4c). At all sites the water holding capacity of the BSC was significantly higher than in the underlying soils. Fig. 4 Soil characteristics at all four sites: 5-Fluoracil chemical structure a soil compaction; b soil fractions; c water holding capacity of soils (lines in bars show standard deviation) Bacterial diversity Non-photosynthetic

bacteria were only quite recently considered as important BSC-organisms (Garcia-Pichel et al. 2003; Castillo-Monroy et al. 2011) and their important role in the nitrogen budget of BSCs has been addressed in several recent works (Green et al. 2008; Brankatschk et al. 2012; Barger et al. 2013). In our investigation so far, we found a shared fraction (potential core microbiome) comprising 125 operational taxonomic units (OTUs based on presence/absence data) across BSCs from the four investigation sites (Fig. 5). Relative composition analysis across the learn more four sites revealed the Alphaproteobacteria as the dominating group, followed by the Actinobacteria (Fig. 5). The small number of shared OTUs among sites in comparison to the total number of OTUs suggests a minimal core microbiome (Maier

et al. 2014). Fig. 5 Core microbiome (125 OTUs) based on 10 samples per location processed in QIIME (sequences were denoised, assigned to OTUs at a 98 % similarity threshold, rarified to 732 reads) OTUs found at all four locations were considered part of the core Cyanobacterial and green algal diversity The vast majority of the bacterial diversity is non-photosynthetic bacteria. Cyanobacteria contribute only 1.6 % of the bacterial diversity (Fig. 5). Nevertheless, their contribution to biomass and especially their role in establishing BSCs is suggested to be reciprocal to their diversity (Campbell 1979; Campbell et al. 1989; Belnap et al. 2003a). To date, we have found nineteen different species/genera at all sites, with Gössenheim

having the lowest number (7) compared to Hochtor (10), Öland (11) and Tabernas (13), despite the latter having the lowest coverage of light and dark BSCs. Species of the genera Microcoleus, the functionally most important genus in forming the initial crusts (Belnap and Gardner 1993; Malam et al. 1999) and Nostoc, Ribonucleotide reductase important nitrogen fixers (Beyschlag et al. 2008; Maqubela et al. 2008), were present at all four sites. At Hochtor an extensive blackish to brown crust (Fig. 6g), often misidentified as the green algal lichen Toniniopsis obscura (Peer et al. 2010), was found to consist of cyanobacteria species (Gleocapsa spp. Nostoc sp. and others) with only few unicellular green algae (Fig. 6h). Peer et al. (2010) published a list of cyanobacteria and green algae found in the BSCs at the Hochtor locality based on classical morphological determination. They found six filamentous and one unicellular Cyanobacteria and 34 mostly unicellular green algal species. Fig. 6 Biological soil crusts and typical lichens.