Microbial diversity, autotrophic-and heterotrophic processes in Mn-rich marine sediments

Sebastian Silva-Solar¹Marit Ruane van Erk^1,2Gilad Antler³Subhajit Basu¹Dirk de Beer^1*

• ¹Max-Planck-Institut fur marine Mikrobiologie, Bremen, Germany
• ²Radboud Universiteit, Nijmegen, Netherlands
• ³Ben-Gurion University of the Negev, Be'er Sheva, Israel

A main aim of microbial ecology is to link microbial communities with the geochemical processes they drive. Although huge progress has been achieved in understanding microbial metabolisms in the environment by molecular and -omics techniques, the information these approaches give about the geochemical processes is limited. Here, we combined geochemical and 16S rRNA amplicon sequencing analyses to spatially link dominant microbial taxa with geochemical processes in deep-sea sediments of the Gulf of Aqaba. The fine-grained aeolian sediments are diffusion controlled, and the geochemical processes are stratified in well-defined zones. A narrow brown band enriched in solid-phase Mn-oxide was observed at 2-3 cm depth, where downward diffusing O2 and upward diffusing Mn2+ overlapped. Here members of the family Scalinduaceae peaked, suggesting involvement in Mn oxidation or -reduction. Geochemical depth profiles and fluxes showed aerobic heterotrophy, nitrification, aerobic Mn-oxidation, denitrification, and anaerobic Fe oxidation to occur in sequence from upper to deeper sediment layers. Although chemoautotrophic processes were significant, 90% of the redox processing was driven by heterotrophy on organic carbon input. The microbial community composition also showed a clear stratification. The oxic zone was dominated by aerobic consortia, while anaerobes were largely absent, as expected. Remarkably, the anoxic zone was inhabited by many aerobic bacteria, in addition to the anaerobic community. Dominant families above the band were Woeseiaceae, Nitrospiraceae, and Nitrosococcaceae, whereas below the band Desulfosarcinaceae, Desulfatiglandaceae, Methylomirabilaceae, and Anaerolineaceae dominated. While Scalinduaceae peaked in the Mn-oxide band, none of the other dominant families were exclusively linked to zones with specific geochemical processes. Our approach did not resolve relations between the geochemical processes and the dominant members of the community unambiguously, but characterized a unique habitat and leads to the formulation of new hypotheses.