Significant Reduction of Photosynthesis in the Harmful Dinoflagellate Margalefidinium polykrikoides by Co-culturing with the Marine Green Alga Tetraselmis suecica

The marine dinoflagellate Margalefidinium polykrikoides is one of the notable taxa that are responsible for harmful algal blooms. During the bloom, multiple interactions between M. polykrikoides and other phytoplankton may either enhance or inhibit their growth; however, these interactions are poorly understood. Herein we investigated the interactions of M. polykrikoides and green algae Tetraselmis suecica by co-culture experiments. The growth of M. polykrikoides decreased significantly when cultured in high and low concentrations of T. suecica (TsH and TsL, respectively). Nutrient uptakes of M. polykrikoides were found to be decreased in the co-cultured samples. TsH showed a significantly higher PO₄ uptake than the other conditions. In addition, TsL, TsH, and filtered T. suecica medium (FTsM) treatments resulted in a significant decrease in chlorophyll autofluorescence (CAF) and an increase in reactive oxygen species (ROS) in M. polykrikoides. Moreover, T. suecica cell treatment significantly down-regulated the photosynthetic genes (MppsbA2 and MppsaB) in M. polykrikoides, while stress-related genes (MpHsp70, MpHsp90, and MpMCA) were up-regulated, particularly when exposed to FTsM. These suggest that the growth and photosynthetic metabolism of M. polykrikoides were suppressed by T. suecica, which may be due to reduced photosynthetic activity as a result of competition for nutrient and the shading effect.