Relationship between porewater organic carbon content, sulphate reduction and nitrogen fixation (acetylene reduction) in the rhizosphere of Zostera noltii

Depth of profiles of nitrogen fixation, measured as acetylene reduction rate (ARR), sulphate reduction, ammonium concentration and porewater volatile fatty acids (VFA) levels were measured in Zostera noltii colonized sediments in the Bassin d'Arcachon in March 1994. ARR were measurable throughout sediment profiles. The addition of sodium molybdate, an inhibitor of sulphate reduction, caused ARR to fall by 75 per cent, indicating that sulphate-reducing bacteria were responsible for most nitrogen fixation. Peak ARR coincided with that of sulphate-reduction; a relatively constant relationship of 40 mole sulphate reduced per mole of acetylene reduced was seen throughout the profiles. This suggested that at least 17 per cent of the adenosine triphosphate yield of sulphate reduction was needed to support the measured rates nitrogen fixation. Acetate accounted for 90 per cent of the porewater VFA as measured by high performance liquid chromatography (HPLC). Comparison of HPLC measured concentrations with those obtained from an enzymic technique suggested that 10 per cent of the total porewater acetate pool was unavailable for microbial metabolism. Profiles of acetate concentrations followed those of ARR and sulphate reduction, indicating that acetate was a likely substrate. There are 52 references.