Microbial Communities in Coastal Sediments - Chapter 3: Environmental variables and factors regulating microbial structure and functions

Coastal sediments are chemically complex with steep gradients of redox potential, pH, and substrate availability that contribute to the formation of large number of microhabitats. The structural heterogeneity of sediments allows resource partitioning, thus creating new niches and enhancing diversification into distinct ecological species. Spatial changes in microbial community structure and abundance also depend on dispersal. Temporal scales are important for microbial communities because with short-generation times, microbes show rapid growth. In coastal sediments there is an increasing substrate limitation of the active bacteria with depth, because dissolved organic carbon, including dissolved carbohydrates, becomes increasingly unavailable to the microorganisms with increasing depth. This results in vertical depth gradients of organic compounds that serve as energy sources for microorganisms. Microbial associations are the result of a coevolution process that leads to adaptation and specialization, allowing the microbes to occupy different niches. This helps them to overcome various biotic and abiotic stresses in the environment as well as exchange of growth factors, genetic information, and molecular signaling. Coevolution of different species has resulted in a variety of relationships such as mutualistic, competitive, antagonistic, pathogenic, and parasitic relationships. Highest microbial activity and metabolism take place in regions where the sediment is well mixed due to hydrological influence. Among the various physicochemical factors that influence microbial distribution and availability, the predominant factors are pH, salinity, redox potential, organic carbon, and nutrients. Biological factors that influence microbial structure and activity in coastal sediments include the various interactions between microorganisms involving trophic interactions such as competition, predation, and parasitism; between microbes and plants as well as the influence of benthic animals in the sediments.