Photosynthesis of the cyanobacterial soil‐crust lichen Collema tenax from arid lands in southern Utah, USA: role of water content on light and temperature responses of CO2 exchange

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Abstract
1. The gelatinous cyanobacterial Collema tenax is a dominant lichen of biotic soil crusts in the western United States. In laboratory experiments, we studied CO2 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 CO2 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.