Volume 10, Part 2, May 1996

HOW DO LICHENS SURVIVE SEVERE DROUGHTCONDITIONS?

Lichens flourish in a wide range of habitats,many of which are regarded as severe environ-ments. There are over 13,500 species of lich-enized fungi and they are usually long-livedorganisms, growing very slowly (a few mm ayear). They occur world-wide, doing best underconditions where little physical disturbanceoccurs, and are able to tolerate high levels ofphysiological stress such as low humidity, tem-perature extremes and low light conditions, inextremely exposed habitats. Most species arehighly drought tolerant and only a very few canwithstand submersion in water for any length oftime. In general, lichens are unusual organismsin that their development, and the maintenanceof healthy active thalli, is favoured by alternatingperiods of wetting and drying.

Lichenized fungi are much more tolerant todrying than are non-lichenised species and mostare able to survive in very dry conditions forweeks or even months. However, lichens do notconserve water and have very little control overwater loss. In times of drought water is lost byevaporation but thalli can endure remarkably lowwater contents, down to as little as 2-5% dryweight. As drying out occurs levels of metabolicactivity decline and the thallus becomes physio-logically inactive. In some environments lichensspend much of the year in a dry state and thelack of physiological activity may explain, atleast in part, the slow proliferation of thalli.

The vegetative part of a lichen, known as thethallus, is usually composed of two partners inclose contact, a fungal component (mycobiont)and either a green alga or a cyanobacterium(photobiont). The associates are self-supporting;the fungus (mainly species of ascomycetes and afew basidiomycetes) provides an anchor and pro-tection from environmental influences whilstobtaining its entire supply of carbohydrate fromthe photosynthetic activities of the partner(green algae e.g. Trebouxia, and Pseudotrebouxiaor cyanobacteria e.g. Nostoc, Scytonema andTrentepohlia). The structure of thalli is veryvaried ranging in complexity from fairly loose

aggregations of cells of the partners (Leprariaspp.) to simple layered structures (species ofLecanora, Parmelia, Peltigera, Xanthoria),through to multilayered and more elaboratearrangements (Usnea spp., Ramelina spp.). Thefungal partner often forms an upper layer (cor-tex) covering the thallus and thus provides someprotection from the prevailing environment. Thephotobiont usually comprises only a small pro-portion of the total thallus and is restricted to ashallow layer just below the upper surface.Central portions (medulla) of the thallus may bemore or less densely filled with fungal hyphae.In some lichens the arrangement of algal cellsand fungal hyphae in the inner layers of the thal-lus are quite loose and the structure thereforedries out quickly. Others have denser cellulararrangements and dry out more slowly. Somecomponent hyphae, especially those in morestructured thalli, are embedded in matrix mate-rials (probably polysaccharides) which bind thestructure together and enhance the mechanicalstrength.

The water content of the thallus reflects thatof the surrounding environment (lichens arepoikilohydrous) and much is lost by evaporationalthough some water is bound into moleculeswithin the body of the thallus and into thematrix materials. However, lichens are able toabsorb water from the environment remarkablyquickly. Water vapour from the air is utilisedextremely efficiently. Indeed, this is oftenenough to maintain metabolic activity even inthe absence of liquid water supplies.Additionally, when supplies of liquid waterbecome available (rain, dew, run-off) absorptioninto the thallus is very rapid (5-30 min to becomefully saturated) and physiological processes arere-initiated very quickly. Many lichens take upwater through both the upper and lower surfaces.Much of the water absorbed is taken into thecentral medulla and into the gelatinous matrix ofthe cortex. Some water may be retained in inter-cellular spaces (hydrophobic compounds aresometimes found in the medulla) and also, in

•

Volume 10, Part 2, May 1996

more saturated thalli, within the cell walls ofcomponent hyphae. It is interesting that, formany lichens, a prolonged state of full water sat-uration (100 - 300% dry biomass) can causemetabolic damage. Healthy activity is main-tained best in thalli subjected to alternating peri-ods of wetting and drying.

Under conducive conditions, in the presenceof the fungal partner, the photobiont cells arestimulated to release large amounts of photosyn-thates. Cyanobacterial symbionts liberate prod-ucts which are converted to glucose and thenreleased to the fungal cells. Green algal photo-bionts liberate carbohydrate products which areconverted to polyols (sugar alcohols such asribitol, erythritol and sorbitol) and then releasedfor use by the fungus. These are rapidly convert-ed to fungal carbohydrates (mannitol and ara-bitol) which cannot be used by the photobionts.This sets up a one-way flow of carbohydrates tothe fungal partner and the fungal polyols act asstorage compounds which are slowly depleted intimes of nutrient stress. High concentrations ofmannitol also act to lower the osmotic potentialin the thallus, thus assisting with water accumu-lation. Additionally, under conditions of waterstress polyols can replace water in molecules inlichen thalli, acting to protect proteins in mem-branes and thereby maintaining membraneintegrity.

In dry thalli physiological activity is very low.On rewetting there is a rapid rise in respirationrate (and a concomitant loss of CO2, which is aphysical reaction) followed by a subsequentdecline to the normal rate for the conditions.

Some solutes, such as polyols and phosphates,can be lost from the component cells at thistime, as a result of changes in membrane perme-ability, and there may also be an accompanyingloss of carbon although some of that may bereabsorbed as the water balance is restored.

In exposed situations the surface of a lichenthallus can quickly become very hot during peri-ods of sunshine, particularly since there is littlecooling by water evaporation. Many lichens arerelatively tolerant to high temperatures and arequite resistant to thermal damage but withstandsuch elevated temperatures only when the thal-lus is dry. High light levels are also very damag-ing. In some species the thick walls of thecortical cells, which are often pigmented, protectthe underlying photobiont as the thallus driesout. Some further protection is also given wherethalli roll up when dry (e.g. Chrondropsis semi-viridis). On rehydration the cells expand and theavailable radiant energy reaches the photobiont.Photosynthesis can resume at about 65-95%water saturation although the actual levels varywith species.

Although some lichens are composed of only arelatively few cells a fine balance is maintainedbetween the activities of the component organ-isms and the environment, making themremarkably tolerant and extremely responsive tochanges in the prevailing conditions.

Susan IsaacDepartment of Genetics & Microbiology

University of Liverpool, Liverpool, L69 3BX

MYCOPHAGY ON THE INTERNET

•

Mycophagists who are Internet browsers mightbe expected to know their fungi fairly well, butthe search for information coupled with anexpression of journalistic flair can lead to a livelyinterpretation. Thus, a recent article in TheTimes gave a stimulating satirical interpretationof the Slovenian Web Page, which describes thepicking of wild mushrooms as 'a national sport'and the descriptions as 'a piece of entertaining'.

Checking the site <http://www.ijs.si/globe>provides some reassurance, for the familiar sci-entific names appear alongside the almostunpronounceable Slovenian equivalent.Grouping them into 'The Good' or 'The Bad andthe Ugly' helps.

A wide range of dishes can be made from, forinstance, Jesenski goban, 'one of the safestmushrooms' (yes you guessed - it's Boletusedulis) among about 20 others in the Good cate-gory. On the other hand, different interpreta-tions of culinary value and the gastronomic riskfactors associated with the 'Bad and the Ugly'provide light entertainment. About 15 speciesare listed, culminating in a reference to Astraeushygrometricus (as it happens, a fungus to be fea-tured in the next issue of the Mycologist), whichis described as 'a really ugly one ....from outerspace....not edible, not even poisonous, justyeechh' (sic)!

GHadley