Rapid assessment of heavy metal-induced toxicity in water using micro-algal bioassay based on photosynthetic oxygen evolution

Toxicity assessment using microalgae adopted various endpoint measurements like mortality rate, photosynthetic activity, chlorophyll content, enzymatic activity, cell density, 14C assimilation, and phosphate uptake. These algal toxicity tests usually require 3-4 days of exposure time and laborious work to measure these endpoints parameters. In the present study, we described a simple and rapid toxicity assessment procedure using photosynthetic oxygen evolution as an endpoint measurement to determine heavy metal-induced toxicity. Oxygen evolution in gaseous phase was measured over a 12 h exposure time. The toxicity of six selected heavy metals was assessed. Concentrations of 1.02, 1.91, 0.46, 3.2, 7.5, and 65.6 mg/L were obtained as EC50 values for mercury, silver, cyanide, copper, cadmium, and hexavalent chromium, respectively, for microalgae (Chlorella vulgaris). Our results demonstrated that hexavalent chromium is less sensitive depending on the conditions (low exposure time, pH 6, and high initial biomass). Toxicities in decreasing order of strength were cyanide > mercury > silver > copper > cadmium > hexavalent chromium. Results obtained in the current and earlier studies demonstrate that photosynthetic oxygen evolution offers an alternative endpoint measurement to assess toxicity since it allows sensitive and rapid detection of heavy metal-induced toxicity in water.