Résumé
The high biodiversity observed in coastal lagoons can often be related to a very high degree of environmental heterogeneity (beta-biodiversity). In this article, I describe many physical factors that contribute to create environmental heterogeneity and different habitats within the Bassin d'Arcachon, a mesotidal lagoon in S.W. France. The microbial biodiversity has been studied in this coastal lagoon by culture-dependent and culture-independent techniques. The sediments of the meadows of the seagrass Zostera noltii have received a specific focus. During the last 15 years, several novel bacterial species have been isolated from these sediments and obtained in culture. Among these, five different species have been fully described as novel species according the Bacteriological Code and two of them represent also the type strain for a novel genus. The bacterial biodiversity has also been studied by molecular ecology techniques. An early study was focused on water column bacteria and a comparison has been made with the same community from the more eutrophic Prevost lagoon (France, Mediterranean coast). A clone library was also constructed for 16-S rRNA genes isolated and amplified from the 1-2 cm depth layer of the Z. noltii sediment. These studies showed a high degree of biodiversity and the importance of sulfate-reducing bacteria in the sediment, pointing to species that have not been obtained in culture. Collectively, these data show that there is still a large discrepancy between culture-dependent and culture-independent prokaryotic biodiversity. It is important to link the functional biogeochemical approach with the biodiversity approach. For the Bassin d'Arcachon studies, the biogeochemical approach has successfully nurtured the biodiversity studies, while the reverse has not yet been the case as the biogeochemical studies have lumped prokaryotes in functional groups.