Background: The rise of multidrug-resistant (MDR) pathogens has intensified the search for new bioactive compounds from natural sources. Cyanobacteria, particularly the genus Fischerella, are promising producers of secondary metabolites, and environmental factors, such as salinity, can modulate their metabolic output.
Materials and Methods: This study investigated how distinct salinity stress regimes modulate growth, antibacterial activity, and metabolic output in Fischerella sp. FS 18. Cultures were assigned to four treatments: untreated control, 24-h pre-exposure to sodium chloride (NaCl) (Pre-24), 96-h pre-exposure (Pre-96), and chronic NaCl exposure for 44 days. Antibacterial activity of ethanolic, propanolic, and butanolic extracts was evaluated against Bacillus cereus and Pseudomonas aeruginosa using disk diffusion. Metabolic profiling was performed using gas chromatography-mass spectrometry (GC-MS).
Results: Control and Pre-24 supported the highest biomass (6.89±0.42 and 6.66±0.26 g/L), while prolonged or continuous salinity significantly impaired growth (Pre-96: 4.27±0.31 g/L; NaCl: 3.82±0.28 g/L). The antibacterial activity was observed only in butanolic extracts against B. cereus, with inhibition zones ranging from 10.0±0.0 mm to 13.0±0.6 mm. Both strains were resistant to beta-lactam antibiotics but susceptible to the butanolic extract. Gas chromatography-mass spectrometry (GC-MS) revealed treatment-dependent biochemical variation, with the greatest chemical richness in Pre-96 (81 compounds). Bis(2-ethylhexyl) phthalate was predominant across treatments (28.61-60.74%).
Conclusion: Salinity acts as a metabolic modulator in Fischerella sp. FS 18, generating a trade-off between biomass production and chemical diversity. Short-term salinity priming may enrich bioactive metabolite profiles with limited growth penalty. Immunological effects were not examined and remain speculative.