Background
The authors write that cadmium "is a non-essential, non-beneficial element with a high toxic potential," which when taken up by plants, "even in very low concentrations ... may be toxic to sensitive organisms." In particular, they report that cadmium has been demonstrated to cause "inhibition or inactivation of many enzymes, thereby disturbing the growth, respiration, or photosynthesis in plant cells and algae (Tukendorf and Baszynski, 1991; Sanita di Toppi and Gabbrielli, 1999; Prasad et al., 2001; Faller et al., 2005)."

What was done
Tukaj et al. grew the unicellular green alga Scenedesmus armatus (strain B1-76) for periods of one, two and three days in batch cultures that contained a 93µM concentration of cadmium and were continuously bubbled with air of either 0.1% or 2% (v/v) CO2 - equivalent to approximately 1,000 and 20,000 ppm CO2, respectively - while making a number of measurements of algal properties and physiological processes.

What was learned
The four Polish scientists found that the density of the cultures grown for 3 days at 2% CO2 "was markedly higher in comparison to cultures grown at 0.1% CO2 concentration mainly due to the growth rate acceleration during the first day of culture." After 24 hours of cadmium exposure, for example, they found that "growth was inhibited to about 49% at 0.1% CO2, whereas at 2% CO2 only to about 74% of the controls." In addition, they report that "cadmium inhibited the rate of oxygen evolution (70% of control) of cells cultured at 0.1% CO2 [but] had no effect on the rate of oxygen evolution of cells cultured at 2% CO2."

What it means
The researchers say their results suggest that the protective mechanism(s) directed against cadmium was (were) "more efficient in algae cultured under elevated CO2 than those cultured under low level of CO2." In further support of this suggestion, they note that "the main detoxifying strategy of plants contaminated by heavy metals is the production of phytochelatins (PCs)," as described by Cobbett (2000); and in this regard they report that "cells grown at 2% CO2 - after 24 hours of exposure - produced much more PCs than cells cultured at 0.1% CO2." In fact, their data indicate that the CO2-induced phytochelatin enhancement of their study was in excess of ten-fold. Consequently, they conclude that "algae living in conditions of elevated CO2 are better protected against cadmium than those at ordinary CO2 level."

References
Cobbett, C.S. 2000. Phytochelatins and their roles in heavy metal detoxification. Plant Physiology 123: 825-832.

Faller, P., Kienzler, K. and Krieger-Liszkay, A. 2005. Mechanism of Cd²⁺ toxicity: Cd²⁺ inhibits photoactivation of Photosystem II by competitive binding to the essential Ca²⁺ site. Biochimica et Biophysica Acta 1706: 158-164.

Pradad, M.N.V., Malec, P., Waloszek, A., Bojko, M. and Strzalka, K. 2001. Physiological responses of Lemna trisulca L. (duckweed) to cadmium and copper bioaccumulation. Plant Science 161: 881-889.

Sanita di Toppi, L. and Gabbrielli, R. 1999. Response to cadmium in higher plants. Environmental and Experimental Botany 41: 105-130.

Tukendorf, A. and Baszynski, T. 1991. The in vivo effect of cadmium on photochemical activities in chloroplast of runner bean plants. Acta Physiologiae Plantarum 13: 51-57.