In polar deserts, Cyanobacteria are often the primary colonisers of permafrost soils in areas where melt water flushes occur through snow melt or retreated glaciers (Kastovska et al 2005, Turicchia et al 2005, Komarek et al. 2012). The colonisation by cyanobacteria increases soil stability and nutrient concentrations through, for example, nitrogen fixation. Communities can be visible in form of terrestrial crusts with a dark coloration, which is due to the UVR-screening pigment scytonemin. Cyanobacteria are also often identified in biofilms below and within rocks, where the microclimate gives protection against environmental stresses such as UV radiation, temperature extremes, desiccation and wind scouring, and trapped moisture provides an additional source of water. Depending on the spatial location, they are called hypolithic (beneath rocks), endolithic (in pore spaces of rocks), chasmoendolithic (in cracks and fissures of rocks), or cryptoendolithic (in the pore space between mineral grains forming sedimentary rocksand). As euendolithic micro-organisms, some can even bore actively into the rock, and inhabite the resultant hole (Vincent 1988, Cockell and Herrera 2008, Omelon 2008, Ziolkowski et al. 2013).
Hypolithic communities
If cyanobacteria and other phototrophs can live below the rock surface, depends on the optical characteristics of the rocks and the level of available photosynthetically active radiation. In the polar desert of the High Canadian Arctic, hypolithic cyanobacterial communities can be found, as part of microbial communities on rock-margins under translucent rocks such as quartz, visible through a mm-cm thick biofilm (Omelon 2008). The Arctic hypoliths are often dominated by cyanobacteria such as Gloeocapsa and Chroococcidiopsis, with unicellular algal chlorophytes also being present (Cockell and Stockes 2004, 2006). Hypolithic communities can be abundant in the Arctic and primary production was estimated to be close to 1g C m-2 year-1. This is similar to the primary production of macroscopic lichens and bryophytes communities In polar deserts (Omelon et al 2006), and contributes to the carbon cycling in such polar soil ecosystems.
In symbiotic relationships
Cyanobacteria containing lichens are part of the soil biodiversity in the Arctic, with e.g. Peltigera as a common example. The cyanobacterial partner can cover a large proportion of the nitrogen requirements of the fungi and subsequently contribute to the terrestrial nitrogen cycling in Arctic environments. . The photobionts also provide fixed carbon for the mycobiont (ref?).
Cyanobacteria also forms association with mosses, and can grow both endophytic and epiphytic on them. In the Arctic, epiphytic cyanobacteria-moss associations are common, e.g. and have been described for mosses such as Sphagnum, Pleurozium and Hlyocomium. Such association are usually with nitrogen-fixing cyanobacteria, and nitrogen fixation activity depends on the size and diversity of cyanobacteria population in the moss, as well as on environmental factors (Gentili et al 2005, Zielke et al 2005). Nitrogen fixation by the cyanobacteria can be an important source for nitrogen for the moss, as well as an important contributor to the systems nitrogen budget.
Nitrogen enriched soils from cyanobacterial nitrogen-fixation in lichens and mosses can facilitate the growth of other microbial communities and help the colonization and growth of higher plants in nitrogen depleted Arctic environments (Bliss and Gold 1994). However the transfer efficiency of nitrogen from the mosses to the environment is not well understood. T he moss can be very efficient in retaining nitrogen and decomposition of dead leaf material can be very slow, which might reduce the amount of nitrogen that actually becomes available for other organisms (DeLuca and Zackrisson 2007). For both Arcticcyanolichen and moss-cyanobacteria association, it was shown that nitrogen fixation is affected by water availability. In regions with low precipitation during the growing season, nitrogen fixation is limited to periods of snowmelt, when water is sufficiently available, or is restricted to habitats that stay wet during summer (Zielke et al. 2005, ref for lichen).
