Identity, dynamics and association-networks of suboxic freshwater bacterioplankton in stratified humic lakes

Humic lakes play a central role in the carbon cycle of the boreal landscape. These systems typically stratify in summer and their hypolimnetic microbial communities modulate the sediment burial of biogenic organic matter. The composition of bacterial communities in these suboxic habitats was studied by pyrosequencing of 16S rRNA amplicons from 5 lakes with variable dissolved organic carbon concentration. Bacterioplankton communities in the hypolimnetic waters were clearly different from the surface layer with candidate division OD1, Chlorobi and Bacteroidetes as dominant community members. Hypolimnetic waters contained more operational taxonomic units than surface waters and such diversity was also positively related to high levels of dissolved organic compounds. Several dominant OTUs were annotated tothe OD1 candidate division. Overall reads annotated as OD1 increased in dominance with DOC concentration in the suboxic hypolimnia of the lakes. Time-series data from lake Alinen Mustajärvi suggest that bacterioplankton of the OD1 candidate division are abundant and consistent members of the suboxic hypolimnion in this lake. Network analyses of such temporal data revealed contrasting ecological roles among the most abundant taxa, also within the candidate division OD1. Furthermore, our data suggests that the suboxic hypolimnetic bacterioplankton communities are more diverse but less dynamics than communities from the surface layer of stratified humic lakes.