| The majority of organisms can be grouped into those relying solely on photosynthesis (phototrophy) or those relying solely on the assimilation of organic substance (heterotrophy) to meet their requirements for energy and carbon. However, a special life history trait exists in which organisms combine both phototrophy and heterotrophy. Such "Mixotrophy" is a widespread phenomenon in aquatic habitats and is observed in many protozoan and metazoan organisms. The mixotrophic characteristics of three Harmful Algal Bloom species such as Heterosigma akashiwo, Alexandrium sp. and Pseudo-nitzschia sp. were studied. And the mixotrophic capability of the three HAB species in nutrient-limited and nutrient-replete cultures were also investigated, using the Nannochloropsis oculata deris and organic medium as prey. The results showed that Heterosigma akashiwo and Alexandrium sp. can use Nannochloropsis oculata deris and organic medium to meet their requirements. There were significantly higher biomass under nutrient-limited and even though in the nutrient-replete culture than the control. It shows Heterosigma akashiwo and Alexandrium sp. were mixotrophic species. One phenomenon under mixotrophy condition is that the growth cycle is about 15-20d, longer than that under phototrophic condition, and the logarithmic phase is prolonged. Pseudo-nitzschia sp. almost can't survive in nutrient-limited culture, it depends on mineral nutrients. Besides, the effects of irradiance on mixotrophic capability of Alexandrium sp. were also studied. 7 irradiance such as 30, 60, 90, 110, 140, 170 and 210μ mol·m-2·s-1 were tested, and the results showed that the irradiance of 90μ. mol·m-2·s-1 is better for the growth of Alexandrium sp. It suggested that lower irradiance can stimulate the feeding of Alexandrium sp. on Nannochloropsis oculata deris. It's in accordance with the previously study of others.
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