StimulatedProrocentrum donghaiensecell growth byin-situmariculture dissolved organic matter

Mixotrophic dinoflagellates frequently cause harmful algal blooms (HAB) in eutrophic mariculture waters that contain diverse excreted dissolved organic matter (DOM). The phagotrophy and the utilization of single organic small molecules have been extensively investigated in the bloom-forming mixotrophic dinoflagellates. However, their ability to utilize thein-situDOM via absorbtrophy still remains unexplored. Here we examined the growth promotion effect of thein-situmariculture DOM onProrocentrum donghaiense, a representative HAB-forming species in coastal waters. Our results showed that the cell growth and photosynthesis ofP. donghaiensewere significantly promoted underin-situDOM culture conditions. Additionally, parallel cultures were set up to disclose the potential role of the bacterioplankton in the free-living community (helper), where they aid in the remineralization of thein-situDOM, and the phycosphere community (competitor), where they compete against the algal host to acquire nutrients from thein-situDOM. Meanwhile, we determined the cellular stoichiometry under different culture conditions, showing that mariculture DOM can shape cellular stoichiometry significantly. Elevated cellular N (84.96%) and P (48.3%) were observed in spring DOM groups compared with the control groups. For the first time, this study quantifies the efficient utilization of thein-situDOM via absorbtrophy, indicating the vital role in the outbreak and maintenance of HAB events.