CANCELLED - Finding and sensing environmental drivers of freshwater microbial communities

Background/Question/Methods

Aquatic bacteria play key roles in biogeochemical cycles and in other ways shape the biotic and abiotic features of aquatic ecosystems. It is clear that these elusive and largely unculturable  ecosystem engineers host a tremendous genetic and metabolic diversity, but limited by methods we have only just begun to make an inventory of the bacteria that reside in lakes, oceans and other aquatic biomes. Attempts to define the ecology and function of these aquatic bacteria is also still in its infancy, but temporal dynamics of phylogenetically defined populations is one promising approach to define phylogenetic levels of ecological coherence, identify linkages among populations and features of their abiotic environment. In the present study, the temporal dynamics of lake bacterioplankton communities and populations were traced by multiplexed pyrosequencing of bacterial marker genes. The relevant timescales ranged from days to seasons and both metabolically defined functional guilds and the combined bacterial communities were in focus. This study design enabled us to address pressing questions about whether or not phylogenetic relatedness and taxonomy are good proxies for ecological traits or biogeochemical function in aquatic bacteria as well as the role environmental drivers play in shaping the indigenous bacterioplankton community.

Results/Conclusions

Global freshwater phylogenies of marker genes enabled identification of at least 100 putative freshwater clusters of bacteria for subsequent time series analysis. Population clustering based on temporal dynamics clearly showed that while some higher phylogenetic groups, notably within the Actinobacteria, display coherent temporal dynamics, most others do not. Network analysis based on local similarity analysis reveals complex community networks with tightly connected clusters of co-occurring populations with strong positive or negative linkages to temperature, nutrient availability and phytoplankton primary producers. We suggest that such clusters may represent ecologically defined groups of bacterioplankton that respond to the same array of critical environmental driver variables. Alternatively these co-occurring populations may feature synergistic interactions and one example of this will be presented for chitin degrading bacteria where freeliving Actinobacteria profit from surplus chitin metabolites released by chitinolytic Flavobacteria attached to chitinous particles. Further time series-analysis of chitin degraders as a functional guild revealed strong resource control of  the chitinolytic community, where not only the amount of substrate, but also the chemical speciation of substrate matters.