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Abstract

Spring phytoplankton blooms in temperate environments contribute
disproportionately to global marine productivity. Bloom-derived organic
matter, much of it occurring as polysaccharides, fuels biogeochemical
cycles driven by interacting autotrophic and heterotrophic communities.
We tracked changes in the mode of polysaccharide utilization by
heterotrophic bacteria during the course of a diatom-dominated bloom in
the German Bight, North Sea. Polysaccharides can be taken up in a
'selfish' mode, where initial hydrolysis is coupled to transport into
the periplasm, such that little to no low-molecular weight (LMW)
products are externally released to the environment. Alternatively,
polysaccharides hydrolyzed by cell-surface attached or free
extracellular enzymes (external hydrolysis) yield LMW products available
to the wider bacterioplankton community. In the early bloom phase,
selfish activity was accompanied by low extracellular hydrolysis rates
of a few polysaccharides. As the bloom progressed, selfish uptake
increased markedly, and external hydrolysis rates increased, but only
for a limited range of substrates. The late bloom phase was
characterized by high external hydrolysis rates of a broad range of
polysaccharides and reduced selfish uptake of polysaccharides, except
for laminarin. Substrate utilization mode is related both to substrate
structural complexity and to the bloom-stage dependent composition of
the heterotrophic bacterial community.