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

## TL;DR

During a spring algal bloom in the North Sea, bacteria that reduce sulfate thrive in low-oxygen areas linked to diatoms, influencing carbon cycling.

## Contribution

This study reveals active sulfate-reducing bacteria in algal particle microbiomes during a shallow-water bloom, suggesting anoxic niches and benthic-pelagic coupling.

## Key findings

- Network analysis identified bacterial modules co-occurring with diatoms and dinoflagellates.
- Metaproteomes confirmed sulfate-reduction enzymes in shallow, particle-rich environments.
- Anoxic niches in phytoplankton particles support sulfate-reducing bacteria, linking benthic and pelagic processes.

## Abstract

Phytoplankton blooms fuel marine food webs with labile dissolved carbon and also lead to the formation of particulate organic matter composed of living and dead algal cells. These particles contribute to carbon sequestration and are sites of intense algal-bacterial interactions, providing 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 a shallow location (6-10 m depth) 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 and in sediments. Our results indicate the presence of sufficiently anoxic niches in the particle fraction of an active phytoplankton bloom to sustain sulfate reduction, and an important role of benthic-pelagic coupling for microbiomes in shallow environments. Our findings may have implications for the understanding of algal-bacterial interactions and carbon export during blooms in shallow-water coastal areas.

During an algal bloom in the North Sea, bacteria living alongside oxygen producing diatoms exploit anoxic niches to reduce sulfate.