Sulfur bacteria in sediments of two coastal ecosystems: the Bassin d'Arcachon and the Etang du Prévost, France

Enumeration of the functional groups of sulfur bacteria was performed in the sediments in the Bassin d'Arcachon, a mesotidal lagoon with strong tidal currents and dominant populations of seagrass (Zostera noltii), and in the Etang du Prevost, a shallow lagoon with moderate tidal fluctuations and dominant populations of floating seaweed (Ulva sp.). In addition, data were collected on the distribution of oxygen and sulfide at the water-sediment interface during diel cycles. Bacterial enumeration studies revealed highest numbers in the top two cm of the sediments for three functional groups of sulfur bacteria, these being the sulfate-reducing bacteria (SRB), the colorless sulfur bacteria (CSB), and the phototrophic sulfur bacteria (PSB). In both systems high numbers of SRB were encountered, suggesting ample availability of organic matter. A comparison between different sites in each ecosystem showed that sediments overlain by more stagnant water were dominated by PSB, whereas those overlain by more oxygenated water were dominated by CSB. Important factors are the physical forces induced by tidal currents and the degree of daily exchange of water between the lagoons and the sea. These factors may explain the differences observed between the two systems with regard to the development of anoxic conditions, more so than the level of eutrophication. It appears that rooted plants play an important role in the introduction of oxygen into the sediments, thus enhancing the competitive position of CSB compared to PSB. Mini-electrodes studies revealed high concentrations of free sulfide at the inner site of the Etang du Prevost but very low concentrations at the inner station of the Bassin d'Arcachon, which may be explained by the high iron input of the latter rather than by differences in the rate of sulfide production