Bioavailability of zinc in marine systems through time

Abstract

Zinc is a marine nutrient that may have been limited in the early oceans. Estimates of marine zinc availability through time suggest that values were instead near-modern during the Proterozoic eon. The redox state of the oceans strongly influences the concentration of dissolved trace metals in sea water. Changes in the redox state of the oceans are thought to have limited the availability of some trace metals in the past, particularly during the Proterozoic eon, 2,500 to 542 million years ago1,2,3,4. Of these trace metals, zinc (Zn) is of particular importance to eukaryotic organisms, because it is essential for a wide range of basic cellular functions. It has been suggested5 that during the Proterozoic, marine environments were broadly euxinic—that is, anoxic and sulphidic—which would have resulted in low Zn availability. Low Zn bioavailability could therefore be responsible for an observed delay in eukaryote diversification2. Here we present a compilation of Zn abundance data from black shales deposited under euxinic conditions from the Precambrian time to the present. We show that these values track first-order trends in seawater Zn availability. Contrary to previous estimates6, we find that Zn concentrations during the Proterozoic were similar to modern concentrations, supporting recent studies7,8 that call for limited euxinia at this time. Instead, we propose that predominantly anoxic and iron-rich deep oceans, combined with large hydrothermal fluxes of Zn, maintained high levels of dissolved Zn throughout the oceans. We thus suggest that the protracted diversification of eukaryotic Zn-binding proteins was not a result of Znbiolimitation.