Iron availability is suggested to be a primary factor limiting nitrogen fixation in the oceans. This hypothesis is principally based on cost-benefit analyses of iron quotas in the dominant nitrogen-fixing cyanobacteria, Trichodesmium spp., in the contemporary oceans. Although previous studies with Trichodesmium have indicated that iron availability enhanced nitrogen fixation and photosynthesis, no clear relationship has been reported between cellular iron quotas and nitrogen fixation. We re-examined the proposed link between iron availability and nitrogen fixation in laboratory isolates and natural populations collected from coastal waters north of Australia. In laboratory cultures grown under iron-limiting conditions, we measured a decline in cellular iron quotas, photochemical quantum yields, the relative abundance of photosystem I to photosystem II reaction centers, and rates of nitrogen fixation. Nitrogen fixation displayed a critical threshold of the dissolved sum of total inorganic Fe species ([FeЈ]) of ca. log[FeЈ] ϭ Ϫ9.7. Field populations of Trichodesmium, collected during bloom conditions, showed high iron quotas consistent with high nitrogen fixation rates. Using seasonal maps of aeolian iron fluxes and model-derived maps of surface water total dissolved Fe, we calculated the potential of nitrogen fixation by Trichodesmium in the global ocean. Our results suggest that in 75% of the global ocean, iron availability limits nitrogen fixation by this organism. Given present trends in the hydrological cycle, we suggest that iron fluxes will be even more limiting in the coming century.Nitrogen fixation by planktonic prokaryotes is a major source of new nitrogen for the oceans (Capone et al. 1997;. In all cyanobacteria, the enzyme responsible for this process, nitrogenase, consists of two proteins, an iron 1 Corresponding author (irfrank@imcs.rutgers.edu). AcknowledgmentsWe thank J. and Y. Reinfelder for their help with the tracer experiments, iron solubility, and MINEQLϩ calculations; H. M. Geller and N. Goldman from the Department of Pharmacology, Robert Wood Johnson Medical School, for help and access to the confocal laser; P. Ludden for the antibody to nitrogenase; D. Kolber for generating the global maps; K. Wyman for invaluable lab assistance; and D. Capone, E. Carpenter, and the crew of the R/V Ewing for enabling our field sampling. We also thank J. Prospero, F. M. M. Morel, and two anonymous reviewers for their comments.

In nearly a dozen open‐ocean fertilization experiments conducted by more than 100 researchers from nearly 20 countries, adding iron at the sea surface has led to distinct increases in photosynthesis rates and biomass. These experiments confirmed the hypothesis proposed by the late John Martin [Martin, 1990] that dissolved iron concentration is a key variable that controls phytoplankton processes in ocean surface waters However, the measurement of dissolved iron concentration in seawater remains a difficult task [Bruland and Rue, 2001] with significant interlaboratory differences apparent at times. The availability of a seawater reference solution with well‐known dissolved iron (Fe) concentrations similar to open‐ocean values, which could be used for the calibration of equipment or other tasks, would greatly alleviate these problems [National Research Council (NRC), 2002[.

Significance Cadmium (Cd), a trace nutrient for marine algae, has a marine distribution that closely matches the macronutrients nitrate and phosphate. Sedimentary microfossil records of Cd provide reconstructions of past ocean nutrient distributions that facilitate understanding the role of the oceans in the carbon cycle and climate change. However, incomplete knowledge of processes that control the addition and removal of Cd in the ocean, and Cd’s variability relative to major nutrients, limit use of the paleoceanographic proxy. We present coupled data of Cd concentration and isotopic composition in seawater and suspended marine particles, indicating direct removal of Cd via coprecipitation with sulfide in oxygen-deficient waters. Thus, the marine Cd cycle may be highly sensitive to the extent of global oceanic oxygen depletion.

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There is considerable interest in the biogeochemical cycling of cadmium (Cd) and phosphate (PO 4 ) in surface waters, driven in part by the ongoing development of a paleonutrient proxy that utilizes Cd preserved in fossil planktonic foraminifera to determine past PO 4 utilization efficiencies in ocean surface waters. The present article reports the results of a field study into the effects of Fe limitation on the Cd : P composition of natural assemblages of marine phytoplankton in the Antarctic Zone of the Pacific sector of the Southern Ocean. Iron enrichment to shipboard incubation bottles led to increases in community growth rate and final biomass. After 10.7 d of incubation, the climax community was dominated by large diatoms of the genus Fragillariopsis, Pseudonitzschia, and Nitzschia. Direct measurements of phytoplankton metal : P ratios from controlled shipboard experiments indicate that Cd : P, Co : P, and Zn : P ratios decreased from control values with increasing initial dissolved Fe concentrations in the incubation bottles, by factors of ϳ2-10 at highest Fe additions. We suggest that the effect of Fe limitation on resident diatoms is to decrease growth rate, leading to elevated cellular Cd content. The dissolved Cd : P ratio in iron-limited surface waters of the Southern Ocean may, therefore, respond to the supply of Fe to the resident phytoplankton community, which has implications for the developing paleonutrient proxy. We suggest that the biological uptake of Cd and P is independent of the dissolved Cd : PO 4 ratio. As a consequence, the results argue against the use of empirical Rayleigh fractionation models or models with fixed phytoplankton uptake ratios to account for regional variability in surface water dissolved Cd : PO 4 .The distribution of cadmium in the ocean mimics that of the algal nutrient phosphate (PO 4 ) (Boyle et al. 1976;Bruland et al. 1978;de Baar et al. 1994). Like PO 4 , vertical distributions are typified by surface depletions that increase rapidly to maximum concentrations in the main thermocline

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I report two vertical profiles of dissolved cadmium (Cd) and phosphate (PO 4 ) from the Bering Sea: one from a high-nutrient, low-chlorophyll (HNLC) area, in which phytoplankton growth is limited by iron (Fe) availability, and one in highly productive waters near the continental shelf, where Fe is sufficient. At both stations, dissolved Cd and PO 4 display nutrient-like profiles and are strongly correlated with depth. The surface-water dissolved Cd : PO 4 ratio in the Fe-limited HNLC (0.21 6 0.03 nmol mmol 21 ) is significantly lower than the ratio in the productive Fe-replete station (0.31 6 0.02 nmol mmol 21 ). A simple model based on the results of previously published laboratory culture studies by others and field incubation experiments with natural phytoplankton assemblages is proposed relating the availability of Fe to the Cd : phosphorus content of phytoplankton, the dissolved Cd : PO 4 of ocean surface waters, and the slope of Cd : PO 4 in the nutricline. The model is consistent with available data and suggests that the nonlinearity or kink in the global dissolved Cd versus PO 4 relationship exists because of chronic Fe-limiting conditions in high-latitude HNLC areas in the modern ocean.