Nitrogen fixation in seagrass meadows: Regulation, plant–bacteria interactions and significance to primary productivity

Abstract

The rhizosphere sediments of seagrasses are generally a site of intense nitrogen fixation activity and this can provide a significant source of “new” nitrogen for the growth of the plants. In this paper, I review the data concerning nitrogen fixation in seagrass ecosystems, the transfer of the fixed nitrogen from the bacteria to the plants and its contribution to the overall productivity of seagrasses in different climatic zones. The relationship between the plants and diazotrophic heterotrophic bacteria in the rhizosphere is discussed, particularly focusing on the potentially important role of nitrogen‐fixing, sulphate‐reducing bacteria. The regulation of nitrogen fixation rates in the rhizosphere by photosynthetically driven oxygen and fixed carbon release by the plant roots and rhizomes, and the availability of ammonium in the porewater, is assessed. Finally, the hypothesis that a mutualistic or symbiotic association exists between the seagrasses and heterotrophic nitrogen fixers in the rhizosphere, based on the mutual exchange of fixed carbon and nitrogen, is discussed.