Abstract
Trichodesmium spp. is a colonial diazotrophic cyanobacterium that occurs in the oligotrophic tropics and subtropics. Because of its ability to fix atmospheric N 2 , it is likely to be growth limited by P or Fe, and it has been hypothesized that limitation differs among different ocean basins. Two assays used as indices of P limitation or stress in Trichodesmium spp. are uptake of 33 PO 4 3‐ to determine maximal P uptake ( V max ) and hydrolysis of P from methylumbelliferone phosphate to estimate alkaline phosphatase activity (APA). The kinetics of PO 4 3‐ uptake were determined for Trichodesmium spp. colonies in the North Pacific, North Atlantic, and in waters north of Australia, whereas APA was determined in the North Pacific and North Atlantic. Trichodesmium spp. V max was significantly greater (~fourfold or more) in the North Atlantic compared with the North Pacific and waters north of Australia when normalized to both chlorophyll a content and number of trichomes per colony. APA in the North Atlantic was also greater than in the North Pacific. The half‐saturation constant for PO 4 3‐ uptake ( K s ) was not significantly different among the three locations. These data indicate that Trichodesmium spp. is more strongly P limited in the North Atlantic compared with the North Pacific or waters along the north coast of Australia. We suggest that the Trichodesmium spp. communities in the North Pacific and waters north of Australia are primarily Fe rather than P stressed and that these differences reflect differing relative inputs and availability of two major controlling variables for diazotrophy, P and Fe, in these geographically divergent areas.