Effect of algal-algal interactions in toxicity testing bioassays -

Abstract

This study examines single and multispecies algal bioassays using copper toxicity. Three green (S. subspicatus, S. quadricauda and A. angustus) and one blue-green algae species (O.prolifera) were selected based on their morphology and availability. Single and multispecies toxicity bioassays were conducted based on cell density as per the standard toxicity testing, and on equivalent surface area to avoid the confounding effect on toxicity of metal binding sites. Cells were counted and differentiated using investigative microscopic imaging.Toxicity tests of single species based on cell density showed a higher copper sulfate toxicity on the blue-green algae O.prolifera (96h-EC50=126 µg-L), followed by S. subspicatus (96h-EC50=198 µg-L), S. quadricauda (96h-EC50=359 µg-L), and A. angustus (96h-EC50=1809 µg-L), while toxicity tests based on surface area showed increased copper toxicity with increasing algal surface area. Algae cell surface area of 47, 108, 742, 2348 µm2 resulted in respective 96h-EC50 values of 1123, 692, 190, and 126 µg-L as CuSO4. In multispecies control bioassays, the growth of A. angustus was significantly (p-value0.05) inhibited in the presence of S. subspicatus, S. quadricauda and O.prolifera in both cell density and surface area based tests. This was in part associated to nutrients limitation, including CO2, as demonstrated by an increase in the culture pH. Toxic exudates released by O. prolifera could have also contributed to the growth inhibition of A. angustus. As compared to single species bioassays, O. prolifera, and S. quadricauda showed a decreased sensitivity to copper sulfate in both cell density and surface area based multispecies tests. However, for the algae specie with the smallest surface area, S. subspicatus, different toxicity trends were observed whereby the algae EC50 value decreased from 1123 µg-L in single species to 461.6 µg-L in multispecies bioassays based on surface area, and increased from 198 µg-L in single speci