Nitrogen Fixation in a Salt Marsh

Salt marshes appear to be ideal sites for the growth of free-living nitrogen-fixing microorganisms. Each zone of the marsh contains anaerobic mud in which nitrogen-fixing bacteria can develop and there are areas of bare mud ripe for colonization by blue-green algae. The lower zones of salt marshes, in common with most marine habitats, are deficient in available combined nitrogen and therefore any contribution made by nitrogenfixing organisms to the nitrogen status of the marsh is likely to be significant. A number of workers have suggested that nitrogen-fixing blue-green algae contribute to the nitrogen in salt marshes (Stewart & Pugh 1963; Wood 1965; Ranwell 1972; Chapman & Chapman 1973; Fogg 1973) but little experimental work has been carried out.