Interrelationships between Rates of Microbial Production, Exopolymer Production, Microbial Biomass, and Sediment Stability in Biofilms of Intertidal Sediments

Abstract

The upper few millimeters of intertidal sediment supports a varied biomass of microbial consortia and microphytobenthos. Many of these organisms release extracellular polymers into the surrounding sediment matrix that can result in sediment cohesion and the increased stability of the sediment. The relationship between the heterotrophic and autotrophic components of these biofilms is not well understood. A combination of mesocosm and field investigations were used to investigate the relationship between microbial production rate (algae and bacteria), the extracellular carbohydrates, biomass, and stability in conjunction with a variety of environmental factors. An inverse relationship was found between rates of algal production and sediment stability both in the field and in laboratory mesocosms, though the relationship was significant only in the field (P < 0.001). Stability of sediments increased with increasing bacterial production rate (P < 0.001). Positive correlations were found between sediment stability and a range of other variables, including algal biomass (P < 0.001), colloidal-S EPS (P < 0.001), colloidal-S carbohydrate (P < 0.01), colloidal-S EDTA (P < 0.01), and sediment water content (P < 0.001). Using the data acquired, a preliminary model was developed to predict changes in sediment stability. Chlorophyll a, water content, and colloidal-S EPS were found to be the most important predictors of stability in intact cores incubated under laboratory conditions. Differences observed in patterns of the surface (0–2 mm) distribution of colloidal-S carbohydrate and chlorophyll a when expressed on a dry weight or areal basis were attributed to effects of dewatering and concomitant changes in wet bulk density. The polymeric carbohydrate (colloidal-S EPS) component of the biofilms was not found to be a constant fraction of the colloidal-S carbohydrate extract, varying from 16 to 58%, and the percentage of polymer decreased logarithmically as chlorophyll a concentrations increased and the biofilms matured (P < 0.001). Changes in the relationships between these variables over the period of biofilm development and maturation highlight the difficulties in their use to predict sediment stability. Exopolymer concentrations were more closely correlated with algal biomass than with bacterial numbers. Rates of algal carbon fixation were considerably greater than those for bacteria, suggesting that the algae have a much greater potential for exopolymer production. It is suggested that the microphytobenthos secretions make a more important contribution to sediment stability.