| Nitrogen (N2) fixation is an important component of the marine nitrogen (N) cycle, as it provides a major source of new N to the oligotrophic ocean, which often N limited. This input could increase global N inventories, leading to the sequestration of atmospheric CO2 via the biological pump. Any changes in N2 fixation can therefore theoretically change the carbon (C) sequestration capacity of the ocean. N2 fixing organisms, like the filamentous cyanobacterium Trichodesmium spp., are inherently not N limited. One would thus expect diazotrophs to thrive in the oligotrophic oceans; this is not always the case. Something must be limiting N2 fixation, and both (P) and iron (Fe) are possible limiting nutrients. However it is not yet clear which nutrient limits N2 fixers in different locations. With this in mind, a study was undertaken to understand where P can be limiting to Trichodesmium spp. and the relationship between the P cycle and N2 fixers.;Two assays used as indices of P limitation in Trichodesmium spp. are uptake of 33PO43- to determine maximal P uptake (Vmax) and hydrolysis of P from methylumbelliferone-phosphate to estimate alkaline phosphatase activity (APA). The kinetics of PO4 3- uptake were determined for Trichodesmium spp. colonies in the North Pacific, North Atlantic and in waters north of Australia while APA was determined in the North Pacific and North Atlantic. Trichodesmium spp. Vmax was significantly greater (∼4 fold or more) in the North Atlantic compared to the North Pacific and waters north of Australia when normalized to both Chl a content and number of trichomes per colony. APA in the North Atlantic was also greater than in the North Pacific. Our results indicate that Trichodesmium spp. is more strongly P limited in the North Atlantic compared to the North Pacific or waters along the north coast of Australia. Because P stress is important in the North Atlantic and Trichodesmium spp. colonies are very large, the ability of Trichodesmium spp. to compete for and acquire P is extremely important there. Nano- and pico-plankton present in bulk water samples have a half saturation constant (Ks) for PO43- nearly 30 times lower than that of Trichodesmium spp and account for >99% of PO43- uptake in these waters. However, while chl a normalized alkaline phosphatase activity (APA) in bulk water was an order of magnitude greater than in Trichodesmium spp., Trichodesmium contributes substantially to total APA in the water. Trichodesmium is outcompeted for inorganic P, but colonies can satisfy their P needs by supplementing PO4 3- uptake with P cleaved from DOP via alkaline phosphatase. Having shown that the Trichodesmium spp. in the North Atlantic are P stressed and must exploit the DOP pool to compete with nano- and picoplankton, the question remains if the rest of the osmotroph community is P stressed in the North Atlantic, and what the spatial pattern is over the entire basin. Despite the fact that SRP is elevated in the Amazon River plume in the far western North Atlantic compared to the rest of the basin, PO4 3- turnover times, considered an indicator of P stress, increase from 100h (less stressed) in the east. This could be due to surface water enrichment in NO3- relative to PO 43- above the Redfield ratio of 16:1. Indeed, DIN:SRP ratios in subsurface waters were ∼60 in the western basin and decreased to ∼16 in the eastern basin. N enrichment without a concurrent enrichment in P may be a result of N2 fixation, and published data does indicate that Trichodesmium spp. is more abundant and active in the western basin than the eastern basin. Trichodesmium spp. Vmax of PO43- uptake was also high in the west and low in the east, indicating that, by enriching waters in N relative to P, this diazotroph is increasing the competition for P between all organisms of the osmotrophic community, including themselves. |
