Multicellularity in Synechocystis PCC 6803

Abstract

PhD thesisSynechocystis sp. PCC6803 is a unicellular freshwater cyanobacterium and has been used as a model organism for a wide variety of celullar processes, including biofilm formation. This thesis discusses the natural aptitude of Synechocystis to form multicellular communities, both on surfaces and as free-floating aggregates termed ’flocs’. An assay is presented to assess Synechocystis flocculation via imaging, gaining quantitative data via image analysis. Synechocystis flocculation is shown to be influenced by the presence of cell surface structures and the degree of piliation with type IV pili through an unclear mechanism, being abrogated completely in some mutants such as a deletion mutant in the RNA chaperone hfq. The gene for the major pilin, pilA1, is dispensable for flocculation, but the minor pilin operon pilA9slr2019 and the pilus extension motor PilB1 are shown to be essential. Other surface structures explored include the S-layer and a putative chaperone-usher system. This work established that wild-type Synechocystis are capable of compensating for the presence of the non-flocculating Δhfq mutant and incorporate the Δhfq cells into flocs up to a certain degree. Using mutants expressing a fluorescently tagged Vipp1 protein we explore the distribution and incorporation of non-flocculating cells into the flocs using fluorescence microscopy. Furthermore, we reveal that cells incorporated into flocs tend to suffer from stress, likely some form of nutrient stress, in their denser areas. Various environmental conditions, particularly the ability of blue/green light photoreceptors Cph2 and PixJ to modulate flocculation in response to the blue:green light ratio, are discussed. Flocculation is shown to be dependent on external factors such as nutrient levels