Marine particle microbiomes during a spring diatom bloom contain active sulfate-reducing bacteria

Phytoplankton 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 contribute to carbon sequestration, yet are also sites of intense algal-bacterial interactions and provide diverse niches for microbes to thrive. We analyzed 16S and 18S ribosomal RNA gene amplicon sequences obtained from 51 time points and metaproteomes from 3 time points during a spring phytoplankton bloom in the North Sea. Particulate fractions larger than 10 µm diameter were collected at near daily intervals between early March and late May in 2018. Network analysis identified two major modules representing bacteria co-occurring with diatoms and with dinoflagellates, respectively. The diatom network module included known sulfate-reducing Desulfobacterota as well as potentially sulfur-oxidizing Ectothiorhodospiraceae . Metaproteome analyses confirmed presence of key enzymes involved in dissimilatory sulfate reduction, a process known to occur in sinking particles at greater depths. Our results indicate the presence of sufficiently anoxic niches in the particle fraction of an active phytoplankton bloom to sustain sulfate reduction, which may have implications for algal-bacterial interactions and carbon export during blooms.