Meta-analyses of environmental sequence data identify anoxia and salinity as parameters shaping ciliate communities

Abstract

Nearly 6000 SSU rRNA sequences of ciliated protists, compiled from 50 marine and terrestrial sampling sites worldwide were analysed to identify environmental barriers spatially relating ciliate communities. The hypotheses were: (i) oxygen is a dispersal barrier for strict anaerobes, allowing allopatric speciation in isolated habitats and resulting in high diversity; (ii) as salinity affects metabolism it may lead to distinct clusters with evolutionarily separated lineages; (iii) different oceanic realms support significantly different ciliate communities. Based on the α-diversity of the ciliate sequences in each habitat and the shared diversity among different habitats, ecological and geographic boundaries were revealed. Community similarities between pairs of habitats and all habitats were depicted with a metric distance matrix. Anoxia and salinity emerged as decisive determinants structuring the communities, with anoxia largely overlaying the effect of other environmental parameters. Measurements of α-diversity suggest a tremendous diversity of ciliates in anoxic environments, exceeding the one in normoxic environments. Salinity also exerts a high selection pressure on ciliates, contributing to community structure, composition and distribution patterns. However, marine–freshwater transitions seem to be more recent in ciliates compared with most other protistan taxa. The theory of island biogeography seems to apply to anoxic habitats, making these sites promising targets for the discovery of novel diversity.