Summary1. The gelatinous cyanobacterial Collema tenax is a dominant lichen of biotic soil crusts in the western United States. In laboratory experiments, we studied CO 2 exchange of this species as dependent on water content (WC), light and temperature. Results are compared with performance of green-algal lichens of the same site investigated earlier. 2.As compared with published data, photosynthetic capacity of C. tenax is higher than that of other cyanobacterial and green-algal soil-crust species studied. At all temperatures and photon flux densities of ecological relevance, net photosynthesis (NP) shows a strong depression at high degrees of hydration; maximal apparent quantum-use efficiency of CO 2 fixation is also reduced. Water requirements (moisture compensation point, WC for maximal NP) are higher than that of the green-algal lichens. Collema tenax exhibits extreme 'sun plant' features and is adapted to high thallus temperatures. 3. Erratic rain showers are the main source of moisture for soil crusts on the Colorado Plateau, quickly saturating the lichens with liquid water. High water-holding capacity of C. tenax ensures extended phases of favourable hydration at conditions of high light and temperature after the rain for substantial photosynthetic production. Under such conditions the cyanobacterial lichen appears superior over its green-algal competitors, which seem better adapted to habitats with high air humidity, dew or fog as prevailing source of moisture.

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Lichens, being poikilohydric, have varying thallus water contents (We) and show a complex interaction between net photosynthesis (NP) and we. NP can be depressed at low WC (desiccation effects) and, in some species, also at high we. In the latter case the depression is normally ascribed to increased CO 2 diffusion resistances through water blockage. Recently, an earlier explanation, that the depression at high WC is due to recycling of CO 2 from increased dark respiration processes (DR),has been given renewed prominence.The two explanations were distinguished by the concurrent use of gas exchange and chlorophyll fluorescence techniques to investigate NP: WC relationships in the lichens Peftigera feucophlebia (green algal) and P.neckeri (cyanobacterial). Both species had a distinct optimal WC for NP with depressed values at low and high We. The maximal quantum yield for both CO 2 fixation (initial slope of light response curves of NP) and photosystem II (fluorescence signals of dark-adapted thalli) was depressed only at low WC and remained high at optimal and greater we. In contrast, the relative electron transport rate (ETR, derived from fluorescence signals of thalli in the light) tracked NP and was depressed at low and high We. The depression of both NP and ETR at high WC (not that at low We) could be prevented by using elevated external CO 2 concentrations. A single, linear relationship was found between all values of gross photosynthesis (NP + DR) and ETR regardless of external CO 2 concentration or We.Our results show that, for these lichens, the depression in NP at high WC is a real fall in photosynthetic rate of the photobionts and is not due to recycling of CO 2 , The removal of the depression in NP and ETR at high WC by using elevated external CO 2 levels allows us to conclude that an additional CO 2 diffusion resistance is present.

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Background and Aims Biological soil crusts cover about one third of the terrestrial soil surfaces in drylands, fulfilling highly important ecosystem services. Their relevance to global carbon cycling, however, is still under debate. Methods We utilized CO 2 gas exchange measurements to investigate the net photosynthetic response of combined cyanobacteria/cyanolichen-, chlorolichen-and moss-dominated biocrusts and their isolated photoautotrophic components to light, temperature, and water. The results were compared with field studies to evaluate their compatibility. Results Different biocrust types responded similarly, being inhibited by limited and excess water, saturated by increasing light intensities, and having optimum temperatures. Cyanobacteria/cyanolichen-dominated biocrusts reached their water optimum at lowest contents (0.52-0.78 mm H 2 O), were saturated at highest light intensities, and had a comparably high temperature optimum at 37°C. Chlorolichen-dominated crusts had a medium water optimum (0.75-1.15 mm H 2 O), medium saturating light intensities and a moderate temperature optimum of 22°C. Moss-dominated biocrusts had the highest water optimum (1.76-2.38 mm H 2 O), lowest saturating light intensities, and a similar temperature optimum at 22°C. Isolated photoautotrophs responded similar to complete crusts, only isolated moss stems revealed much lower respiration rates compared to complete crusts. Conclusions In addition to their overall functional similarities, cyanobacteria/cyanolichen-dominated biocrusts appeared to be best adapted to predicted climate change of increasing temperatures and smaller precipitation events, followed by chlorolichen-dominated biocrusts. Mossdominated biocrusts needed by far the largest amounts of water, thus likely being prone to anticipated climate change.