The Biological Role of Cadmium
3-D structure of a single repeat of CDCA1
Cadmium, an element which has been thought to be only toxic to organisms, behaves exactly like a nutrient in the sea: it is depleted to very low concentrations as result of biological uptake at the surface and remineralized at depth. Because of its excellent correlation with phosphate, cadmium is used as a paleotracer for nutrients. Over the past several years, we have demonstrated that cadmium is an important micronutrient for marine phytoplankton. In particular, we have discovered that the diatom Thalassiosira weissflogii possesses a Cd-carbonic anhydrase, CdCA, which is involved in the acquisition of inorganic carbon for photosynthesis. We have obtained the full DNA sequence for this enzyme and characterized its active center by X-ray spectroscopy (Lane et al. 2005). Recently, we have been able to over-express an active form of the enzyme (the first known Cd enzyme) and, in collaboration with Yigong Shi’s group in Molecular Biology, obtained its crystal structure (Xu et al. in review).
We have also shown that many diatom species possess closely homologous versions of the CdCA enzyme found in T. weissflogii (Park et al. 2007). Together with our ongoing work on the effects of CO2, Cd and Zn concentrations on the expression of CdCA, these results will serve as the foundation of upcoming field work on the presence and regulation of this enzyme in the surface ocean.
Cadmium also plays a beneficial role for the growth of the coccolithophore Emiliania huxleyi at low Zn and/or Co concentration, although the biochemical basis for this effect is still unknown. We have quantified the inter-replacement of Zn, Co and Cd in this organism and shown that such metal replacement should occur in some oceanic regions (Xu et al. 2007).
Princeton University
Department of Geosciences