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

Abstract Phytoplankton blooms initiate bacterioplankton blooms, from which bacterial biomass is released via grazing zooplankton and viral lysis. Bacterial consumption of algal biomass during blooms is well studied, but little is known about the simultaneous reuse of bacterial necromass. Alpha- and beta-glucans are abundant dissolved organic macromolecules during blooms. We demonstrate algal laminarin-fueled alpha-glucan synthesis in marine Bacteroidota strains, as well as bacterial reuse of these alpha-glucans as major carbon source in vitro and during a diatom-dominated bloom. We highlight two types of genomic loci and the encoded protein machineries with structurally distinct SusD substrate-binding proteins that may target alpha-glucans of different complexities. It is demonstrated that these encoded machineries can be specifically induced by extracted alpha-glucan-rich bacterial polysaccharides. This bacterial alpha-glucan synthesis and recycling from bacterial necromass constitutes a large-scale intra-population energy conservation mechanism redirecting substantial amounts of carbon in an essential part of the microbial loop.