Microbial community diversity, structure and assembly across oxygen gradients in meromictic marine lakes, Palau.

Microbial communities consume oxygen, alter biogeochemistry and compress habitat in aquatic ecosystems, yet our understanding of these microbial-biogeochemical-ecological interactions is limited by a lack of systematic analyses of low-oxygen ecosystems. Marine lakes provide an ideal comparative system, as they range from well-mixed holomictic lakes to stratified, anoxic, meromictic lakes that vary in their vertical extent of anoxia. We examined microbial communities inhabiting six marine lakes and one ocean site using pyrosequencing of 16S rRNA genes. Microbial richness and evenness was typically highest in the anoxic monimolimnion of meromictic lakes, with common marine bacteria present in mixolimnion communities replaced by anoxygenic phototrophs, sulfate-reducing bacteria and SAR406 in the monimolimnion. These sharp changes in community structure were linked to environmental gradients (constrained variation in redundancy analysis = 68%-76%) - particularly oxygen and pH. However, in those lakes with the steepest oxygen gradients, salinity and dissolved nutrients were important secondary constraining variables, indicating that subtle but substantive differences in microbial communities occur within similar low-oxygen habitats. Deterministic processes were a dominant influence on whole community assembly (all nearest taxon index values >4), demonstrating that the strong environmental gradients present in meromictic marine lakes drive microbial community assembly.