Environmental stress drives ecosystems toward new equilibria in which deterministic selection increasingly governs bacterial community assembly. In mangrove sediments, these processes regulate biogeochemical pathways that underpin blue-carbon storage. Yet most studies evaluate environmental drivers in isolation, overlooking how interacting filters operate across seasonal, spatial, and developmental gradients. This study adopts a salinity-centred framework to examine how physicochemical constraints structure sediment bacterial communities in arid mangroves and how their effects are modulated by forest age, tidal zonation, vegetation cover, and seasonal temperature.
Surface sediments (0–5 cm) were collected in winter and fall from upper (UIZ) and lower (LIZ) intertidal zones across three forested mangrove sites in the United Arab Emirates representing a developmental gradient (JA < 10 years, UAQ > 14 years, RK > 30 years). Microhabitats included Avicennia marina rhizosphere sediments, and adjacent non-vegetated sediments. Physicochemical analyses (soil texture, organic matter, electrical conductivity, pH, total carbon, total nitrogen) were integrated with 16S rRNA gene sequencing, diversity metrics, multivariate beta-diversity analyses, and co-occurrence network modelling.
Sediment electrical conductivity (EC) emerged as the primary driver of bacterial community shifts, increasing significantly from winter to fall across all sites (p < 0.001) and showing the strongest seasonal and spatial contrasts at the oldest forest (RK). Salinity differentiation among microhabitats intensified with forest age, particularly between UIZ and LIZ zones, indicating progressive salt accumulation linked to vegetation development and tidal modification. Bacterial alpha diversity (ACE, Shannon, evenness, Simpson) declined systematically with forest age, supporting intensified environmental filtering under persistent hypersaline conditions. Beta-diversity analyses revealed increasing compositional and phylogenetic separation among sites and microhabitats, with tidal zonation and vegetation-associated sediments becoming more distinct in older forests. Seasonal effects were strongest at the youngest site, where taxonomic turnover and abundance shifts were pronounced, whereas older forests exhibited greater phylogenetic structuring and reduced within-group variability. Network analyses reinforced these patterns. With increasing forest age, bacterial co-occurrence networks became denser and less modular, indicating tighter ecological coupling and reduced community flexibility. Integration of forest development, tidal position, and vegetation buffering enhanced salinity accumulation, promoting convergence toward stabilized, salinity-filtered bacterial assemblages.
Collectively, these findings position salinity as the central axis of bacterial assembly in arid mangroves, with secondary filters modulating its local expression. Long-term forest development strengthens deterministic selection, reduces diversity, and stabilizes interaction networks, providing a mechanistic foundation for blue-carbon management and restoration under intensifying climatic stress.