Effect of trace metal availability on coccolithophorid calcification

The deposition of atmospheric dust into the ocean has varied considerably over geological time. Because some of the trace metals contained in dust are essential plant nutrients which can limit phytoplankton growth in parts of the ocean, it has been suggested that variations in dust supply to the surface ocean might influence primary production. Whereas the role of trace metal availability in photosynthetic carbon fixation has received considerable attention, its effect on biogenic calcification is virtually unknown. The production of both particulate organic carbon and calcium carbonate (CaCO₃) drives the ocean's biological carbon pump. The ratio of particulate organic carbon to CaCO₃ export, the so-called rain ratio, is one of the factors determining CO₂ sequestration in the deep ocean. Here we investigate the influence of the essential trace metals iron and zinc on the prominent CaCO₃-producing microalga Emiliania huxleyi. We show that whereas at low iron concentrations growth and calcification are equally reduced, low zinc concentrations result in a de-coupling of the two processes. Despite the reduced growth rate of zinc-limited cells, CaCO₃ production rates per cell remain unaffected, thus leading to highly calcified cells. These results suggest that changes in dust deposition can affect biogenic calcification in oceanic regions characterized by trace metal limitation, with possible consequences for CO₂ partitioning between the atmosphere and the ocean.