Microbial polysaccharide utilization in sandy surface sediments

The shallow waters over continental shelves are a hotspot of marine primary production. In these regions, significant amounts of carbon fixed in the water column is exported to the sediment and, together with benthic primary production, feeds heterotrophic microbes. Much of the organic matter is comprised of polysaccharides produced by algae, which can be very complex. Since the organic matter is not accumulating, utilization of polysaccharides by benthic microbes must be highly relevant for the marine carbon cycle. In this thesis, I first studied the dynamics of benthic microbial communities in sands by assessing taxonomic changes over the seasonal cycle in temperate and polar regions. Compared to the rapid substrate driven successions of coastal bacterioplankton communities during and after spring blooms, there was very little seasonal change in the taxonomic composition of the benthic bacterial community. This comparison indicated fundamental differences in the ecological mechanisms driving pelagic and benthic communities. Since no taxonomic succession was observed, transcriptional activity was investigated. Here, clear seasonal changes were identified for several Gammaproteobacteria and Bacteroidia species. However, these species represented only a minor fraction of the community, with the majority showing relatively constant activity. Assessing the temporal changes in polysaccharides revealed α- and β-glucans as important substrates in winter and in spring, respectively, indicating that recycling processes might stabilize the community. Incubations with fluorescently labeled polysaccharides provided further evidence for β-glucan (laminarin) and animal-derived glycoprotein (mucin) utilization. The dominance of extracellular hydrolysis over selfish uptake in incubations and the high concentration of glucose-depleted dissolved organic matter in the pore water of sands, indicates that benthic microbes preferentially utilize glucose during the degradation of particulate organic matter, with the remaining glycans being released in dissolved form into the overlying pelagic system.