22,40) It has a predomi-nant effect on productivity.
In this paper, the irradiance I at a point within the cul-
ture at a distance x from the illuminated surface is assumed
to approximately obey the Lambert-Beers law
I I0 exp l q x : (2.1)
I0 is the intensity on the flat wall, l is the attenuation coeffi-
cient, and q is the culture density. As light is attenuated rap-
idly in a high-density culture, its intensity is appreciable
only in a thin layer close to the irradiated reactor surface.
The thickness of this layer (the photic zone) is of the
order of [1/l(q)] (
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move rapidly in and out of the photic zone, ensuring that as
many cells as possible receive high light intensity for short
periods of time. Reactor thickness, cell concentration, the
degree of turbulence and light intensity, determine the fre-
quency of the cycles of cell motion between the illuminated
and dark parts of the reactor, as well as the fraction of the
cycle during which a cell is exposed to a high photon flux.
Cell motion can be qualitatively described as a randomwalk, characterized by a macroscopic diffusion constant, D.
Estimation of D is based on two areas in fluid dynamics
where turbulence-induced diffusion occurs. First, the qual-
itative description of randomly moving eddies has been
developed in the context of turbulent fluid motion (see, e.g.,
Ref. 63 and references therein). Denoting the characteristic
eddy size by l and the average eddy velocity by u0, D is
given by
D ffi lu0=2: (2.4)In the literature on two-phase, or bubbly, flow, bubble-
induced turbulence is characterized by a diffusion constant,given by a phenomenological relation6774
D ffi kblu0: (2.5)In Eq. (2.5), l is the characteristic bubble size, u0 is the aver-
age bubble velocity, b is the void fraction (fraction of fluid
volume occupied by bubbles) and k is an empirical coeffi-
cient of the order of 0.6. Assuming typical a bubble size of
l 0.2 cm and bubble velocity, u0, %3050 cm/s,2329 onefinds that D 35 cm2/s. For l 0.5 cm, D would be7.512.5 cm2/s.
Denoting the thickness of the reactor by L, a measure of
the average crossing time for a cell is
TCROSS L2=2D
: (2.6)
ForL 1 cm, and D 10 cm2/s, TCROSS 50 ms. If the reac-tor is equally illuminated on both sides, so that, due to sym-
metry, the thickness to be crossed is 0.5 cm, TCROSS is
reduced to 12.5 ms.
C. Physiological time scales
When the light-dark frequency (characterized by an av-
erage crossing time of the order of magnitude of TCROSS) is
of the same order of magnitude as physiological time scales,which control large-scale biomass production, one expects a
high production rate.
This paper does not deal with the details of the photo-
synthetic process. Rather, the process is viewed as a black
box, for the following reasons. First, the goal is to offer a
simple, qualitative picture that provides understanding of the
observed characteristics of large-scale algal biomass produc-
tion, rather than a detailed numerical model. In a detailed
description of the photosynthetic process one must take into
account, for example, that a photosystem I needs two pho-
tons to reduce NADP, and a photosystem II needs fourphotos to split one molecule of water. These processes occur
on extremely short time scales (microseconds). The time
scales that seem to control observed large-scale biomass pro-
duction are several to several tens of milliseconds. Hence, in
comparison with these long time scales, the net chemical
reactions occur instantaneously. It, therefore, makes sense
to distinguish between these short time scales and the overall
production-cycle time. It is the latter that affects the mac-
roscopic observations.
Estimates of the overall production-cycle time in contin-uous illumination experiments vary over a wide range of val-
ues, depending on algae species, culture history (i.e.,
acclimation to growth conditions) and type of bioreactor.
The following are estimates based on dilute culture experi-
ments: 115 ms,75,76 7700 ms,3 50 ms2 s,2 and 130
ms4 s.16
In the spirit of the approximation proposed above, the
overall production-cycle time may be written as:
TPR TCOLL TR: (2.7)
TCOLL is the time required to collect the necessary number of
photons (8, if quantum efficiency is accounted for). TCOLLdepends inversely on I, the radiation intensity, to which cells
are exposed. TR, is an effective reaction time. It is the time
from the moment a reaction center has absorbed all the
required photons, goes through the (fast!) chemical reactions,
until it is available again for exploiting impinging photons.
There are many works that have collected information
regarding the exposure of algal cells to short duration light
pulses. However, a picture of the characteristics of cells that
have been acclimated to pulse exposure does not exist. It is
still not even clear yet whether the cells are acclimated to the
high irradiance during a pulse or to the time-averaged inten-
sity. Thus, estimates of the time scales offered in the follow-
ing are, at best, rough estimates of yet unknown quantities.
D. Estimating TR
That the time scale, TR, may provide a reasonable quali-
tative description of the production cycle is inferred from in-
formation, collected many years ago, in experiments, in
which dilute algal cultures were exposed to continuous
radiation.18,18,30,44,45,61 In plots of the P-E curve (photosyn-
thetic activity rate, measured in terms of, for example, oxy-
gen generation) the production rate increases linearly at low
radiation, and reaches saturation at 20%25% of one sun in-
tensity. (In the PAR, one sun represents a flux of 2000 lmol
photons m
2
s
1
). These observations are readily understoodin terms of the two time scales in Eq. (2.7). At low radiation
levels, photon collection takes a long time, so that TCOLL
TR, and, hence, TPR % TCOLL. As a result, the production rategrows linearly with radiation intensity, I. At high irradiance
levels, the photon collection time has becomes so short that
it can be neglected in Eq. (2.7). Equation (2.7) is then
reduced to TPR % TR. Saturation implies that the TR has aconstant average value. The simplest interpretation is that,
although the actual photosynthetic process is very fast, the
reaction center becomes available again for exploiting im-
pinging photons after a much longer time, TR.
The time required to collect the number of photons
needed for a single photosynthetic cycle varies. It depends
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on light intensity and on the chlorophyll antenna absorption
cross-section area. For photosystems I and II, the minimum
achievable chlorophyll antenna absorption cross-section area
for capturing photons is approximately 15 nm2.64 This esti-
mate was based on the product of the minimum number of
chlorophyll molecules per photo system, 95 and 37 chloro-
phyll molecules for photo systems I and II, respectively,65
and the absorption cross section of a chlorophyll molecule.41
Assuming the above quoted cross-section area, under 2000
lmol photons m2
s1
radiation (PAR in one sun), the num-
ber of quanta absorbed per reaction center per second is of
the order of (16) 103 photons/s.64 To estimate TR, we usethe value of light intensity, at which saturation of the P-E
curve sets in (approx. 20%25% of one sun). Collecting 8 (6
plus accommodation for quantum efficiency) photons at this
rate yields an upper bound for TR of 330 ms. Hence, values
of several ms to a few tens of ms have been used in the pres-
ent study. This range of values is in agreement with the com-
mon understanding that whole photosynthetic process takes
several milliseconds (for a review see Ref. 77).
E. What happens under very low irradianceMaximum charging waiting time
Experiments under continuous illumination indicate that
the production rate vanishes at very low, but non-vanishing,
radiation levels. The light intensity, at which this happens,
the compensation point, is not known accurately, as there is
ample scatter in production-rate plots at low photon fluxes.
The compensation point seems to vary, depending on growth
conditions and algal species.66 The interpretation given is
that, at this low irradiance, the photosynthetic and respiration
rates are equal, so that the net biomass production vanishes.
However, a totally different scenario emerges in thin
flat-plate reactors as well as in small-diameter tubular reac-
tors at ultrahigh culture densities. In such systems, the cells
wonder most of the time in the bulk of the reactor volume,
where the radiation intensity is extremely low; so low that it
may be under the compensation point. For example, for cul-
ture density of 30 dry-weight gr/l, Eqs. (2.1)(2.3) yield that
at a distance of 2 mm from the illuminated reactor wall, the
irradiance has fallen to 0.4% of the incoming intensity.
Based of a flux of (16) 103 photons/s per reaction cen-ter,64 this corresponds to 424 photons/s per reaction center.
Hence, collection of, say, 8 photons takes anywhere between
300 ms to 2 s.Let us assume that a reaction center has already col-
lected, say, 3 of the required photons. Will it wait for the
remaining 3 (or 5, if quantum efficiency is accounted for)
indefinitely? Clearly not. What will happen to the photons al-
ready collected? Obviously, unless the remaining photons
are collected within some limited time, the collected ones
will be lost, for example, by deexcitation of electronic levels
or recombination processes, the released energy reradiated or
converted into heat. In summary, if photon collection is too
slow, other processes may compete for the same photons.
The literature on photon energy losses from, e.g., photo-
system II, indicates that there are loss processes that take
nanoseconds, picoseconds, microseconds, millisecond, sec-
onds, and even tens of seconds (for a review, see Ref. 78).
The fastest loss processes occur over time spans much
shorter than the characteristic time span of the photosyn-
thetic process (which is of the order of several ms77); hence,
may be lumped into a factor that determines the efficiency of
the photosynthetic process. The very slow processes take
seconds to tens of seconds. Over such a long of time, a ran-
domly moving cell will have visited the photic zone quite afew times, and collected the required photons. Hence, it
seems that only loss processes occurring over time scales of
milliseconds will have a predominant effect on large-scale
biomass production. An example for such a process is
delayed chlorophyll a fluorescence.79,80
The time scales for various loss processes may be differ-
ent, and may, perhaps, also have a statistical nature. As the
detailed theory for this scenario does not exist, we propose
to represent the time scales for photon losses by one time
scale, TW, the maximum photon charging waiting time. It
represents the (average) time over which a reaction center
does not lose a partial number of photons to other processes.
To estimate TW, would require detailed modeling that is out-side the scope of this paper. However, a rough estimate of
TWmay be obtained as follows.
At irradiance levels, I, above the compensation point,
the net production rate is positive and linear in I. As I is
increased, the photon collection time, TCOLL, becomes
smaller. Hence, the value of TCOLL at the compensation point
provides an estimate of the order of magnitude of TW. To ac-
complish one photosynthetic cycle, the cell needs 8 photons
when quantum efficiency is accounted for. Consequently, the
lower bound forTW is
TW ! TCOLL At compensation point
8 photonsCompensation point photon flux on reaction center
:
(2.8)
For the green alga Chlorella pyrenoidosa the compensation
point irradiance is about 6 lmol photons m2 sl under
light-flash illumination, and about 9 lmol photons m2 sl
under continuous LED light.20 Under full sun irradiance
(a PAR flux of 2000 lmol photons m2 sl), the estimated
flux is (16) 103 photons/s per reaction center.64 A flux of6 lmol photons m2 sl,20 therefore, corresponds to a flux of
(320) photons/s per reaction center. If, as some experiments
seem to indicate, the compensation point is as high as 2% ofone sun, this corresponds to 100 photons/s per reaction cen-
ter. For a photon flux on a reaction center of 3100 photons/
s, this corresponds to TW! 80 ms3 s.The important role played by TW may be seen as fol-
lows. In ultrahigh density cultures, light intensity decreases
exponentially into the depth of the culture. Cells in the inte-
rior of the bioreactor (the majority of cells) are exposed to
extremely low radiation. Hence, these cells require a long
time to collect photons. If their migration through the dark
culture takes longer than TW, then they may lose photons al-
ready collected. Only if they manage to reach the (thin)
photic zone within a time shorter than TW, is the probability
to collect the required number of photons in a sufficiently
034904-5 A. K. Gebremariam and Y. Zarmi J. Appl. Phys. 111, 034904 (2012)
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short time high. This explains the advantage of thin reactors.
If the reactor is thick, so that the crossing time [Eq. ( 2.6)] is
longer than TW, then a sizable fraction of the cells does not
participate in the production process. Thus, TW plays a cru-
cial role in reducing biomass production at high culture den-
sities when reactor thickness is increased.
To attain a significant increase in productivity, the aver-
age crossing time of the reactor width, TCROSS, ought tobecome comparable to or shorter than TW. A doubly efficient
way to achieve this goal is exposure of both sides of the reac-
tor to light, as in Fig. 1. For example, in symmetric illumina-
tion, productivity is increased due to the higher photon flux,
as well as due to overcoming the limitation enforced by the
effect of TW: The distance to be crossed by cells is halved,
resulting in reduction ofTCROSS by a factor of 4.
III. THE MODEL
The assumptions made in order to ensure simplicity of
the model made in the model are:
1) The culture has been operating for a sufficiently longtime, so that the acclimation process need not be
accounted for.
2) Pigment content is constant.
3) Denoting the position of a cell by x, it executes a lateral
random walk with reflective boundaries over the interval
0xL. The random walk is characterized by a diffu-sion coefficient D.
4) Irradiance attenuation through the culture is determined
by Eqs. (2.1)(2.3).
5) The number of photons that need to be collected for one
photosynthetic cycle is set at 8 (6 2 for quantumefficiency).
6) The calculation assumes illumination on both sides of the
reactor.
As the cell moves randomly through the reactor, its reac-
tion centers are continuously hit by the photon flux. Owing
to the random motion of the cell, the irradiance, to which
reaction centers are exposed, varies randomly. Once a reac-
tion center is available for processing photons, the number of
photons it collects in a given time span is
n at2
t1
I0elx t dt=E: (3.1)
where a is the effective area of a reaction center, and E is the(average) energy of a single photon. If the required number
of photons is not collected within a time span shorter than
TW, then the photons already collected are lost. If 8 photons
are collected within TCOLLTW, the reaction center enters areaction phase, during which it does not respond to pho-
tons that continue to hit it. Once this phase is over (its dura-
tion is TR, the reaction time), biomass is produced, and the
reaction center returns to photon collection. Cell density is
assumed to be constant in space (due to mixing), and in time
(corresponding to continuous harvesting).
The simulation was written in MATLAB. Time was di-
vided into short time intervals. The length of the time inter-
val, Dt, was selected so that it was appreciably shorter than
all relevant time scales and that statistically significant results
were obtained, in particular, that the statistical samples were
sufficiently large so that smooth steady-state curves were
obtained. The random motion of the cell was generated by a
normalized random number generator. The step, Dx, was gen-
erated by a normal distribution with zero mean, and standard
deviation
r ffiffiffiffiffiffiffiffiffiffiffi
2DDtp
(3.2)
T, the total physical operation time of the reactor was chosen
sufficiently long, again, to ensure smooth and statistically
significant results. The main factor in choosing both Dt and
T was the need to ensure a sizable sample also at the highest
densities we considered, for which the rate of production-
cycle completion is low.
The simulation takes one cell and lets it move randomly
through the reactor throughout the operation time, T. Every
time a production cycle is completed by a reaction center, a
counter is increased by 1. The result of one run is N, the total
number of production cycles per reaction center. The flowchart of the numerical simulation is shown in Appendix A.
A. Calculated quantity - J
The volumetric production rate, R (gr/l s1) is given by
R g N=nPR nCentersnCellsqT
eq gnCentersnCellsnPRT
J eq:J qN (3.3)
In Eq. (3.3), g (gr) is the (average) amount of biomass gener-
ated every nPR photosynthesis cycles, nCenters is the (average)
number of reaction centers per cell, nCells is the (average)number of cells per 1 gr of dry-weight biomass, q is the dry-
weight density (gr/l), N is the total number of production
cycles per reaction center, and T is the total operation time
of the bioreactor. The subtracted term represents the loss of
biomass owing to respiration, where e (s1) is the loss rate.
The coefficients multiplying J in Eq. (3.3) amount to a
constant, which may depend on algae species, acclimation
process and other factors. Therefore, in Figs. 28 we show J
[its units are: (Cycles per reaction center) (gr/l)] versus q,as a measure of the volumetric production rate. The respira-
tion term has not been included, as it is small, and affects the
production rate only slightly at low densities, and at very
high densities.
B. Very long TW
We first consider the unrealistic limit of TW 1, i.e.,there are no photon loss processes. The purpose is twofold.
First, this limit serves as a test of the credibility of our nu-
merical simulation, because, in two limits, of very low and
of unrealistically high culture densities, the expected depend-
ence of productivity on culture density can be derived inde-
pendently, using simple arguments. Second, the predictions
in this limit accentuate the crucial role played by photon loss
processes, the effect of which is qualitatively described in
terms of the single time scale, TW, when the latter is assigned
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values in the range of milliseconds, as is demonstrated in
Sec. IV.
In the limit of extremely high cell densities, the photic
zone is so narrow that the time spent by cells within that
zone does not allow them to collect the required number of
photons in a single visit. Based on Eqs. (2.1) and (2.2), the
thickness of the photic zone is of the order of
lph % 1= aq : (3.4)
Hence, the time spent in the in the photic zone is of the order
of
Tph % lph 2= 2D : (3.5)For a cell dry-weight density of 50 gr/l, and D 10 cm2 s1,one finds Tph % 0.02 ms. Even at a full sun intensity, corre-sponding to a flux of the order of (16) 103 photons/s (Ref.64) per reaction center, this does not allow a center to collect
even a single photon. Most of the remainder of the reactor
volume is dark. So dark that irradiance levels are lower even
than the compensation point. As there is no time limitation on
photon collection, a cell may move back and forth for as long
a time as needed until its reaction centers have collected the
required number of photons. Consequently, photon collection
is determined by the average radiation intensity, give by
I aI0 1 eaqL
= aqL : (3.6)To collect the required number of n photons, n, the col-
lection time, TCOLL, is then given by
TCOLL nI naqL
aI0 1 eaqL : (3.7)
Equation (2.7) for the total production-cycle time, there-
fore, yields
TPR
TR
TCOLL
TR
naqL
aI0 1 eaqL
: (3.8)
The volumetric production rate, R, is proportional to
R / qTPR
qTR naqLaI0 1eaqL
q1
aI0
naL: (3.9)
Namely, at extremely high densities, R tends to a constant,
which is inversely proportional to reactor thickness, and is
independentofTR.At very low densities, the radiation intensity throughout
the reactor is approximately uniform. Equation (3.9) is then
replaced by a linear dependence on culture density, given by
R q0
q
TR n= aI0 : (3.10)
Hence, as expected, low values of TR yield higher pro-
duction rates.
Figure 2 demonstrates that these expectations are indeed
born out for TW 3000 s, D 10 cm2/s and TR 0.1 and1 ms. The quantity J[see Eq. (3.3)] is proportional to the vol-
umetric production rate. At ultra high densities, the asymp-
totic limit of Eq. (3.9) is attained; the volumetric production
rate is inversely proportional to reactor thickness, so that the
areal production rate does not depend on the thickness, and
becomes independent ofTR. At low densities, the lower value
of TR yields higher production rates, as predicted by Eq.
(3.10).
For intermediate densities, neither approximation
described above is valid. However, it is expected that the
density dependence ofJ will vary smoothly from the low- to
the high-density prediction, which it does. Calculations per-
formed for other values ofTR and D yield similar results.
IV. RESULTS
To obtain statistically significant results for all densities
considered (including the highest values) the physical run-
ning time of the production process was fixed at T 2400 s.For Figs. 27, the calculation assumes illumination on both
sides of the reactor with a photon flux of 104 photons/s hit-
ting a reaction center (roughly, the equivalent of the PAR of
1.5 suns64
).
When TW, the maximum charging waiting time, has re-
alistic values (several ms to a few tens of ms), at high culture
densities, a cell must reach the photic zone (thin layer of
high radiation intensity near an illuminated wall) by the time
it has completed the previous production cycle, and is ready
again to exploit impinging photons. This means that it is ad-
vantageous to have an average crossing time, TCROSS that is
comparable to the reaction time, TR. However, before one
reaches reactor thicknesses sufficiently small to attain this
goal, TCROSS has to be comparable to, or shorter than TW.
Otherwise, many cells will collect only a fraction of the
required 8 photons, and then lose them because they will be
wondering for too much time in the dark part of the reactor
volume. Namely, one expects significant improvement in
productivity when
Tacross%
L2= 2D
TW: (4.1)
FIG. 2. Jof Eq. (3.3) vs cell culture density for different reactor thicknesses.
TW 3000 s, D 10 cm2/s, TR 0.1 ms (full symbols), TR 1 ms (opensymbols).
034904-7 A. K. Gebremariam and Y. Zarmi J. Appl. Phys. 111, 034904 (2012)
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Based on experiments, estimates of TW may vary between
about 80 ms and 3 s (see Sec. II E). As accurate information
regarding values of the different parameters does not exist,
we performed simulations for several combinations of pa-
rameters, to see how the motion of cells across the bioreactor
affects productivity. The results are presented for progres-
sively smaller values of TW. Figures 37 show Jof Eq. (3.3),
as a representative for the volumetric production rate for two
values of TR: 1 and 15 ms. In Figs. 35, values of the diffu-
sion coefficient D and of the photon loss time scale, TW,
were chosen, for which one has TCROSSTW, whereas Figs.6 and 7 correspond to the case TCROSS> TW. These figures
indicate the following unique features of biomass productionin bioreactors with a small optical depth:
1) The interplay among the three relevant time scales deter-
mines the characteristics of the production process.
2) TW, the time scale for photon losses, plays an important
role. Compared to the unrealistic case of no loss mecha-
nisms (corresponding to extremely large TW), presentedin Fig. 2, when TW is not extremely long, the overall pro-
duction rate is decreased significantly.
3) As cells may lose photons if the required number (8) is
not collected within TW, the reduction of the production
rate at the highest densities leads to the emergence of an
optimal culture density.
4) As expected, when TR (the time a reaction center is not
available for processing impinging photons) is increased,
the production rate is reduced.
The features observed in the experiments reported in
Refs. 22 and 33 are:
1) The optimal density increases as reactor thickness is
reduced.
2) The optimal volumetric production rate grows faster than
(1/L) as L is reduced. Consequently, the optimal produc-
tion rate per unit area grows as reactor thickness is
reduced.
FIG. 3. J of Eq. (3.3) vs culture density for different reactor thicknesses.
TW 80 ms, D 25 cm2/s, TR 15 ms (open symbols), TR 1 ms (fullsymbols).
FIG. 4. J of Eq. (3.3) vs culture density for different reactor thicknesses.
TW
80 ms, D
40 cm
2/s, TR
15 ms (open symbols), TR
1 ms (full
symbols).
FIG. 5. J of Eq. (3.3) vs culture density for different reactor thicknesses.
TW 50 ms, D 40 cm2/s, TR 15 ms (open symbols), TR1 ms (fullsymbols).
FIG. 6. J of Eq. (3.3) vs culture density for different reactor thicknesses.
TW
15 ms, D
10 cm
2/s, TR
15 ms (open symbols), TR
1 ms (full
symbols).
034904-8 A. K. Gebremariam and Y. Zarmi J. Appl. Phys. 111, 034904 (2012)
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Both are obtained when TCROSSTW (Figs. 35).However, these two characteristics are not reproduced
in Figs. 6 and 7, which correspond to the case TCROSS>TW.The optimal density becomes, essentially, independent of re-
actor thickness; the volumetric production rate is roughly
inversely proportional to reactor thickness L, so that, to a
good approximation, the production rate per unit area
becomes independent of reactor thickness. As TW is reduced,so does the production rate. The effect is particularly dra-
matic in Fig. 7. Although the diffusion coefficient, D, has a
high value (40 cm2/s), so that the crossing time for a 1 cm
thick reactor is of the order of 10 ms [see Eq. (2.6)], TCROSS TW. As a result, the production rates and the optimal den-
sity are all are very low. These results are expected, because
when TCROSS>TW, most cells do reach the photic zone insufficiently short a time span in order to collect photons.
A. No saturation at radiation levels far above one sun
In continuous radiation experiments on dilute cultures, it
is invariably found that the photosynthetic rate reaches satu-
ration at radiation levels of the order of 20%30% of one
sun. It is also quite well understood that saturation is indica-
tive of the existence of TR, the reaction time scale, which
does not represent the time scales characteristic of the elec-
tronic energy conversion process (order of microseconds)
but a physiological time scale that characterizes the rate at
which a reaction center completes a photosynthesis cycle
until it is ready again to exploit impinging photons. If all
cells are exposed to high continuous irradiance, say equiva-
lent to one sun, then time is divided into segments of length
TR. At the beginning of each segment photons are collected
in a very short time span, of the order of microseconds oreven less. While the production cycle takes place, the reac-
tion center is not available to processing impinging photons,
until the time TR has passed. This is saturation. In a flat-plate
reactor, saturation is expected to be deferred to higher radia-
tion levels because cells spend a very short time in the photic
zone, so that they are exposed to light flashes, rather than to
continuous irradiation.
The results, shown in Fig. 8, bear out this expectation.
The reactor thickness was L 1 cm, and the reaction timewas TR 15 ms. The PAR intensities used were 6, 10, and30 photons/ms per reaction center, and applied only on one
side of the reactor. (The lowest of the three is roughly equiv-
alent to the PAR of one sun.64
) The volumetric production
rate grows with radiation intensity, not exhibiting any signs
of saturation; the optimal rate is still proportional to the radi-
ation intensity.
Figure 8 demonstrates the detrimental effect of the
relative sizes of TW and TR on biomass production. ForTW 50 ms, one has TCROSS % 10 ms TW, whereas forTW 5 ms, one has TCROSS % 50 ms TW. The reduction inproductivity in the latter case is the result of the fact that a
significant fraction of the cells does not reach the photic
zone before TW is over. It migrates through the dark part of
the culture, collecting photons very slowly, and losing them.
V. DISCUSSION
The model proposed here provides a simple description
of biomass production of unicellular algae in high-density
cultures in flat-plate bioreactors. The assumptions on whichthe model is based are of two categories: fluid characteristics
and cell characteristics. The motion of the cells across the re-
actor is a random walk, generated owing to bubble-induced
turbulence. The latter is caused by the erratic motion of ris-
ing air bubbles. The attenuation of light through the fluid is
assumed to follow the exponential Lambert-Beers law. The
photosynthetic process is treated as a black box character-
ized by two physiological time scales. When reactor thick-
ness is reduced to the order of 1 cm, the time scale for
crossing the reactor thickness by the randomly moving cells
becomes comparable to the physiological time scales. The
interplay among these time scales generates the observed
phenomena. These include the increase of both optimal
FIG. 7. J of Eq. (3.2) vs culture density for different reactor thicknesses.
TW 1 ms, D 40 cm2
/s, TR 15 ms.
FIG. 8. J of Eq. (3.3) vs culture density for different radiation intensities.
L 1 cm, TR 15 ms; reactor irradiated on one side. TW 50 ms, D 40cm
2/s (full symbols); TW 5 ms, D 10 cm2/s (open symbols).
034904-9 A. K. Gebremariam and Y. Zarmi J. Appl. Phys. 111, 034904 (2012)
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density and of optimal production rate per unit area with
reduction of reactor thickness. The most important character-
istic is the linear increase of production rates with radiation
intensities significantly exceeding one sun, showing no sign
of saturation.
The model is sufficiently flexible to allow for parame-
ter values, which may correspond to algae species different
from the ones for which flat-plate experiments have beenperformed, or for new species, the parameters of which
have been affected by genetic engineering. A conclusion
of practical relevance is the advantage in illuminating the
reactor on both sides, as in Fig. 1. Productivity is
increased not only due to the higher photon flux, but also
due to overcoming the limitation enforced by the effect of
TW. For example, in symmetric illumination, the distance
to be crossed by cells is halved, resulting in reduction of
TCROSS by a factor of 4, facilitating implementation of the
requirement of Eq. (4.1).
Very little is known about all the relevant parameters.
The fluid-related processes are: the turbulence-induced diffu-
sion, characterized by the coefficient, D, and radiationattenuation through the culture. The theoretical basis for the
evaluation ofD is based on approximations and on phenome-
nological assumptions. The extent, to which the Lambert-
Beer exponential law provides an accurate description of the
attenuation process, is not known. Assuming Eq. (2.1) for
light attenuation, the coefficient, l, is not known. It is not
known whether l is indeed linear in density, as assumed in
Eq. (2.2). If a linear relation does hold, the value of the coef-
ficient a in Eq. (2.2) is not known precisely. The imprecision
in these factors may be the reason why the optimal densities
obtained in our results are higher than observed densities. In
this paper, a%
1 l gr1 cm1 has been used. Values quoted
in the literature for densities appreciably lower from the ones
covered in the flat-plate experiments varied in the range of
11.5 l gr1 cm1, with rather large standard deviations.
Choosing a % 2 l gr1 cm1 would place all relevant den-sities in our calculations comfortably within the range of
experimentally observed values.
The physiological time scales: the reaction time, TR,
and the maximum-charging waiting time, TW are also
poorly known. The existence of the reaction time scale,
TR, may be inferred with some degree of certitude from
the fact that the photosynthetic rate under continuous illu-
mination of dilute cultures, reaches saturation when the
radiation flux is 20%30% of one sun. However, whethervanishing of the biomass production rate at very low (but,
possibly, non-vanishing) radiation intensities can be inter-
preted in terms of another time scale, the maximum-
charging waiting time, TW, is definitely an open question.
Even if this turns out to be a reasonable description, the
value of TW is not known at all.
Finally, the fact that the simple model proposed in this
paper has the capability to reproduce the qualitative features
of experimental results calls for a whole series of experi-
ments aimed at determining the relevant parameters for dif-
ferent algae species and (possible) variation in these
parameters when the light regime is changed from continu-
ous irradiation to light flashes.
APPENDIX A
Flow chart of simulation model described in Sec. III.
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