The oceanic cycle of cadmium is still poorly understood, despite its importance for phytoplankton growth and paleoceanographic applications. As for other elements that are biologically recycled, variations in isotopic composition may bring unique insights. This article presents i) a protocol for the measurement of cadmium isotopic composition (Cd IC) in seawater and in phytoplankton cells; ii) the first Cd IC data in seawater, from two full depth stations, in the northwest Pacific and the northwest Mediterranean Sea; iii) the first Cd IC data in phytoplankton cells, cultured in vitro. The Cd IC variation range in seawater found at these stations is not greater than 1.5 ε Cd/amu units, only slightly larger than the mean uncertainty of measurement (0.8 ε Cd/amu ). Nevertheless, systematic variations of the Cd IC and concentration in the upper 300m of the northwest Pacific suggest the occurrence of Cd isotopic fractionation by phytoplankton uptake, with a fractionation factor of 1.6±1.4 ε Cd/amu units. This result is supported by the culture experiment data suggesting that freshwater phytoplankton (Chlamydomonas reinhardtii and Chlorella sp.) preferentially take up light Cd isotopes, with a fractionation factor of 3.4±1.4 ε Cd/amu units. Systematic variations of the Cd IC and hydrographic data between 300 and 700m in the northwest Pacific have been tentatively attributed to the mixing of the mesothermal (temperature maximum) water (ε Cd/amu =-0.9±0.8) with the North Pacific Intermediate Water (ε Cd/amu =0.5±0.8). In contrast, no significant Cd IC variation is found in the northwest Mediterranean Sea. This observation was attributed to the small surface Cd depletion by phytoplankton uptake and the similar Cd IC of the different water masses found at this site.Overall, these data suggest that i) phytoplankton uptake fractionates Cd isotopic composition to a measurable degree (fractionation factors of 1.6 and 3.4 ε Cd/amu units, for the in situ and culture experiment data, respectively), ii) an open ocean profile of Cd IC shows upper water column variations consistent with preferential uptake and regeneration of light Cd isotopes, and iii) different water masses may have different Cd IC. This isotopic system could therefore provide information on phytoplankton Cd uptake and on water mass trajectories and mixing in some areas of the ocean. However, the very small Cd IC variations found in this study indicate that applications of Cd isotopic composition to reveal aspects of the present or past Cd oceanic cycle will be very challenging and may require further analytical 3 improvements. Better precision could possibly be obtained with larger seawater samples, a better chemical separation of tin and a more accurate mass bias correction through the use of the double spiking technique.4

We investigated the effect of phosphate bioavailability on cellular metal quotas in two species of freshwater phytoplankton (the eukaryote Chlorella sp. UTCC522 and the cyanobacterium Microcystis sp. LE3), grown in semicontinuous culture over four controlled levels of phosphate availability, encompassing phosphorus (P) deplete to P replete conditions. P limitation caused reduced growth rate, high C : P (up to 1800 mol mol 21 ), and increased alkaline phosphatase (APase) activity. Low P availability led to enriched cobalt (Co), cadmium (Cd), and zinc (Zn) in Chlorella (up to 2.8-fold, 1.7-fold, and 1.8-fold, respectively, normalized to cellular N, relative to P-replete control) but resulted in enriched Co and nickel (Ni) in Microcystis (up to 4.4-fold and 3.0-fold). In contrast, cellular iron (Fe), manganese (Mn) and copper (Cu) were largely unchanged (6,20%) in both organisms. Cd and Co may substitute for Zn in the APase of Chlorella while in Microcystis the dominant phosphatase may be strictly Co-requiring, as has been reported for other prokaryotes and is consistent with its evolutionary emergence before the oxygenation of the atmosphere, when Co was relatively abundant in natural waters. By extension, the absolute Co requirement of the important marine cyanobacteria Synechococus and Prochlorococcus may be related in part to widespread depletion of orthophosphate (PO 3{ 4 ) in the oligotrophic surface ocean. The enrichment of Ni in Microcystis may indicate increased activity of Ni-requiring superoxide dismutase under P limitation or, speculatively, a co-uptake of Ni and Co by a shared transport system. These results shed light on the interaction between trace metals, macronutrient availability, and phytoplankton assemblage composition, and suggest intensified biological cycling of Zn, Cd, Co, and Ni in low-P freshwater and marine systems.Phosphorus (P) is an essential nutrient that all organisms require for energy transport, construction of membranes, and storage and replication of genetic information. P has been documented to limit community production in marine systems ranging from restricted seas (Krom et al. 1991;Nausch and Wasmund 2004) to the open ocean, including oligotrophic regions of the North Atlantic and the North Pacific (Cotner et al. 1997;Karl 1997;Wu et al. 2000). In addition, P is limiting in many large lakes, as most recently demonstrated for Lake Superior (Sterner et al. 2004). Low P availability has a substantial effect on the biochemistry and physiology of phytoplankton. Severe P limitation makes phytoplankton incapable of producing nucleic acids and leads to a decrease in the rate of protein synthesis, which in turn inhibits cell division and decreases rates of light utilization and carbon fixation (Cembella et al. 1984;Falkowski and Raven 1997). While these physiological effects have been well studied, the effect of P availability on trace metal uptake in phytoplankton is poorly understood. While changes in metalloenzyme activities may result from variations in P availability, th...