Redox stability regulates community structure of active microbes at the sediment-water interface

Changes in redox conditions occur in a wide range of microbial habitats, in particular at the sediment-water interface (SWI) of aquatic systems. A mesocosm study using intact sediment cores from Lake Stechlin (Germany) was performed to investigate the impact of redox changes on microbial communities at the SWI. The SWI was exposed to permanent oxic (OX) or anoxic (ANOX) or to variable (VR) redox conditions, and for molecular analysis sediment samples were taken at the start and after seven days of the treatment. We performed 16S rRNA amplicon sequencing to identify redox-specific changes in the composition of metabolically active microbes. Generally, the community of active microbes in the VR cores was similar to in the OX cores, but differed significantly from the ANOX cores. Interestingly, VR conditions resulted in a high fraction of a Crenothrix-like microorganism increasing in read abundance from 4 to 5% initially, up to 69% over the experimental period. This implies that periodic redox fluctuations select for specific bacteria in environments such as seiches-affected sediments of stratified lakes. In Lake Stechlin sediment cores, these redox fluctuations lead to increased activities of specific microorganisms and high organic matter turnover rates with profound implications for aquatic organic matter cycling.