Ocean Acidification: Effect on the Availability of Divalent Trace Metals to Phytoplankton

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The ongoing changes in seawater chemistry caused by the dissolution of anthropogenic CO2 into surface seawater will affect the growth of phytoplankton and, hence, the functioning of marine food webs. One effect of the acidification (lowering of pH) caused by the increasing CO2 is a change in the chemical speciation and bioavailability of essential trace metals. Recent work with natural seawater samples has shown an unexpected result: a decrease at low pH in the bioavailability of zinc (Zn), a metal that plays key enzymatic roles in phytoplankton. Zinc, like other essential metals, is known to be bound to organic complexing agents in surface seawater. Acidification is expected to decrease the extent of Zn complexation and thus augment its availability to phytoplankton. Such increase in Zn biovailability is indeed seen upon acidification of laboratory media in which Zn is bound to a known complexing agent.In this project, a research team at Princeton University will test whether the paradoxical decrease in Zn biovailabilty seen in acidified samples of natural seawater is explained by a shift of Zn from weak and bioavailable organic complexes to strong unavailable complexes. Proof of principle for such a "two-ligand mechanism" has been obtained in laboratory systems and is generalizable in principle to other bio-useful trace metals such as manganese, cobalt, nickel copper and cadmium. The experiments will involve parallel chemical and biological experiments with natural surface seawater samples at different pHs: (i) electrochemical measurements of Zn and Cd complexation by weak and strong ligands, and ( ii) Zn and Cd uptake experiments with model phytoplankton species and natural populations. Broader impacts: This project is directly relevant to an issue of great concern to society and will further our understanding of the possible ecological effects of global ocean acidification, an ineluctable consequence of the ongoing increase in atmospheric CO2. It will also augment our understanding of the processes that control the bioavailability of essential trace metals to marine phytoplankton. The results of the research are thus expected to generate wide interest and some will likely be published in journals with broad dissemination and high impact. They will be also incorporated into two of the courses taught by the Principal Investigator at the undergraduate and graduate levels and in a "Science day" event at a local elementary school.