Specialist-generalist trade-offs structure sediment microbial community assembly and interactions along an extreme salinity gradient

As climate change intensifies salinisation in many coastal wetlands, microbial responses will govern future ecosystem health. Microorganisms drive essential ecosystem functions, such as nutrient cycling and energy flow. Coastal lagoons often exhibit large salinity gradients, and hypersaline (> 40 g/L salt) conditions are now proliferating worldwide. As such, coastal lagoons offer ideal settings to study the effects of broad salinity gradients on microbial communities.This study examines how salinity shapes sedimentary microbial communities along an estuarine-hypersaline gradient in the Coorong Lagoon, South Australia. Communities were analysed based on habitat specialisation, revealing that hypersalinity promotes deterministic assembly, favouring specialists among both Archaea (Thermoplasmata, Halobacterota) and Bacteria (Balaneolaceae, Halanaerobium, Desulfopila, Desulfotignum). Estuarine specialists included Anderseniella and Sulfitobacter, with generalists less affected by salinity changes. Microbial community networks showed increased community complexity at high (100-150 g/L) and low (0-40 g/L) salinity extremes, with intermediate salinity exhibiting less complexity, suggesting community reorganisation under osmotic stress.These results highlight the importance of salinity in determining community structure and the varied roles of specialists and generalists in supporting ecosystem function. This provides new insight into how ecosystems may respond to climate-driven salinisation, crucial for making predictions and informing mitigation strategies.