The Biogeochemical Cycle of Cadmium in Subantarctic Water: Studies of Carbonic Anhydrase Enzyme Activity

Abstract

Carbonic anhydrase is a zinc-containing enzyme that catalyzes the reversible reaction between carbon dioxide hydration and bicarbonate dehydration and is used for inorganic carbon acquisition by phytoplankton. In regions of the oceans where zinc is depleted, diatoms may use cadmium as the catalytic metal centre in carbonic anhydrase. Cadmium carbonic anhydrase (CdCA) readily exchanges cadmium and zinc at its active site (Xu et al., 2008) an apparently unique adaptation to oceanic life (Xu et al., 2008). Although CdCA was initially isolated as a Cd enzyme, it is actually a cabalistic enzyme, i.e. it can use either Zn or Cd for catalysis and spontaneously exchanges the two metals.

Cadmium in the open ocean has a labile nutrient-like profile (Boyle et al., 1976; Bruland et al., 1978) in which concentrations are depleted at the surface, increasing to a broad maximum at the bottom of the main thermocline and remain more or less constant at greater depths. This labile nutrient-like profile of Cd is a reflection of its likely involvement in phytoplankton uptake in the surface water followed by an increase with depth because of the decomposition of biogenic particles and subsequent remineralization. The similarity between Cd and the labile nutrients implies that most of the Cd uptake is associated with the more labile organic tissues of the phytoplankton.

However, Cd is unusually depleted in certain regions of the ocean, particularly in subantarctic waters close to New Zealand (Frew and Hunter, 1992), apparently because of enhanced seasonal biological uptake. Recent work in our group indicates that Cd levels decline significantly (almost 50-fold) during summer compared to winter (Gault-Ringold et al., 2012) as a result of iron-induced biological growth in Subantarctic waters off the Otago Shelf. This decrease coincides with the drawdown of Zn and macronutrients (NO3–, PO43–, Si(OH)4). This evidence strongly suggests that low Cd concentrations are dependent on biological uptake in this water.

In the present study both incubation and field experiments were conducted to determine the biological role of Cd on the growth of phytoplankton and its bio-uptake mechanism into the diatoms. In incubation experiments two axenic diatoms (Thalassiosira pseudonana and Nitzschia sp.) species were grown in chemically defined Zn and Cd manipulated media. The growth results of both species demonstrated a beneficial effect of Cd on the growth of phytoplankton. However CA activity of cultured experiments was also measured to aid the interpretation of the field observations of CA activity of phytoplankton.

In the field study, measurements have been made of the factors involved in seasonal Cd cycling along with nutrients, chl-a and trace metals off the southeast coast of New Zealand (Otago transect). We collected water samples along the Otago transect bimonthly over one year. Carbonic anhydrase activity was assayed using spectrophotometric and electrometric methods of all collected field samples. Measured CA activity showed strong identical seasonality with trace metals and nutrient concentrations. High CA activity was observed in the winter and low CA activity in the summer; this trend was the reverse of the trend for chl-a concentrations. Chl-a and other supporting field data strongly suggest the biological uptake of Cd in this study. However CA activity data from the field experiment indicates that the uptake of Cd by phytoplankton in the water along the Otago transect does not involve CdCA.

The cadmium uptake mechanism was studied in diatom (Thalassiosira pseudonana) cultures grown in high and low Zn treatment with added same Cd concentration. Cellular Zn and Cd concentrations were measured in diatom pellets grown in both treatments. This showed that Cd uptake was independent of Zn concentrations. Diatom extracts were fractionated using gel permeation chromatography for CA activity, Zn and Cd analysis of pooled fractions. However there was no correlation found between CA activity and Zn or Cd.

From these results presented in this thesis, we conclude that Cd has a biological effect on the growth of diatoms. Uptake of Cd is not associated with stimulating CA enzyme activity, and finally, it was confirmed that Cd is taken up independently of Zn.