SULFUR BACTERIA IN SEDIMENTS OF 2 COASTAL ECOSYSTEMS - THE BASSIN-DARCACHON AND THE ETANG-DU-PREVOST, FRANCE

Enumeration of the functional groups of sulfur bacteria was performed in the sediments in the Bassin d'Arcachon, a mesotidal lagoon with str ong tidal currents and dominant populations of seagrass (Zostera nolti i), 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 sul fide at the water-sediment interface during diel cycles. Bacterial enu meration studies revealed highest numbers in the top two cm of the sed iments 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 numb ers 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 a nd the degree of daily exchange of water between the lagoons and the s ea. 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 a n important role in the introduction of oxygen into the sediments, thu s enhancing the competitive position of CSB compared to PSB. Mini-elec trodes studies revealed high concentrations of free sulfide at the inn er site of the Etang du Prevost but very low concentrations at the inn er station of the Bassin d'Arcachon, which may be explained by the hig h iron input of the latter rather than by differences in the rate of s ulfide production.