In marine sediments, heterotrophic microbes are central to organic matter exchange and transformation processes, ultimately affecting how much organic matter is sequestered. To date, little is known about how these communities change in composition and substrate utilisation across seasons. Here, we sampled surface sediments from Isfjorden, Svalbard, in winter, spring, and summer 2021 to 2022. We fractionated the sediment into the pore water (PW), cells which are loosely attached to grains (LA), and those firmly attached to grains (FA). We then assessed seasonal shifts in the community composition of the fractions through 16S rRNA gene sequencing, and quantified hydrolysis rates of fluorescently-labelled laminarin and mucin, a common polysaccharide and glycoprotein in the marine environment. Across seasons, cell numbers in the bulk sediment were 2-4x10⁸ cells/mL; up to 90% were in the FA fraction. Cell counts in PW and LA fluctuated more, with an increase in spring for PW (4% +/- 2%) and in spring and summer for LA (9% +/- 3%) In PW and LA, there were more Colwellia, Polaribacter, Yoonia-Loktanella in spring and summer, while there were more Lutimonas in winter. In FA, Woeseia and Maribacter stayed abundant across seasons. In winter, mucin was hydrolysed and consumed between 1 and 2 days in PW and LA, while in FA, hydrolysis only peaked after 7 days. Summer hydrolysis rates for laminarin were higher in PW and LA compared to winter. This study demonstrates that bacterial fractions within surface sediments respond differently to seasonal changes and substrate input.

Primary Presenter: Chyrene Moncada, Max Planck Institute for Marine Microbiology (cmoncada@mpi-bremen.de)

Authors:

Chyrene Moncada, Max Planck Institute for Marine Microbiology  (cmoncada@mpi-bremen.de)

Carol Arnosti, University of North Carolina - Chapel Hill  (arnosti@email.unc.edu)

Rudolf Amann, Max Planck Institute for Marine Microbiology  (ramann@mpi-bremen.de)

Katrin Knittel, Max Planck Institute for Marine Microbiology  (kknittel@mpi-bremen.de)