Mixotrophic dinoflagellates are well known to conduct phagotrophy to support cell growth, indicating that they are able to catabolize macromolecules. Therefore, we propose that high concentration and complex DOM released from the dense mariculture may serve as alternative nutrient sources to support rapid cell growth and lead to the outbreak of HABs. However, the effect of in-situ DOM on mixotrophic dinoflagellate still remain unclear. Here we use seawater collected from abalone mariculture area as the sole medium to grow Prorocentrum donghaiense. Besides cell growth and physiological responses, we also investigated and compared the potential contribution between in-situ free bacterioplankton and phycosphere bacteria with regards to the DOM utilization. Our results showed that, compared with the control group grown in artificial seawater, the cell density, photochemical efficiency Fv/Fm, cellular chlorophyll a content and the average growth rate of P. donghaiense were significantly promoted under in-situ DOM culture conditions. Remineralization of in-situ bacterioplankton can facilitate the DOM utilization, meanwhile there may be potential competition between phycosphere bacteria and algae cell in DOM cultures. This study illustrates that mixotrophic dinoflagellates are capable of utilizing in-situ DOM as alternative nutrient source and microbes may play a dual role under different scenarios. Our results provide further insights for addressing the important role of mariculture eutrophication in the outbreak and maintenance of HAB events.