| Summary: | Three critically important alterations of Salish Sea waters − reduced salinity, increased temperature and elevated CO2 levels, were examined in controlled laboratory studies of an impactful harmful algal bloom species on sustainable aquaculture efforts in British Columbia and Washington. Our results demonstrate the capability of the raphidophyte Heterosigma akashiwo (Y. Hada) Y. Hada ex Y. Hara et M. Chihara, to grow at varying levels of salinity, temperature and CO2-induced acidity (pH), and provide evidence of the potential ichthyotoxicity of this raphidophyte estimated from a gill cell assay approach that quantifies cytotoxicity. A non-axenic strain of H. akashiwo isolated from Puget Sound, WA was exposed to a combination of three salinity (32, 20 and 10) and five temperature (14.7, 18.4, 21.4, 24.4 and 27.8°C) conditions, as well as two pH levels (8.1 and 7.4). Laboratory findings demonstrate that cell permeability and cytotoxicity are strongly correlated in unialgal cultures of H. akashiwo, which both increase as salinity decreases from 32 to 10. Specific growth rates were found to increase with increasing temperature (14.7-24.4°C) for cultures grown at salinities of 10 (0.7-1.1 d-1), 20 (1.0-1.5 d-1) and 32 (0.7-1.2 d-1), with the fastest growth rates occurring at the salinity of 20. Furthermore, over a range of environmentally realistic lower salinities (10 and 20), neither temperature nor specific growth rate were correlated with cytotoxicity. However, the 400% increase in acidity experienced by cultures grown at salinity of 32 and pH 7.4, results in faster exponential growth rates, and 2-3 fold increases in cytotoxicity as these flagellated raphidophytes enter their N-limited stationary phase of growth. These laboratory results reveal the capacity of H. akashiwo to increase its growth potential and become more toxic not only at reduced salinities, but also in more acidic waters − environmental conditions expected in the Salish Sea due to CO2-induced ocean acidification and greenhouse warming.
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