Nitrogenase Activity of Heterotrophic Bacteria Associated with Roots of Eelgrass Zostera marina

Abstract

Nitrogenase activity associated with the seagrass Zostera marina (eelgrass) was examined by the acetylene-reduction assay method. Most of the rhizome-root complex samples showed significant levels of nitrogenase activity after an initial lag period of 8 to 12 hours under aerobic or anaerobic conditions. However, separated rhizomes and roots rarely showed significant nitrogenase activity. Glucose addition had little effect on the activity of rhizomes but remarkably enhanced the activity of roots, which suggests that heterotrophic nitrogen-fixing bacteria are predominately associated with the roots rather than the rhizomes and also that the rhizome-root complexes show significant nitrogenase activity only when the rhizomes contain adequate energy source the for root-associated nitrogen-fixing bacteria. Most of the root-associated nitrogen-fixing bacteria were probably distributed on the surface and not inside the roots.