Nitrogen Fixation in the Marine Environment

Abstract

Cyanobacteria of the genus Oscillatoria (Trichodesmium) account for annual inputs of nitrogen to the world's oceans of about 4.8 x 10¹² grams while benthic environments contribute 15 x 10¹² grams. The sum of these inputs is one-fifth of current estimates of nitrogen fixation in terrestrial environments and one-half of the present rate of industrial synthesis of ammonia. When the total of all nitrogen inputs to the sea is compared with estimated losses through denitrification, the marine nitrogen cycle approximates a steady state. Oceanic nitrogen fixation can supply less than 0.3 percent of the calculated demand of marine phytoplankton. The minor contribution by nitrogen fixation to the overall nitrogen economy of the sea is not consistent with the supposition that nitrogen is the primary limiting nutrient and suggests that factors other than nitrogen availability limit phytoplankton growth rates.