Polysaccharide utilization loci and associated genes in marine Bacteroidetes - compositional diversity and ecological relevance

Abstract

The synthesis of marine organic carbon compounds by photosynthetic macroalgae, microalgae (phytoplankton) and bacteria provide a basis for life in the ocean. In marine surface waters this primary production is largely dominated by microalgae and is especially pronounced during spring phytoplankton blooms. During and after these often diatom-dominated blooms, increased amounts of organic matter are released into the surrounding waters. Here, the organic matter, rich in polysaccharides, can trigger blooms of heterotrophic bacteria. Marine members of the Bacteroidetes are consistently found related to such bloom events. These bacteria are regularly detected as the first responders to thrive after phytoplankton spring blooms in temperate coastal regions and are often equipped with a variety of polysaccharide utilization gene clusters. These gene clusters, termed polysaccharide utilization loci (PULs), encode enzymes for the extracellular hydrolysis of polysaccharides and the subsequent uptake of oligosaccharides into the periplasm, where they are shielded from competing bacteria. This mechanism allows for rapid uptake and substrate hoarding, and thus could be one reason why Bacteroidetes are often seen as the first responders of the bacterioplankton community. The investigation of the so far largely unknown diversity and the ecological relevance of PULs in marine Bacteroidetes was the major goal of the work presented here. We could show that genomes of Bacteroidetes isolates from the North Sea, with free-living to micro- and macro-algae associated lifestyles, harboured a variety of these loci predicted to target in total 18 different substrate classes. Overall PUL repertoires of these isolates showed considerable intra-genus and inter-genus, variations suggesting that Bacteroidetes species harbour distinct glycan niches, independent of their phylogenetic relationships. By investigating the PUL repertoires of uncultured free-living Bacteroidetes during three consecutive years of spring phytoplankton blooms at the North Sea island of Helgoland, I could further reveal that the set of targeted substrates during these bloom events was dominated by only five of the substrate classes targeted by the isolates. These were the diatom storage polysaccharide laminarin, alpha-glucans, alginates, as well as substrates rich in alpha-mannans and sulfated xylans. In addition to this constrained set of substrate classes targeted by the free-living Bacteroidetes community, I could show that the species diversity during these blooms was limited and dominated by only 27 abundant and recurrent species that carried a limited number of abundant PULs. The majority of these PULs were targeting laminarin and alpha-glucan substrates, which were likely targeted during the entire time of the blooms. The less frequent PULs, targeting alpha-mannans and sulfated xylans, were predominantly detected during mid- and late- bloom phases, suggesting a relevance of these two substrate classes in the later phases of phytoplankton blooms. Overall these findings highlight the recurrence of a few specialized Bacteroidetes species and the environmental relevance of specific polysaccharide substrate classes during spring phytoplankton blooms. However, for some of these substrate classes the origin, structural details and their abundance during blooms are as yet largely unknown. To further shed light on the polysaccharide niches of abundant key-players, these findings can serve as a guide for future laboratory studies.