Environmental factors shaping microbial community structure in salt marsh sediments

Abstract

We examined benthic microbial communities in 3 contrasting subtidal salt marsh sedi- ments over the course of a year to investigate the relationship between environmental conditions and benthic microbial community structure. Samples were collected monthly from a high-energy sandy beach, a tidal creek bed, and a Spartina alterniflora marsh border. The concentrations and biomasses of benthic microalgae (BMA), total and potentially active bacteria (measured by an enzyme-activated fluorogenic compound), heterotrophic protists, and metazoan meiofauna were measured at each location. Sediment grain size and porewater pH explained most of the variability in biomass distrib- utions; variations in benthic biomass did not correlate well with temperature. There was a seasonal shift from a BMA-dominated community in the spring and summer months to bacteria-dominated communities in autumn at all locations, when inactive bacteria were most abundant. When normal- ized to sediment porewater volume, benthic protists concentrations were not significantly related to sediment porosity. Benthic protist porewater concentrations (mean: 3.4 × 10 3 , range: 0.1 to 9.3 × 10 3 protists ml -1 ) were comparable to protist concentrations in the water column. In contrast, bacteria were several orders of magnitude more concentrated in the sediments (mean: 4.2 × 10 9 ; range: 0.6 to 16 × 10 9 bacteria ml -1 ) than the water column. Low abundances of protists relative to bacteria appear to contribute to long bacterial turnover times, especially in fine-grained sediments. We hypothesize that both grazing by meiofauna and low anaerobic growth efficiency lead to relatively low biomass of bacterivorous protists.