RELEASE OF DISSOLVED ORGANIC NITROGEN BY MARINE DIAZOTROPHIC CYANOBACTERIA, TRICHODESMIUM SPP

Trichodesmium sp. is a filamentous, colonial cyanobacterium which cont ributes substantially to the input of nitrogen in tropical and subtrop ical oceanic waters through nitrogen fixation (N-2 fixation). We appli ed a N-15 tracer technique to assess the rate of release of dissolved organic nitrogen (DON) from this cyanobacterium and compared those rat es with rates of N-2 fixation determined for the same assemblages at t he same times of day, Rates of release of DON showed considerable vari ation within replicate experiments and were variable depending on time of day and duration of time course experiments. On average, rates of DON release were ca. 50% the rates of N-2 fixation. We also fractionat ed the DON released by using ultrafiltration and found that 60 to 80% of the total organic release was of the size class < 10,000 Da. The re lease of these organic compounds by Trichodesmium spp. is likely a sig nificant source of new nitrogen for the associated bacteria or the non -nitrogen-fixing filaments of the Trichodesmium colonies.