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