Nitrogen fixation (acetylene reduction) in stony corals: evidence for coral-bacteria interactions

Nitrogen fixation, as measured by acetylene reduct~on, has been detected to be associated with various hermatypic corals. Experiments were carried out on the massive coral Favia f a v u s both in situ and in the laboratory. Nitrogen fixation activity was found to be light dependent and fully inhibited by 5 X 10-6 M DCMU [3-(3,4-dichloropheny1)-1,l-dimethylurea Addition of glucose restored nitrogen fixation activity both in the dark and in the presence of DCMU. Removal of the coral tissue prevented acetylene reduction, while addition of glucose to the coral skeleton restored this activity. Bacteria isolated from the coral skeleton were found by dot blotting to contain the nif H gene. These results suggest that nitrogen-fixing bacteria found in the skeleton of corals benefit from organic carbon excreted by the coral tissue. The interaction between the nitrogen-fixing organisms and the coral may be of major lrnportance for the nitrogen budget of the corals.