Effects of monospecific banks of salt marsh vegetation on sediment bacterial communities.

The aim of this study was to understand if two species of salt marsh plants, widely distributed in European estuaries (Spartina maritima and Halimione portulacoides) differently influence the distribution, activity, and metabolic physiology of sediment bacterial communities in monospecific banks, in comparison with uncolonized sediment (control). Microbiological descriptors of abundance and activity were assessed along vertical profiles of sediments. Rates of activity of the extracellular enzymes beta-glucosidase, alpha-glucosidase, aminopeptidase, arylsulfatase, and phosphatase were generally higher in the vegetation banks in relation to control sediments where they were also less variable with depth. This is interpreted as an indirect effect related to supply of plant-derived polymeric substrates for bacterial growth. Parameters related to sediment texture (grain size, percent of fines or water content) showed significant relations with cell abundance or maximum hydrolysis rates, pointing to an indirect effect of plant colonization exerted through the modification of sediment physical properties. The profiles of utilization of sole-carbon-source (Biolog Ecoplates) showed that only the communities from the upper sediment layer of the S. maritima and the H. portulacoides banks exhibit consistent differences in terms of physiological profiles. Bacterial communities in control sediments exhibited the lowest physiological variability between surface and sub-surface communities. The results indicate that microbial colonization and organic matter decomposition are enhanced under the influence of salt marsh plants and confirm that plant coverage is a major determinant of the processes of organic matter recycling in intertidal estuarine sediments.