Enumeration and relative importance of acetylene-reducing (nitrogen-fixing) bacteria in a delaware salt marsh

Three groups of N(2)-fixing bacteria were enumerated from the top 1 cm of the surface in four vegetational areas in a Delaware salt marsh. The results over the 9-month sampling period showed that there were no discernible seasonal patterns for any of the groups enumerated (Azotobacter sp., Clostridium sp., and Desulfovibrio sp.). Azotobacter sp. was present in numbers of 10 per g of dry mud, whereas the two anaerobic fixers were present in much lower numbers (10 to 10 per g of dry mud). There were no differences in the numbers of each group among the different vegetational areas, indicating that there was a heterogeneous population of N(2) fixers present. Additional studies indicate that the activity of sulfate reducers (Desulfovibrio sp.) may account for as much as 50% of the total observed acetylene reduction activity. Oxygen was found to exert little effect on the observed acetylene reduction activity, indicating that stable aerobic and anaerobic microenvironments exist in the surface layer of marsh sediments.