Along with other microorganisms such as heterotrophic bacteria, archaea and fungi, as well as with macroscopic lichens and bryophytes, cyanobacteria and algae are the most important phototrophic components of BSCs (Elbert et al. 2012). These communities can be characterized as “ecosystem engineers” forming water-stable aggregates that have important, multifunctional ecological Everolimus roles in primary production, nitrogen (N) cycling, mineralization, water Rapamycin in vitro retention, and stabilization of soils (Evans and Johansen 1999; Lewis 2007; Reynolds et al. 2001). A recent review on BSCs clearly demonstrated their important
ecological contribution to global carbon (C) fixation (about 7 % of terrestrial vegetation) and nitrogen (N) fixation (about 46 % of terrestrial biological N fixation) (Elbert et al. 2012). Although the ecological structure and function of BSC communities from subtropical to polar regions have been studied in recent decades (Belnap and Lange 2001; Büdel 2005), less is known about similar communities living in high alpine habitats such as the Alps (Türk and Gärtner 2001). BSCs from the Alps have been described from bare mineral PLX3397 soils, soil gaps between higher plants, underneath higher plants, peat, plant debris,
and even on fluvioglacial deposits up to the nival zone (Ettl and Gärtner 1995; Reisigl 1964; Türk and Gärtner 2001). However, most studies on aeroterrestrial algae have focused on classical systematics (Ettl and Gärtner 1995). Soil algae of alpine habitats are members of various groups of the Xanthophyta, Eustigmatophyta,
Chlorophyta and Streptophyta; in this review we focus on green algae from the last two divisions. Environmental conditions for alpine biological soil crust communities In the Alps, a relatively large proportion of the landscape lies in the subalpine, alpine and nival zones. Here the abiotic conditions show dramatic gradients and extensive patterns of small-scale habitats (Körner 2003; Larcher 2012). Over short elevational distances, the thermal gradients reflect the climate across vast latitudinal distances, resulting in a compression of life zones (Körner 2003; Larcher and Wagner 2009). The steep abiotic gradients CYTH4 include wide diurnal temperature fluctuations, occasional frost in summer, intense irradiation even at low temperatures, a large increase in ultraviolet radiation (UVR) with altitude, and high impacts by wind or storms that produce short-term drought and abrasion. Therefore, high mountains are extreme habitats, which set selective boundaries/limits to the altitudinal distributions of BSCs. In addition to the altitudinal gradients, the chemistry of the underlying rocks (e.g., limestone or silicate) influences soil formation and properties (e.g., pH value), and consequently the settlement and ecology of all primary producers. Organisms living in alpine regions must be well adapted to these extreme conditions to assure their long-term survival.