Regulatory toxicity testing with algae commonly use 72- or 96-h inhibition of growth by the green algae Selenastrum, Chlorella, or Scenedesmus. The long incubation period is sufficient for the algae to change the composition of the test medium. Organism-induced changes include the release of organics and the uptake of nutrients with accompanying pH changes. Both the released organics and the variable pH can change the chemical speciation of the toxicants; this may alter their toxicity. In addition, the competitive interactions between hydrogen ions and toxicants can affect toxicity as the pH varies. It is shown here that cadmium (Cd) toxicity to Selenastrum capricornutum increases by up to eight times for every unit of pH increase. Copper (Cu) with its different speciation pattern does not vary as much as Cd, and the pattern is different. Because the toxicity of the two metals vary differently with pH, Cd is 500 times less toxic than Cu at pH 6, but Cd is twice as toxic as Cu at pH 10.
The present regulatory protocols call for nutrient-sufficient conditions, but algae are likely to be limited by either nitrogen (N) or phosphorus (P) in nature. Here, Selenastrum has been grown in chemostats with both N and P approaching limiting levels. Inhibition of N and P uptake, rather than growth, has been used to assess toxicity in short-term experiments. The advantage of chemostat, rather than batch cultivation (as used in present regulatory tests), is the ability to produce algae of known and well-defined physiological states.