The association of N2-fixing bacteria with sea urchins |
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Authors: | M L Guerinot D G Patriquin |
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Institution: | (1) Department of Biology, Dalhousie University, B3H 4J1 Halifax, Nova Scotia, Canada;(2) Present address: Department of Microbiology, University of Maryland, 20742 College Park, MD, USA |
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Abstract: | Dinitrogen fixation associated with bacteria in the gastrointestinal tract of sea urchins appears to be a widespread phenomenon: sea urchins from the tropics (Diadema antillarum, Echinometra lacunter, Tripneustes ventricosus), the temperature zone (Strongylocentrotus droebachiensis) and the arctic (S. droebachiensis) exhibited nitrogenase activity (C2H2 reduction). Pronounced seasonal variation was found in nitrogenase activity of temperate sea urchins feeding on kelp (Laminaria spp.) and eelgrass (Zostera marina). The mean monthly nitrogenase activity was inversely correlated with the nitrogen content of the sea urchin's food, which varied up to fivefold over the course of a year. The highest rate of nitrogenase activity recorded for a temperate sea urchin during the 14 month sampling period was 11.6 g N fixed g wet wt-1 d-1, with a yearly mean activity of 1.36 g N fixed g wet wt-1 d-1. Studies with 15N confirmed the C2H2 reduction results and showed incorporation of microbially-fixed nitrogen into S. droebachiensis demonstrating that N2 fixation can be a source of N for the sea urchin. Laboratory experiments indicated that part of the sea urchin's (S. droebachiensis) normal gastrointestinal microflora is responsible for the observed nitrogenase activity. |
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