Monitoring of heavy metals in seawater using single chamber foraminiferal sclerochronology

The establishment of sustainable coastal industries requires better temporal and spatial monitoring of heavy metal (HM) pollutants, even at low concentrations and during pulse-release events, before their accumulation becomes hazardous for local ecosystems or for the use of seawater in desalination (for drinking water). Foraminifera, unicellular marine organism, build their shells by sequential addition of chambers made of calcite, which contains impurities reflecting the composition of the growth medium. Therefore, the chemical composition of each chamber reflects HM in the ambient water at the time of calcification. To test the applicability of single-chamber analyses of foraminifera shells as a tool for HM monitoring in seawater, we conducted culturing experiments to calibrate the relationship of zinc and lead in foraminiferal shells and the ambient seawater. Two species of the cosmopolitan genus Amphistegina were cultured under a range of concentrations of both metals and two temperatures. We show that concentrations of zinc and lead in shells increase linearly with metal concentration in seawater with no difference between species and no effect of temperature. Our experiments further show that the concentrations in seawater can be directly inferred from concentrations in the shells at least up to the levels representing chronic pollution for both metals (68 ug/L for Zn and 8 ug/L for Pb). Combined with their high abundances in all marine environments and good preservation of their shells in sediments, our results demonstrate that foraminifera represent a powerful natural recording system offering high spatial and temporal resolution, allowing the establishment of historical baselines and detection of confined or short-term events of HM release, which could escape traditional monitoring methods.