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Institute
Primary producers, alongside heterotrophic bacteria and viruses, modulate the essential global carbon cycle. About half of the Earth’s net primary production originates in the marine environment. By effecting these systems and the burial of carbon, bacteria play a significant role in the world’s climate, especially with regard to rising temperatures and increasing anthropogenic carbon dioxide production.
Particles present substrate-rich niches for particle-associated bacteria, but are rare in the marine system. Particle-associated bacteria, comprising of chemotactic motile free-living and particle-attached bacteria, were shown to have higher respiration rates, were larger in cell and genome size and showed a higher hydrolytic activity of extracellular enzymes compared to the free-living fraction.
Understanding the contribution of particle-associated bacteria to the degradation of algal biomass is essential to understand the marine carbon cycle. However, the identification of this group is difficult and required refinement.
Sequential filtration, the most commonly used technique for the separation of bacterial fractions, provides only access to a part of the particle-associated microbiome, and includes with large and clustered bacteria undesired false-positives. To overcome these limitations, separation by gravity in Imhoff sedimentation cones was explored in this thesis to access, identify and define particle-associated microbiomes, in comparison and conjunction with the established separation techniques like sequential filtration and centrifugation.
The cultivability on agar plates was assessed, aiming at the question which portion of the colony-forming bacteria belong to free-living non-motile or motile bacteria or to particle-attached bacteria. As continuous cultivation on plates often involves loss of cultures, colonies of the original plate were used to obtain partial 16S rRNA sequences of individual colonies and of plate microbiomes.
For future studies on particle-associated bacteria, a representative strain collection was established from particle-attached bacteria retained on 3 μm filters and from particle-associated bacteria collected together with settled algae in sedimentation cones.
To understand the contribution of top-down selection to a yearly recurring bacterioplankton bloom at our sampling site Helgoland, particle-associated strains were included in isolation experiments for flavophages, since Flavobacteriia are among the most important responder to the yearly observed blooms.
Overall, this thesis provides new insights into the isolation and cultivation of particle-associated bacteria – an important, but currently not fully understood fraction of organisms within the marine system.