Responses of blue-green and green algae to streptomycin in unialgal and paired culture

Streptomycin sulfate prevented growth of six blue-green algae at concentrations (0.09 to 0.86 mg/l) substantially lower than needed to prevent growth of 7 of 8 green algae tested. Chlorella vulgaris, Scenedesmus obliquus and Ulothrix sp. grew in active streptomycin concentrations less than 21 mg/l, while Chlamydomonas reinhardtii growth was prevented at concentrations of 0.66 mg/l. Algal growth in sublethal concentrations of streptomycin was slowed or delayed, and the maximum density attained by several species was decreased. These data suggest that attempts to select ‘representative’ species for toxicity testing may have mixed success: any blue-green alga would be a suitable representative of Cyanophyta with respect to streptomycin, but no single green alga would adequately represent the wide range of response to streptomycin seen among the Chlorophyta species tested. To test for interactions between competition and chemical inhibition, paired cultures of S. obliquus with Anabaena cylindrica, Ankistrodesmus sp., C. vulgaris and Selenastrum capricornutum were observed with and without 6.6 mg/l streptomycin. The outcome of competition was consistent with predictions based on single-species assays, where major differences in chemical sensitivity were present, although significant interactions were observed. S. obliquus populations when treated with streptomycin were competitively superior to A. cylindrica and S. capricornutum. Significant interactions between competition and streptomycin treatement were also observed for paired cultures of C. vulgaris and S. obliquus; neither population fit the model of additive effects of chemical inhibition and competition.