Assessment of relative phosphorus limitation of Trichodesmium spp. in the North Pacific, North Atlantic, and the north coast of Australia

Trichodesmium spp. is a colonial diazotrophic cyanobacterium that occurs in the oligotrophic tropics and subtropics. Because of its ability to fix atmospheric N2, 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 PO 4 to determine maximal P uptake (Vmax) and hydrolysis of P from methylumbelliferone phosphate to estimate alkaline phosphatase activity (APA). The kinetics of PO 4 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. Vmax 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 uptake (Ks) 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. N2 fixation is now recognized as an important source of new N in the tropical oligotrophic ocean and comparable with the diffusive flux of nitrate from below the photic zone in those systems (Capone et al. 2005). However, where the upward flux of nitrate co-occurs with carbon dioxide (CO2) at Redfield proportions, that nitrate cannot locally support a net flux of CO2 from atmosphere to ocean (Eppley and Peterson 1979; Michaels et al. 2001). Atmospheric input of N or in situ N2 fixation are additions of N without concurrent CO2 and are required to promote positive C sequestration over a nitrate-supported baseline. N2 fixation in the ocean is thus inextricably linked to the C cycle and possibly even climate change (Michaels et al. 2001). Understanding what controls N2 fixation in the open ocean is a vital part of understanding C cycling in the past, present, and future ocean. N has long been considered the primary limiting nutrient throughout the open ocean (Thomas 1966). Diazotrophs avoid this limitation by exploiting the largest N reservoir on the planet, N2 gas, through the process of N2 fixation. However, there is no analog of this process for P. This led to the conclusion that P is the ultimate limiting nutrient of marine primary production, as any N deficiencies are made up by N2 fixation (Redfield 1958). The assumption that follows is that marine N2 fixation is a P-limited process. Alternatively, Fe may constrain diazotroph growth, as diazotrophs have a higher Fe requirement than other phytoplankton (Raven 1988; Kustka et al. 2003b), and Fe concentrations are generally low in the upper ocean (Wu et al. 2001). This study focuses on the role of P in limiting open-ocean diazotrophs. The colony-forming, diazotrophic cyanobacterium Trichodesmium spp. is cosmopolitan in tropical and subtropical waters and contributes a substantial amount of new N to the areas where it occurs (Capone et al. 2005). Thus, the study of this one organism is important to our understanding of open-ocean N2 fixation. Knowledge of Trichodesmium spp. susceptibility to P limitation or stress in different ocean basins will improve our understanding of its ability to contribute new N to the open ocean. Common indices used to diagnose P stress in phytoplankton are PO 4 uptake kinetics and the activity of alkaline phosphatase—an enzyme that cleaves PO 4 from organic P molecules. Many algal species respond to P stress by altering PO 4 uptake kinetics and alkaline phosphatase activity (APA, Donald et al. 1997), and these measures can be used to infer relative P limitation. Recently, both maximal PO 4 uptake (Vmax, Fu et al. 2005) and APA (Mulholland et al. 2002) were shown to increase with P depletion in Trichodesmium IMS 101 cultures. On the basis of these culture studies, both Vmax and APA can be used to compare relative P limitation or stress in field populations of Trichodesmium spp. 1 Corresponding author ([email protected]). 2 Present address: Department of Earth and Ocean Sciences, University of Liverpool, Liverpool, United Kingdom L69 3GP.