2010; Holzinger et al. 2011; Karsten and Holzinger 2012). While K. crenulatum forms rather long, strong filaments, sometimes growing in rope-like aggregates that support high self-protection against water loss, the coexisting K. dissectum has smaller filaments that easily disintegrate. Fig. 3 Changes in photosynthetic activity (Fv/Fm, optimum quantum yield) in the MAPK inhibitor alpine biological soil crust green alga

Klebsormidium dissectum (SAG 2416) during short-term (<2.5 h) and long-term desiccation (1, 3 weeks), as well as during the recovery phase after rehydration. This species was isolated at 2,350 m a.s.l. (Schönwieskopf, Obergurgl, Tyrol, Austria). The photosynthetic responses are expressed as relative percentages in relation to the control (100 %). Figure modified after Karsten et al. (2013) Fig. 4 Light micrographs of Klebsormidium crenulatum (SAG 2415), a control cells, b desiccated Trametinib at 5 % air relative humidity for 1 day, c plasmolysed in 800 mM sorbitol, d plasmolysed in 2,000 mM sorbitol. b desiccated sample viewed in immersion oil, contraction

of the whole filament visible, c incipient plasmolysis, d advanced plasmolysis. Bars 10 μm. a, c, d reprinted from Kaplan et al. (2012) with permission of Springer Science and Business Media; b reprinted from Holzinger et al. (2011) with permission of the Phycological Society of America Since in the dehydrated state, photosynthesis would be completely blocked, any further excitation energy absorbed cannot be used for electron transport, and hence may result in photoinhibition or even photodamage (Wieners et al. 2012). Axenfeld syndrome Various desiccation-sensitive sites Sapanisertib in the photosynthetic apparatus have been reported: the photosystems, particularly PSII with its oxygen-evolving complex, ATP generating, and carbon assimilation processes (Allakhverdiev et al. 2008; Holzinger and Karsten 2013). Although dehydration effects on the CO2 exchange in alpine BSC algae have to our knowledge not been reported in the literature, there exist some data on the aeroterrestrial

green alga Apatococcus lobatus, one of the most abundant taxa in temperate Europe, which forms conspicuous biofilms on trees and building surfaces (Gustavs et al. 2011). This species forms cell packets surrounded by mucilage, thereby achieving hydration equilibrium with the vapor pressure of the atmosphere (Bertsch 1966). The maximum carbon assimilation in A. lobatus was determined at 97–98 % RH, while at 90 % RH, 50 % of the maximum CO2-uptake was measured. The lower limit of carbon assimilation was estimated at 68 % RH (Bertsch 1966). These data clearly indicated that atmospheric moisture favors CO2-uptake in A. lobatus, compared to liquid water, which inhibits uptake. The water content of Klebsormidium flaccidum also determines the carbon dioxide supply and hence the photosynthetic rate (De Winder et al. 1990).