SEASONAL-VARIATION IN RATES OF HETEROTROPHIC NITROGEN-FIXATION (ACETYLENE-REDUCTION) IN ZOSTERA-NOLTII MEADOWS AND UNCOLONIZED SEDIMENTS OFTHE BASSIN DARCACHON, SOUTH-WEST FRANCE

Nitrogen fixation (acetylene reduction) rates were measured over an an nual cycle in meadows of the seagrass Z. noltii and uncolonised sedime nts of the Bassin d'Arcachon, south-west France, using both slurry and whole core techniques. Measured rates using the slurry technique in Z . noltii colonised sediments were consistently higher than those deter mined in isolated cores. This was probably due to the release of labil e organic carbon sources during preparation of the slurries. Thus, in colonised sediments the whole core technique may provide a more accura te estimate of in situ activity. Acetylene reduction rates measured by the whole core technique in colonised sediments were 1.8 to 4-fold gr eater, dependent upon the season, in the light compared with those mea sured in the dark, indicating that organic carbon released by the plan t roots during photosynthesis was an important factor regulating nitro gen fixation. In contrast acetylene reduction rates in uncolonised sed iments were independent of light. Addition of sodium molybdate, a spec ific inhibitor of sulphate reduction inhibited acetylene reduction act ivity in Z. noltii colonised sediments by >80% as measured by both slu rry and whole core techniques irrespective of the light regime, throug hout the year inferring that sulphate reducing bacteria (SRB) were the dominant component of the nitrogen fixing microflora. A mutualistic r elationship between Z. noltii and nitrogen fixing SRB in the rhizosphe re, based on the exchange of organic carbon and fixed nitrogen is prop osed. In uncolonised sediments sodium molybdate initially severely inh ibited acetylene reduction rates, but the fever of this inhibition dec lined over the course of the year. These data indicate that the nitrog en fixing SRB associated with the Zostera roots and rhizomes were prog ressively replaced by an aerobic population of nitrogen fixers associa ted with the decomposition of this recalcitrant high C:N ratio organic matter. Acetylene and sulphate reduction rates in the seagrass beds s howed distinct summer maxima which correlated with a reduced availabil ity of NH4+ in the sediment and the growth cycle of Z. noltii in the B assin. Overall, these data indicate that acetylene reduction (nitrogen fixation) activity in the rhizosphere of Z. noltii was regulated both by release of organic carbon from the plant roots and maintenance of low ammonium concentrations in the root zone due to efficient ammonium assimilation. Nitrogen fixation rates determined from acetylene reduc tion rates measured by the whole core technique ranged from 0.1 to 7.3 mg N m(-2) d(-1) in the Z. noltii beds and between 0.02 and 3.7 mg N m(-2) d(-1) in uncolonised sediments, dependent upon the season. Nitro gen fixation in the rhizosphere of Z. noltii was calculated to contrib ute between 0.4 and 1.1 g N m(-2) y(-1) or between 6.3 and 12% of the annual fixed nitrogen requirement of the plants. Heterotrophic nitroge n fixation therefore represents a substantial local input of fixed nit rogen to the sediments of this shallow coastal lagoon and contributes to the overall productivity of Z. noltii in this ecosystem.