Nutrient deprivation stimulates soluble extracellular polymeric substances: physiological and biochemical responses in the cyanobacteria Cyanocohniella rudolphia

The effect of macro- and micronutrient deprivation on the cyanobacterium Cyanocohniella rudolphia (BEA 0786B) over 180 days (without medium renewal) was evaluated to intensify the production of soluble extracellular polymeric substances (S-EPS). Growth, pH, cell-free medium viscosity, S-EPS content (phenol‑sulfuric acid assay), Alcian Blue staining, and biochemical/structural markers were monitored to assess EPS release and extracellular matrix dynamics. Progressive nutrient depletion induced a shift from biomass growth toward extracellular-matrix investment. Microcolony formation observed around day 45 preceded a sustained high-production phase, with pronounced matrix densification evident by approximately day 75. Apparent viscosity increased in parallel with S-EPS accumulation and showed a strong correlation with extracellular carbohydrate concentration (R 2 = 0.86), supporting its use as an operational, non-destructive proxy for S-EPS. Alcian Blue staining qualitatively confirmed the presence of acidic polysaccharides. Quantitatively, S-EPS concentrations increased from 0.1 to 0.8 g L −1 , accompanied by an increase in apparent viscosity from 24.7 to 34.7 mPa·s. Morphological and spectroscopic analyses indicated a predominantly carbohydrate-rich extracellular matrix with an increasing protein contribution over time. As an exploratory proof of concept, unprocessed S-EPS present in the spent culture medium enabled LAP-initiated photochemical formation of Au and Ag nanoparticles at 365 nm, with S-EPS acting as a stabilising and co-reducing matrix and conferring pH-responsive optical behaviour. This indicates a practical route toward integration into EPS-rich matrices to engineer stimuli-responsive coatings and biomaterials. Overall, this study demonstrates that prolonged nutrient deprivation without medium renewal is an effective strategy to intensify S-EPS accumulation in C. rudolphia and that apparent viscosity provides a practical in-process parameter for monitoring production and supporting direct valorisation of spent culture medium in saline bioprocesses. • Nutrient deprivation enriches the C. rudolphia spent medium in S-EPS. • Apparent viscosity of cell-free medium is a simple in-process proxy. • Clear harvest window at days 60–180 (viscosity plateau) • Spent medium valorized via solvent-free ultrafiltration and drying. • Exploratory: S-EPS photoforms Au/Ag NPs with pH-responsive optics.