Seasonal variations in nitrogen-fixation (acetylene reduction) and sulphate-reduction rates in the rhizosphere of Zostera noltii: nitrogen fixation by sulphate-reducing bacteria

Nitrogen-fixation (acetylene reduction) rates were measured over an annual cycle in meadows of the seagrass Zostera noltii Hornem in the Bassin d'Arcachon, south-west France, between March 1994 and February 1995, using both slurry and whole-core techniques. Measured rates using the slurry technique consistently overestimated those determined on whole cores, probably due to the release of labile organic carbon sources as a result of root damage during preparation of the slurries. Thus, the whole-core technique may provide a more accurate estimate of in situ activity, since disturbance of physicochemical gradients of oxygen, sulphide, nutrients and the relationship between the plant roots and the rhizosphere microflora is minimised. Rates measured by the whole-core method were 1.8- to 4-fold greater (dependent upon season) in the light than those measured during dark incubations, indicating that organic carbon diffusing from the plant roots during photosynthesis was an important factor in regulating nitrogen fixation in the rhizosphere. Additions of sodium molybdate, a specific inhibitor of sulphate-reducing bacteria (SRB) inhibited acetylene-reduction activity by >80% as measured by both the slurry and whole-core techniques throughout the year, inferring that 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. Acetylene- and sulphate-reduction rates showed distinct summer peaks which correlated with a reduced availability of ammonium in the sediment and the annual growth cycle of Z. noltii in the basin. Overall, these data indicate that acetylene reduction (nitrogen fixation) activity in the rhizosphere of Z. noltii was regulated both by the availability of organic carbon from the plant roots and maintenance of a low NH ₄ ⁺ concentration in the vicinity of the plant roots due to efficient assimilation of NH ₄ ⁺ by Z. noltii during the growth season. Nitrogenfixation rates determined from acetylene-reduction rates measured using the whole-core technique ranged from 0.1 to 7.3 mg N m^-2d^-1, depending on season, and were calculated to contribute between 0.4 and 1.1 g N m^-2yr^-1, or 6.3 to 12% of the annual fixed nitrogen requirement of Z. noltii.