Niches of abundant heterotrophic bacteria during North Sea spring algal blooms

The remineralization of algal biomass by heterotrophic bacteria during spring phytoplankton blooms is a globally important process in carbon cycling. Yet, the ecophysiology of environmentally relevant bacteria occurring during these events is largely unknown. In this thesis, I investigated the niches of the gammaproteobacterial genus Reinekea and the flavobacterial genera Polaribacter and Formosa, which are recurrently abundant during the 2009-2012 North Sea spring algal blooms off Helgoland. Firstly, I studied a Reinekea forsetiia , the representative of North Sea Reinekea clade, using genomic and physiological analyses. The results suggested a versatile opportunistic lifestyle comprising (i) utilization of relevant algal polysaccharides, (ii) potential toxin production, and (iii) strategies to deal with phosphorus limitation. Secondly, temporal dynamics and large niche space of diverse North Sea Polaribacter spp. was investigated using fluorescence in situ hybridization with novel oligonucleotide probes, binning of time-series metagenomes and re-analysis of in situ expression data. These analyses showed the succession of four major Polaribacter clades with varying degradation capacity for high molecular weight compounds and suggested a polysaccharide-driven niche partitioning between these closely-related bacteria. Finally, the polysaccharide niche of North Sea Formosa clade was examined using metaa omic approaches and complementary physiological and biochemical experiments. An efficient laminarin uptake and degradation mechanism, which is coupled to peptide utilization, was revealed. This thesis demonstrated how heterotrophic bacteria employ different ecological strategies to utilize diverse high molecular weight compounds released during spring algal blooms and increased our knowledge on microbially mediated carbon turnover in the surface ocean.