Bacterial interactions in the rhizosphere of seagrass communities in shallow coastal lagoons

Rooted phanerogam communities in the shallow intertidal and subtidal coasta l zone represent productive and healthy ecosystems. Inorganic nutrients are assimilated into seagrass biomass. Much of the organic matter resulting fr om moribund seagrass is rapidly mineralized, principally by bacteria. The m icrobial community of the rhizosphere is also highly active due to the supp ly of organic matter released during photosynthesis. This active sediment c ommunity plays an important role through carbon, nitrogen and phosphorous c ycling in maintaining the stability and productivity of seagrass meadows. O ver the last two decades, however, seagrass meadows in European coastal are as have declined due to increasing pollution. As eutrophication advances a trasition occurs from rooted phanerogram dominated communities to planktoni c algal blooms and/or cyanobacterial blooms. Such changes represent the dec line of a stable, high biodiversity habitat to an unstable one dominated by a few species. These changes of community structure can occur rapidly once the internal nutrient and organic matter control cycles are exceeded. A fi eld investigation was undertaken to establish the spatial distribution of b acterial populations of Zostera noltii colonized and uncolonized sediment i n the Bassin d'Arcachon, France. Bacteria were enumerated using both plate count and MPN techniques for different functional groups as well as determi ning the total bacterial populations present. Nitrogen fixation, ammonifica tion, sulphate reduction rates, as well as alkaline phosphatase activity we re also determined. Colonization of the Z. noltii roots and rhizomes was st udied by light and scanning electron microscopy. Results confirmed that hig her bacterial populations were present in the rhizosphere of Z. noltii comp ared to uncolonized sediments. Furthermore, electron microscopy identified the rhizome as the main site of colonization for a diverse range of morphol ogical groups of bacteria. Sulphate reducing bacteria were identified as th e key group of bacteria involved in N-fixation in the rhizosphere of Z. nol tii. The data will be discussed in relation to the role played by the rhizo sphere microflora in supplying and mobilising nutrients in Z. noltii.