Differences in structure of northern Australian hypolithic communities according to location, rock type, and gross morphology.

Hypolithic communities (under translucent rocks) were compared between a semi-arid site (Wave Hill) and a site with considerably higher rainfall (Lake Bennett) to test the hypothesis that the communities at the higher rainfall site would be more diverse. A total of 153 cyanobacteria operational taxonomic units (OTUs) were identified, and only 50 of those were found at both sites. Of these, only two were core OTUs, as defined as being present in ≥90% of samples, highlighting the extreme differences in the cyanobacterial communities at the two sites. At Wave Hill, we compared the composition of the cyanobacterial components under two different rock types (quartz and prehnite) to determine if the different minerals would result in different hypolithic communities, but no differences were found. Of the 42 core OTUs found at Wave Hill, 22 (52%) were shared between the two rock types. As hypothesised, the diversity of both cyanobacteria and eukaryotes in the hypolithic communities was significantly higher at Lake Bennett. Some hypolithic communities were thin and tightly adhered to the rock surface, but others were thicker and could be peeled off the rock in sheets. However, the two types were not significantly different in OTU composition. Metazoans, primarily nematodes, were ubiquitous, raising the possibility that nematodes may act as vectors to transport the components of hypolithic communities from rock to rock as a mechanism of colonization.