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J Bacteriol. 2019 Jul 1. pii: JB.00344-19. doi: 10.1128/JB.00344-19. [Epub ahead of print]

Factors controlling floc formation and structure in the cyanobacterium Synechocystis sp. PCC 6803.

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School of Biological & Chemical Sciences, Queen Mary University of London, Mile End Road, London E1 4NS, U.K.
University of Freiburg, Institute of Biology III, Schänzlestr. 1, 79104 Freiburg, Germany.


Motile strains of the unicellular cyanobacterium Synechocystis sp. PCC 6803 readily aggregate into flocs, floating multicellular assemblages, when grown in liquid culture. Here, we use confocal imaging to probe the structure of these flocs, and we develop a quantitative assay for floc formation based on fluorescence imaging of 6-well plates. The flocs are formed from strands of linked cells, sometimes packed into dense clusters but also containing voids with very few cells. Cells within the dense clusters show signs of nutrient stress, as judged from the sub-cellular distribution of GFP-tagged Vipp1 protein. We analyzed the effects on flocculation of a series of mutations that alter piliation and motility: Δhfq, ΔpilB1, ΔpilT1, ΔushA and mutants lacking major and minor pilins. The extent of flocculation is increased in the hyperpiliated ΔpilT1 mutant, but active cycles of pilus extension and retraction are not required for flocculation. Deletion of PilA1, the major subunit of type IV pili, has no effect on flocculation, however flocculation is lost in mutants lacking an operon coding for the minor pilins PilA9-11. Therefore, minor pilins appear crucial for flocculation. We show that flocculation is a tightly-regulated process that is promoted by blue light perception by the cyanobacteriochrome Cph2. Floc formation also seems to be a highly co-operative process. A proportion of non-flocculating Δhfq cells can be incorporated into wild-type flocs, but the presence of a high proportion of Δhfq cells disrupts the large-scale architecture of the floc.Importance Some bacteria form flocs: multicellular floating assemblages of many thousands of cells. Flocs have been relatively little-studied compared to surface-adherent biofilms, but flocculation could play many physiological roles, be a crucial factor in marine carbon burial and enable more efficient biotechnological cell-harvesting. We studied floc formation and architecture in the model cyanobacterium Synechocystis sp. PCC 6803, using mutants to identify specific cell-surface structures required for floc formation. We show that floc formation is regulated by blue and green light perceived by the photoreceptor Cph2. The flocs have a characteristic structure based on strands of linked cells aggregating into dense clusters. Cells within the dense clusters show signs of nutrient stress, pointing to a disadvantage of floc formation.

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