Extent and limitations of functional redundancy among bacterial communities towards dissolved organic matter

Abstract: One of the key processes in the carbon cycle on our planet is the degradation of dissolved organic matter (DOM) in aquatic environments. The use of organic matter by bacteria links energy from DOM to higher trophic levels of the ecosystem when bacteria are consumed by other organisms. This is referred to as the microbial loop. In this thesis I examined if the communities were functionally redundant in their ability to utilize organic matter, or if variation in bacterial composition and richness is of importance. To test this overarching question several experiments were conducted that include methods such as illumina sequencing of the 16S rRNA gene for taxonomic identification of bacterial communities, flow cytometry to follow the growth of communities and spectroscopic measurement to describe the composition of the organic matter pool. Initially we demonstrated how to optimally sterilize organic matter for experimental studies in order to preserve its natural complexity. In further experiments we found that bacterial communities are redundant in their utilization of organic matter and can maintain optimal performance towards a range of organic matter pools. Related to this we found that pre-adaptation to organic matter played a small role as communities performed equally well regardless of their environmental history. We saw a small effect of richness and composition of bacterial communities on the efficiency of organic matter use, but conclude that this is of minor importance relative to abiotic factors. Still, we also show that organic matter can put strong selection pressure on bacterial communities with regards to richness and composition. Additionally we found that the supply rate of a carbon compound greatly influenced the energy utilization of the compound, i.e. a higher growth rate can be maintained if substrate is delivered in pulses relative to a continuous flow. Finally we conclude that the variation in bacterial communities is unlikely to have a major influence on carbon cycling in boreal lakes, but to enable a finer understanding, the genetics underlying the carbon utilization needs to be further explored.