Distribution and Function of marine Bacteroidetes

Abstract

Members of the phylum Bacteroidetes play a pivotal role in degrading organic matter and appear everywhere in marine and freshwater systems, from coastal to open ocean, from polar to equatorial, from surface waters down to the deep sea as well as in association with aggregates and with phytoplankton blooms. The studies described in this thesis elaborate on the distribution and function of marine Bacteroidetes. Specifically their association with spring phytoplankton blooms, substrate association by direct surface attachment and their genetic capability of degrading high molecular weight organic matter and in particular polysaccharides were examined. The Bacteroidetes distribution and community structure were analyzed at a temporal scale, by investigating the responses of distinct bacteroidetal clades during and after spring phytoplankton blooms of four consecutive years at the coastal station Helgoland Roads. It could be shown by automated microscopic cell counting that shortly after the chlorophyll a maximum concentration Bacteroidetes increased to more than 50% of the total bacterioplankton community during spring seasons. The Bacteroidetes community comprised only a few dominant genera, which accounted together for more than half of the Bacteroidetes. Each year a distinct succession pattern of the clades Ulvibacter, Formosa A, and Polaribacter was observed with relative abundances of single clades with up to 20%. Furthermore, members of the Bacteroidetes inhabited not only the free-living fraction, but they were also found attached to diatoms. Although a quantification of attached Bacteroidetes was difficult, qualitative observations were made. For example members of this phylum attach frequently to the diatom Chaetoceros spp., which is commonly blooming in spring at Helgoland Roads. The clades Polaribacter and Formosa A were identified as dominating among those Chaetoceros-associated Bacteroidetes. In contrast, Ulvibacter was not found attached to Chaetoceros, but to Asterionella spp., another diatom genus occurring in spring blooms. Since members of Bacteroidetes are the first in responding to algal blooms and attached even to distinct diatom species, we investigated their genetic potential to degrade algal derived organic matter. In particular we searched for the presence of polysaccharide utilization loci (PULs) in fosmids retrieved from two contrasting provinces of the North Atlantic Ocean. In total 14 PULs were identified, six on fosmids from the northern station and eight on fosmids from the southern station. Among those PULs one seems to be involved in xylan degradation and four were identified as potential laminarin degradation PULs. Interestingly, GHs were identified which had been assumed to be unique among terrestrial Flavobacteria, suggesting a higher capability of open ocean Bacteroidetes clades for organic matter degradation than previously anticipated.