Evidence of species recruitment and development of hot desert hypolithic communities.

Hypoliths, photosynthetic microbial assemblages found underneath translucent rocks, are widely distributed within the western region of the Namib Desert and other similar environments. Terminal restriction fragment length polymorphism (T-RFLP) analysis was used to assess the bacterial community structure of hypoliths and surrounding soil (below and adjacent to the hypolithic rock) at a fine scale (10 m radius). Multivariate analysis of T-RFs showed that hypolithic and soil communities were structurally distinct. T-RFLP-derived operational taxonomic units were linked to 16S rRNA gene clone libraries. Applying the ecological concept of 'indicator species', six and nine indicator lineages were identified for hypoliths and soil, respectively. Hypolithic communities were dominated by cyanobacteria affiliated to Pleurocapsales, whereas actinobacteria were prevalent in the soil. These results are consistent with the concept of species sorting and suggest that the bottom of the quartz rocks provides conditions suitable for the development of discrete and demonstrably different microbial assemblages. However, we found strong evidence for neutral assembly processes, as almost 90% of the taxa present in the hypoliths were also detected in the soil. These results suggest that hypolithons do not develop independently from microbial communities found in the surrounding soil, but selectively recruit from local populations.