Résumé
Nitrogen fixation (acetylene reduction) rates were measured over an annual cycle in meadows of the seagrass Z noltii and uncolonised sediments 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 determined in isolated cores. This was probably due to the release of labile organic carbon sources during preparation of the slurries. Thus, in colonised sediments the whole core technique may provide a more accurate estimate of in situ activity. Acetylene reduction rates measured by the whole core technique in colonised sediments were 1.8 to 4-fold greater, dependent upon the season, in the light compared with those measured in the dark, indicating that organic carbon released by the plant roots during photosynthesis was an important factor regulating nitrogen fixation. In contrast acetylene reduction rates in uncolonised sediments were independent of light.
Addition of sodium molybdate, a specific inhibitor of sulphate reduction inhibited acetylene reduction activity in Z. noltii colonised sediments by >80% as measured by both slurry and whole core techniques irrespective of the light regime, throughout the year inferring that sulphate reducing bacteria (SRB) were the dominant component of the nitrogen fixing microflora. A mutualistic relationship between Z. noltii and nitrogen fixing SRB in the rhizosphere, based on the exchange of organic carbon and fixed nitrogen is proposed. In uncolonised sediments sodium molybdate initially severely inhibited acetylene reduction rates, but the level of this inhibition declined over the course of the year. These data indicate that the nitrogen fixing SRB associated with the Zostera roots and rhizomes were progressively replaced by an aerobic population of nitrogen fixers associated with the decomposition of this recalcitrant high C:N ratio organic matter.
Acetylene and sulphate reduction rates in the seagrass beds showed distinct summer maxima which correlated with a reduced availability of NH4+ in the sediment and the growth cycle of Z. noltii in the Bassin. 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 reduction 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. Nitrogen fixation in the rhizosphere of Z. noltii was calculated to contribute 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 nitrogen fixation therefore represents a substantial local input of fixed nitrogen to the sediments of this shallow coastal lagoon and contributes to the overall productivity of Z. noltii in this ecosystem.