Light-Dependent Aggregate Formation and Photosynthetic Acclimation During Biofilm Development in Synechocystis sp. PCC 6803

Biofilm formation is an important adaptive strategy for cyanobacteria in ever-changing environments. In response to shifts in the light regime, the model cyanobacterium Synechocystis sp. PCC 6803 GT-L can switch from a planktonic state to aggregation and phototrophic biofilm formation. Planktonic cultures contain single cells and microcolonies, whereas developing biofilms are dominated by cell aggregates with remarkable heterogeneity. Distribution analysis revealed that aggregate areas increased under higher irradiance (250–400 µmol photons m⁻² s⁻¹) and exhibited a strong wavelength dependence, reaching maximum values under blue-greenish light (460–510 nm) and minimum values under yellow-orange wavelengths (560–590 nm). Extracellular polysaccharide (EPS) production closely followed the aggregation pattern. Biofilm cultures produced significantly higher levels of EPS than planktonic cultures, with cultivation under blue light inducing the highest EPS accumulation. Switching the cultivation light between red (630 nm) and blue (460 nm) demonstrated that EPS production and aggregate formation are reversible and actively regulated by the light regime. Growth rates and EPS production showed the opposite patterns: cultures grown under 630 nm light exhibited the highest growth rates and minimal EPS secretion, whereas cultures grown under 460 nm light showed the lowest growth rates and maximal EPS secretion. 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 enhanced energy dissipation (DI O /RC) relative to planktonic cells. In contrast, biofilm-associated cells maintained relatively high φP O and reduced ABS/RC and DI O /RC when grown under violet-green (430–540 nm) wavelengths, whereas planktonic cells showed excitation-induced stress. These results demonstrate the central role of light-regulated physiological plasticity in cyanobacterial biofilm development.