The effects of aeration on growth and toxicity of Prymnesium parvum grown with and without algal prey

We investigated the effects of aeration on growth and toxicity of the haptophyte Prymnesium parvum in the presence and absence of the algal prey Rhodomonas salina. Batch monocultures of P-limited P. parvum and N and P sufficient R. salina and mixed cultures of the two microalgae were grown with no, low (20) and high (100) ml min⁻¹ aeration for 18 days. Cell growth of P. parvum and R. salina and cell toxicity of P. parvum were studied over the experimental period. The highest specific growth rates of P. parvum were found at low aeration rates. R. salina in monocultures showed typical growth patterns, while R. salina numbers declined rapidly in the mixed cultures. Of the initial cell densities, 98–100% of the R. salina cells were lysed or ingested within 24 h of mixing with P. parvum cells. The maxima P. parvum biomasses were significantly higher in the mixed cultures than in the monocultures. Cell toxicity of P. parvum increased significantly in response to aeration rates and the highest levels were found in the high aeration condition. Availability of prey and resupply of inorganic nutrients decreased P. parvum cell toxicity. Our study suggests that P. parvum is tolerant and is able to grow over a broad range of aeration and associated turbulence effects though low aeration represents an optimal condition for growth. As P. parvum toxicity was higher in the high aeration treatment we suggest that the higher concentrations of oxygen cause more toxins to be produced, as these are oxygen rich compounds. We suggest that oxygenation and turbulence of surface waters caused by mixing may be involved in promoting high toxic P. parvum blooms in shallow lakes and coastal waters.