Sulfate reducing bacteria are active and co-occur with diatoms during a spring phytoplankton bloom

Abstract

ABSTRACTPhytoplankton blooms fuel marine food webs with labile dissolved carbon, but also lead to the formation of particulate organic matter composed of living and dead algal cells. These particles are sites of intense algal-bacterial interactions and provide diverse niches for microbes to thrive. We analyzed co-occurrence networks based on 16S and 18S ribosomal RNA gene amplicon sequences obtained during a spring phytoplankton bloom in the North Sea. Particulate fractions larger than 10 µm diameter were collected at almost daily intervals between early March and late May in 2018. Metaproteomics was used to assess microbial functional activity at three selected time points during the sampling period. Network analysis identified two major modules representing bacteria co-occurring with diatoms and with dinoflagellates, respectively. Bacterial communities associated with these two algal lineages differed in composition, with diatoms distinctly clustering with known sulfate-reducingDesulfobacterotaas well as with potentially sulfur-oxidizingEctothiorhodospiraceae. Metaproteomics confirmed the presence of key enzymes involved in dissimilatory sulfate reduction, a process known to occur in sinking particles at greater depths, but considered of lesser importance in the photic zone. However, our results suggest presence of sufficiently anoxic niches in diatom-derived particles, which may have implications for diatom-bacterial interactions and carbon export during phytoplankton blooms.