Heterotrophic nitrogen fixation in shallow estuarine sediments

Abstract

Nitrogen fixation, as measured by the acetylene reduction method, has been detected in situ in stratified sediment cores from Kingoodie Bay, Tay Estuary, northeast Scotland. Maximum rates of fixation (1.84 ng N/g dry wt sediment/h) in untreated samples were observed in July, 1974 under anaerobic conditions and these could be increased to 3.82 ng N/g/h upon addition of 1%, w/v, glucose. Much lower rates (maximum of 1.07 ng N/g/h) were observed under aerobic conditions. Heterotrophic nitrogen fixing bacteria, identified as members of the genera Azotobacter, Klebsiella, Enterobacter, Desulfovibrio, and Clostridium have been isolated from these sediments. Most probable number counts show that, with the exception of Clostridium sp., these organisms occur in considerable numbers in the sediments (total heterotrophic nitrogen-fixing counts range from 4.23–8.97 × 10 3 organisms/g dry wt sediment). Salinity markedly affects the nitrogen fixing capacity of these bacteria. None of 30 strains of Azotobacter and 9 strains of Klebsiella and Enterobacter examined fix nitrogen in the presence of salt or sea water. In contrast the 12 Desulfovibrio isolates have an obligate salt require- ment when grown in N-free medium whilst 5 Clostridium sp. studied appear to be unaffected by salt concentration. Since Desulfovibrio species are the dominant component (44–50%) of the heterotrophic nitrogen fixing microflora and require salt for fixation it is possible that these bacteria play an important rôle in supplying nitrogen to the nitrogen deficient Kingoodie Bay sediments.