Phototrophic biofilms are key to nutrient cycling in natural environments and bioremediation technologies, but few studies describe biofilm formation by pure (axenic) cultures of a phototrophic microbe. The cyanobacterium Synechocystis sp. strain PCC 6803 (here Synechocystis) is a model microorganism for the study of oxygenic photosynthesis and biofuel production. We report here that wild-type (WT) Synechocystis caused extensive biofilm formation in a 2,000- liter outdoor nonaxenic photobioreactor under conditions attributed to nutrient limitation. We developed a biofilm assay and found that axenic Synechocystis forms biofilms of cells and extracellular material but only when cells are induced by an environmental signal, such as a reduction in the concentration of growth medium BG11. Mutants lacking cell surface structures, namely type IV pili and the S-layer, do not form biofilms. To further characterize the molecular mechanisms of cell-cell binding by Synechocystis, we also developed a rapid (8-h) axenic aggregation assay. Mutants lacking type IV pili were unable to aggregate, but mutants lacking a homolog to Wza, a protein required for type 1 exopolysaccharide export in Escherichia coli, had a superbinding phenotype. In WT cultures, 1.2 × BG11 medium induced aggregation to the same degree as 0.8 × BG11 medium. Overall, our data support that Wza-dependent exopolysaccharide is essential to maintain stable, uniform suspensions of WT Synechocystis cells in unmodified growth medium and that this mechanism is counteracted in a pilus-dependent manner under altered BG11 concentrations.

Original languageEnglish (US)
Article numbere02192-18
JournalApplied and environmental microbiology
Issue number7
StatePublished - Apr 1 2019


  • Aggregation
  • Biofilms
  • Biofuels
  • Biogeochemistry
  • Blooms
  • Cyanobacteria
  • Exopolysaccharide
  • HABs
  • Photobioreactor
  • Synechocystis

ASJC Scopus subject areas

  • Biotechnology
  • Food Science
  • Ecology
  • Applied Microbiology and Biotechnology


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