Toxicity of lead and cadmium to tropical marine phytoplankton

Toxicity of Pb and Cd to three tropical, marine phytoplankton species isolated from the Andaman Sea off Phuket Thailand were determined. The phytoplankton species included one diatom, Chaetoceros calcitrans, one green alga, Chlorella sp., and one chrysophyte, Dunaliella tertiolecta. The test method was a two day mini scale (10 mL) modified International Standard (ISO) growth inhibition test with natural and artificial seawater.

Citric acid was added as a metal chelator instead of the more strongly metal complexing and photodegradable EDTA. Tests were carried out at 26-27 deg C and under continuous white fluorescent light of a 10 to 12 klux intensity, and a 48 h test duration. Concentrations resulting in 50 percent reduced growth rate (EC50) were for C. calcitrans, Chlorella sp. and D. tertiolecta, respectively: Cd in artificial seawater: 3.28, 0.74, and 25.6 mg /L, and in natural seawater: 3.02, 0.32, and 34.6 mg /L .

EC50 values for Pb in artificial seawater were 1.4, 0.12, and 5.25 mg/L d and in natural seawater 0.18, 0.4 and 6.77 mg/L. Pb was consistently more toxic to the algae than Cd, and Chlorella sp was generally most sensitive followed by C. calcitrans while D. teriolecta was the least sensitive. Toxicity levels in the natural and synthetic seawater media were similar except for Pb toxicity with C. calcitrans, which was more sensitive in natural seawater than in the synthetic medium.

The test medium contained a minimum amount of iron and chelator and it appears to have worked although the medium may not be stable in the long-term, which could have been achieved with a large chelator surplus.