The application of algal growth potential techniques to surfactant and zinc toxicity studies

Studies were undertaken to evaluate the suitability of algal growth potential techniques for testing the toxic effects of surfactants and zinc to an algal test organism. The algal growth potential techniques used included both static and continuous-flow algal bioassay procedures. The algal test organism selected was Selenastrum capricornutum Printz. The surfactants tested included two anionic mixtures (linear tridecyl benzene sulfonate and linear dodecyl benzene sulfonate) and a nonionic surfactant (primary alcohol ethoxylate).

The surfactant toxicity studies were conducted by feeding a surfactant-synthetic sewage mixture into a series of bench-scale activated sludge units. Both control and experimental sludge units were operated for each of two residence times. The effluents from the sludge units were bioassayed to evaluate the effects of biodegradation on the toxicity of the surfactants to S. capricornutum. The effluents were bioassayed using static and continuous-flow algal bioassay procedures. Toxic effects to the alga were determined by measuring variations in standing crop levels and maximum specific growth rates. Also, algal bioassays were conducted on selected concentrations of the intact (unbiodegraded) surfactants in algal nutrient medium.

Results of these studies indicated that the effects of biodegradation on the toxicity of surfactants to S. capricornutum were to reduce the surfactant concentrations in the effluents to sublethal levels. For all three surfactants tested, the levels in the effluents were less than or equal to 1 mg/1. Bioassays of the intact surfactants indicated thresholds of toxicity of 10 mg MBAS/1 for both anionic mixtures and 1 mg/1 for the nonionic surfactant. Although the effluents were not toxic to the alga, the bioassay procedures used successfully evaluated the algal growth potentials of the solutions tested. Also, the algal bioassays of the intact surfactants successfully determined the toxic levels of the respective compounds.

Additional studies were undertaken to determine if the toxicity of the heavy metal zinc to S. capricornutum was affected by temperature. Static algal bioassay procedures were used. The zinc concentrations tested included 0.02, 0.06, and 0.10 mg Zn/1. Incubation temperatures selected included 19, 24, and 29° C. Toxic effects again were determined by measuring standing crop and maximum specific growth rate levels.

Results indicated that the threshold of zinc toxicity to S. capricornutum was approximately 0.06 mg Zn/1. Results also indicated that the degree of algal growth inhibition by 0.06 and 0.10 mg Zn/1 was significantly greater at 29° C than at the two lower test temperatures. Thus, increased incubation temperatures apparently increased the degree of algal growth inhibition by a given zinc concentration.

Light and dark bottle studies were used to test the effects of the previously described zinc concentrations and temperatures on the photosynthesis and respiration rates of S. capricornuturn. Results showed that zinc toxicity affected photosynthetic rates but not respiration rates. No differences in photosynthetic rate inhibition by zinc at the three temperatures tested could be demonstrated. This possibly was due to the need for a highly concentrated algal inoculum in the bottles so that measurable oxygen levels would be produced in a short time period. Inoculum concentration is an important factor in toxic responses of algae.