Impact of Salinity on Cell Surface Chemistry of Cyanobacteria From Freshwater, Marine and Alkaline Environments: A Hidden Phosphorus Engine.

Climate change is altering ocean salinity, impacting cyanobacteria, key primary producers with vital ecological roles. While cyanobacterial adaptations to salinity are well studied, molecular cell surface chemistry changes remain underexplored. This study examines the impact of salinity on surface properties of freshwater Synechocystis sp. PCC6803, marine Synechococcus sp. PCC8806 and alkaliphilic Spirulina platensis. Species were cultured under salinities of 2‰, 6‰, 10‰, 30‰ and 60‰. Surface chemical composition and content were characterised using infrared and x-ray photoelectron spectroscopy and potentiometric titration. All strains exhibited salinity-dependent changes in surface charge and functional group expression, reflecting distinct adaptation strategies. In non-marine strains, salinity stress led to decreased phosphoryl signals and increased lipid saturation, consistent with phospholipid replacement and reduced membrane fluidity. We propose membrane phospholipids as a phosphorus reservoir, mobilised to support biosynthesis and ion homeostasis. In contrast, Syn. PCC8806 increased phosphoryl-associated signals and membrane fluidity across salinity conditions, consistent with phosphatidylglycerol enrichment to maintain photosynthetic function. Surface chemical shifts support a model where membrane remodelling is central to species-specific acclimatisation, balancing nutrient conservation with functional integrity. This work enhances understanding of microbial adaptation under osmotic stress and provides insight for predicting cyanobacterial blooms, designing biotechnological systems.