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

Abstract

Phytoplankton blooms represent hotspots of primary production and lead to the formation of particulate organic matter composed of living and dead algal cells. These particles are characterized by steep chemical gradients, for instance in oxygen concentration, that provide diverse ecological niches for specifically adapted microbes to thrive. We analyzed cooccurrence 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 micrometer size were collected at almost daily intervals between early March and late May in 2018. Metaproteomics was used to asses microbial protein expression at three selected time points during the sampling period. Cooccurrence networks identified two major modules representing bacteria cooccurring with diatoms and with dinoflagellates, respectively. Bacterial communities associated with these two algal lineages differed in composition, with diatoms being distinctly clustered with known sulfate-reducing Desulfobacterota as well as with potentially sulfur-oxidizing Ectothiorhodospiraceae. Metaproteomics confirmed expression of respective key enzymes in these taxa. Microbial sulfate reduction is known to occur in sinking particles at greater depths (marine snow), but is considered of lesser importance in the upper photic zone during phytoplankton blooms. However, our results suggest presence of sufficiently anoxic niches in diatom-derived particles, which could enable processes akin to cryptic sulfur cycling in oxygen minimum zones to take place also in the largely oxygen-rich photic zone during phytoplankton blooms. This illustrates the need to gain further insights into the functional roles of phytoplankton-associated bacteria.

Alpha-glucans from bacterial necromass indicate an intra-population loop within the marine carbon cycle

“…Sequences have been submitted to the European Nucleotide Archive under the accession number PRJEB51816. Amplicon Sequence Variants (ASVs) were obtained using DADA2 46 and taxonomically classi ed as described previously 47 . ASVs classi ed as Metazoa were removed before downstream analyses to reduce the effect of crustacean zooplankton on community composition.…”