Seagrasses and local environment control the bacterial community structure and carbon substrate utilization in brackish sediments.

Seagrasses are complex benthic coastal ecosystems that play a crucial role in organic matter cycling and carbon sequestration. However, little is known about how seagrasses influence the structure and carbon utilization potential of benthic bacterial communities. This study examined the bacterial communities in monospecific and mixed meadows of seagrasses and compared with bulk (unvegetated) sediments from Chilika, a brackish water coastal lagoon of India. High-throughput sequencing of 16S rRNA genes revealed a vegetation effect in terms of differences in benthic bacterial community diversity, composition, and abundances in comparison with bulk sediments. Desulfobacterales, Chromatiales, Enterobacteriales, Clostridiales, Vibrionales, and Acidimicrobiales were major taxa that contributed to differences between seagrass and bulk sediments. Seagrasses supported ∼5.94 fold higher bacterial abundances than the bulk due to rich organic carbon stock in their sediments. Co-occurrence network demonstrated much stronger potential interactions and connectedness in seagrass bacterial communities compared to bulk. Chromatiales and Acidimicrobiales were identified as the top two keystone taxa in seagrass bacterial communities, whereas, Dehalococcoidales and Rhizobiales were in bulk communities. Seagrasses and local environmental factors, namely, water depth, water pH, sediment salinity, redox potential, total organic carbon, available nitrogen, sediment texture, sediment pH, and sediment core depth were the major drivers of benthic bacterial community composition. Carbon metabolic profiling revealed that heterotrophic bacteria in seagrass sediments were much more metabolically diverse and active than bulk. The utilization of carbon substrate guilds, namely, amino acids, amines, carboxylic acids, carbohydrates, polymers, and phenolic compounds was enhanced in seagrass sediments. Metabolic mapping predicted higher prevalence of sulfate-reducer and N2 fixation metabolic functions in seagrass sediments. Overall, this study showed that seagrasses control benthic bacterial community composition and diversity, enhance heterotrophic carbon substrate utilization, and play crucial roles in organic matter cycling including degradation of hydrocarbon and xenobiotics in coastal sediments.

Spatio-temporal structuring and assembly of abundant and rare bacteria in the benthic compartment of a marginally eutrophic lagoon.

The investigations on ecological processes that structure abundant and rare sub-communities are limited from the benthic compartments of tropical brackish lagoons. We examined the spatial and temporal patterns in benthic bacterial communities of a brackish lagoon; Chilika. Abundant and rare bacteria showed differences in niche specialization but exhibited similar distance-decay patterns. Abundant bacteria were mostly habitat generalists due to their broader niche breadth, environmental response thresholds, and greater functional redundancy. In contrast, rare bacteria were mostly habitat specialists due to their narrow niche breadth, lower environmental response thresholds, and functional redundancy. The spatial patterns in abundant bacteria were largely shaped by stochastic processes (88.7 %, mostly dispersal limitation). In contrast, rare bacteria were mostly structured by deterministic processes (56.4 %, mostly heterogeneous selection). These findings provided a quantitative assessment of the different forces namely spatial, environmental, and biotic that together structured bacterial communities in the benthic compartment of a marginally eutrophic lagoon.