Exploring minimum conditions allowing the development of oxygenic photogranules

Overview & Synopsis

Photogranulation is the microbially driven phenomenon in which a dense, roughly spherical phototrophic aggregates of several millimeters is formed. These aggregates have a densely populated outer layer of filamentous cyanobacteria embedded in a matrix of extracellular polymeric substances. Naturally occurring photogranules are observed in a wide variety of environments, including cryoconite granules on glacier surfaces and microbialites in marine waters and tropical lagoons. In the laboratory, we can produce oxygenic photogranules from activated sludge used in wastewater treatment in hydrostatic batch cultivations and turbulently mixed reactors. Photogranules have the potential to be applied biotechnology, i.e., in aeration-free wastewater treatment because their high settleability and syntrophic community that closes oxygen and carbon dioxide cycles. Biological and ecological principles of photogranulation may be applied in biotechnology, but how communities assemble and form the 3D photogranule structure is today largely unknown. The aim of my thesis was to find out what is minimally needed for the formation of photogranules. I studied hydrostatic photogranulation in which activated sludge in a hydrostatically incubated vial exposed to light transforms into an oxygenic photogranule over the course of several weeks. To follow the development of photogranules, I developed a high-throughput time-lase imaging assay that allows quantification of biomass contraction over time. Using high-throughput sequencing coupled to qPCR, I then explored the link between microbial community structure and ecosystem spatial organization. I furthermore investigated how spatial structure formation was altered by the addition of filamentous cyanobacteria from the outer layer of mature photogranules, which we hypothesize to play a key role in shaping the 3D structure. I did not find a link between final phototrophic communities and final spatial structures. From the augmentation experiments, it appeared that cyanobacteria that were initially present and steered the development of the photogranule were no longer detected in the final communities. The initial type and abundance of cyanobacteria determines the final ecosystem spatial organization. Cyanobacteria that steered photogranulation are phylogenetically close which hints towards a narrow group of cyanobacteria possessing photogranulation ability. These principles may also apply to the formation of photogranules in natural environments. Cyanobacterial augmentation in bioreactors can be applied to steer photogranulation in bioreactors.