Differential effects of phosphorus limitation on cellular metals in Chlorella and Microcystis

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 mol21), 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, there are at present very few published studies of the effects of P bioavailability on intracellular metal concentrations in phytoplankton. Such effects are central to quantifying changes in overall metalcycling processes that may occur under macronutrient depletion in freshwater and marine systems. Many phytoplankton have evolved mechanisms for acquiring P from dissolved organic matter when the supply of inorganic P (PO 3{ 4 ) is limited, including the globally important marine diazotroph Trichodesmium (Dyhrman et al. 2006). Under conditions of potentially limited concentrations of PO 3{ 4 , a subset of dissolved organic phosphorus (DOP) compounds can serve as a reliable growth substrate (Karl and Bjorkman 2002). Many phytoplankton (and bacteria) can produce alkaline phosphatase (APase), a group of enzymes (monoand di-esterases), that can catalyze the hydrolysis of a broad spectrum of DOP compounds, thereby affording the organism access to the DOP pool, which typically has much higher concentrations 1 Corresponding author. Acknowledgments We appreciate the help of P. Falkowski and J. Ammerman, who provided their lab facilities for phytoplankton incubation monitoring and determination of alkaline phosphatase activity. P. Field and C. Fuller provided generous help in trace metal, organic carbon, and nitrogen analyses. Y. Rosenthal, P. Falkowski, J. Reinfelder, J. Ammerman, and J. Moffett provided very helpful comments on the manuscript. We thank two anonymous reviewers whose comments substantially improved this manuscript. Funding for this project was provided by awards to R. M. Sherrell from the National Science Foundation (OCE-9902660, OCE-0137592, OCE-0352208) and New Jersey Sea Grant (R/ES2002-2). Limnol. Oceanogr., 53(5), 2008, 1790–1804 E 2008, by the American Society of Limnology and Oceanography, Inc.