Nitrogen fixation along a north‐south transect in the eastern Atlantic Ocean

Abstract

Nitrogenase activity, indicative of N 2 fixation, was measured in the surface waters along a north‐south transect in the eastern Atlantic Ocean, from Texel (The Netherlands, 53°N) to Cape Town (South Africa, 35°S) using a sensitive on‐line, near real‐time acetylene reduction assay. From the beginning of January to the end of February 2000 nitrogenase activity was detected in varying rates, but only between 14°N and 13°S latitudes. Dark incubations yielded an average activity of 2.2 (± 2.4) µmol m −2 d −1 N, but light increased the activity to 3.7 (± 2.9) µmol m −2 d −1 N. However, nitrogenase activity in the light was sensitive to O 2 doubling to 7.6 (± 12.7) µmol m −2 d −1 N when the incubation was anaerobic. In the area where N 2 fixation occurred, phosphate concentrations were fourfold lower than in the area where N 2 fixation was absent, while silicate levels were higher. The water temperature in the area with N 2 fixation was 28°C, while in the adjacent area the temperature was 3°C lower, which might have prevented the proliferation of diazotrophic cyanobacteria. Action spectra revealed that chlorophyll a , phycocyanin, and phycoerythrin are the light‐harvesting pigments supporting nitrogenase activity. In one area in the northern latitudes, potential nitrogenase activity was highest during daytime, which is characteristic for Trichodesmium . In areas with a high potential nitrogenase activity, surface waters were dominated by a phycoerythrin‐containing cyanobacterium. Since nitrogenase activities were highest at night, these cells may have been unicellular cyanobacteria like Crocosphaera .