Light-dependent reversible biofilm formation in the model cyanobacterium Synechocystis sp. PCC 6803

In response to light regime changes, the model cyanobacterium Synechocystis sp . PCC 6803 can switch from a planktonic lifestyle to form a phototrophic biofilm. Its planktonic cultures contain single cells and microcolonies, while the formed biofilms are dominated by cell aggregates with remarkable heterogeneity. We fitted the size distribution of the latter with a log-normal function and found that under low-to-moderate light intensities (<100 μmol photons m ⁻² s ⁻¹ ) as well as under yellow-orange (560-590 nm) monochromatic illumination the Synechocystis biofilms are dominated by small aggregates with 10-15 μm ² mean aggregate area, while higher (250-400 μmol photons m ⁻² s ⁻¹ ) light intensities and blue-to-green (460–510 nm) illumination induced broad distribution function curves with 40-50 μm ² mean aggregate area. The determined extracellular polymeric substance (EPS) concentrations were significantly higher in biofilm cultures (~1-3 µg/mL) compared to planktonic ones (<1 µg/mL) under the applied light intensities. In biofilms, violet-to-green (430-540 nm) monochromatic illumination induced the highest (~3-7 µg/mL) observed EPS concentrations, while yellow-orange illumination induced only ~0.5 µg/mL EPS levels, the same as in planktonic cultures, regardless the applied light regime. Growth rates and EPS production showed the opposite pattern: cultures grown under 630 nm light exhibited the highest growth rates and minimal EPS secretion, while cultures grown under 460 nm light showed the lowest growth rate and maximal EPS secretion. The chlorophyll fluorescence parameters derived from OJIP curves revealed that biofilm-associated cells had a smaller pool size of electron acceptors (S _M ), a reduced maximum photochemical efficiency (φP _O ), an increased apparent antenna size of PSII (ABS/RC), and an enhanced energy dissipation (DI _O /RC) relative to planktonic cells.