Extreme Salinity Change Governs Microbial Community Assembly and Interactions

Coastal wetlands are highly vulnerable to climate-driven salinisation, which reshapes critical microbial processes underpinning nutrient cycling and energy flow. We examined how sediment microbial communities vary with salinity across the Coorong Lagoon (South Australia), spanning estuarine (0-40 g L^-1), intermediate (40-100 g L^-1) and hypersaline (100-150 g L^-1) waters. Salinity was found to be the dominant driver of sediment microbial community composition, diversity and assembly. High salinity favoured specialists and homogenous community structures, with generalist bacteria persisting across intermediate salinities and supporting ecosystem resilience. Sulfur and carbon cycling is likely dependent on salinity, as bacterial sulfur-oxidisers were abundant estuarine specialists, whereas methane producers (Archaeal methanogens) and sulfate-reducers were enriched at high salinity. Deterministic microbial community assembly (homogeneous selection) was dominant, increasing at extreme salinity, which acted as a strong environmental filter. Community complexity increased at both high and low salinity ranges, with intermediate salinity exhibiting less complexity, suggesting community reorganisation under osmotic stress. The varied roles of specialists and generalists at different salinities support ecosystem function, where increased heterogeneity and specialisation in hypersaline conditions suggest vulnerability of the community to disturbance. These findings provide insight into how microbially underpinned ecosystems may respond to future climate-driven salinisation, important for making predictions and informing mitigation strategies.