Nitrogen fixation and uptake of combined nitrogenous nutrients by Oscillatoria (Trichodesmium) thiebautii in the western Sargasso Sea 1

In the western Sargasso Sea in early autumn the standing crop of Oscillatoria thiebautii at nine stations averaged 640 × 10 5 cells m −2 with a maximum population of 11.8 × 10 5 cells m −3 at 25 m. Population density at 175 m averaged only 6% of that at 25 m. N 2 fixation was greatest at 1‐m depth, 0.069 pg cell −1 h −1 , and at 50 m decreased to 9% of that at 1 m. Highest recorded N 2 fixation, 0.20 pg cell −1 h −1 , was at the southernmost station. Average N 2 fixed per square meter of sea surface in the upper 50 m was 2.0 µ g N m −2 h −1 . When compared with other N inputs to the surface waters of the western Sargasso Sea, N 2 fixation by Oscillatoria seems negligible. Results of 15 N‐labeled NH 4 + , NO 3 − , and urea uptake experiments indicate that O. thiebautii is barely able to utilize these nutrients at the concentrations present in the euphotic zone. Doubling times ( T d ) for cellular nitrogen by N 2 fixation and NH 4 + uptake together are in the order of 40 to 110 days. An uptake curve for NH 4 + yielded a K. of 6.7 µ g‐atoms N liter −1 , a value about an order of magnitude greater than that found for other open‐ocean phytoplankters. We were unable to isolate any N 2 ‐fixing bacteria from O. thiebautii.