Mixotrophs, acting as both primary producers and consumers, play a critical role in transmitting biomass and other elements along the food chain. Despite their importance in the marine ecosystem, the unique nutrition strategy adds complexity to the investigation of their responses towards global climate changes. This study focused on the functional responses of a predominantly autotrophic dinoflagellate Lepidodinium sp. towards global warming, seawater stoichiometric imbalance, and prey supply. Lepidodinium sp. grown in mixotrophic condition showed a significantly higher thermal sensitivity than in autotrophic condition; mixotrophic nutrition enables Lepidodinium sp. to achieve a higher growth rate under rising temperature. In contrast, less thermal sensitive photosynthesis may constrai...[ Read more ]

Mixotrophs, acting as both primary producers and consumers, play a critical role in transmitting biomass and other elements along the food chain. Despite their importance in the marine ecosystem, the unique nutrition strategy adds complexity to the investigation of their responses towards global climate changes. This study focused on the functional responses of a predominantly autotrophic dinoflagellate Lepidodinium sp. towards global warming, seawater stoichiometric imbalance, and prey supply. Lepidodinium sp. grown in mixotrophic condition showed a significantly higher thermal sensitivity than in autotrophic condition; mixotrophic nutrition enables Lepidodinium sp. to achieve a higher growth rate under rising temperature. In contrast, less thermal sensitive photosynthesis may constrain the growth of Lepidodinium sp. under obligated autotrophic conditions, indicating that, under global warming, mixotrophic dinoflagellates may benefit substantially from mixotrophy when prey supply is sufficient. The C:N ratio of Lepidodinium sp. under mixotrophic and autotrophic growth increased under warming, but the C:N ratio under mixotrophic growth had a larger increase compared to the C:N ratio under autotrophic growth. The ingestion of high C:N ratio prey of Lepidodinium sp. under mixotrophic condition might account for such differences. The increased in their C:N ratios and decrease in cell size of Lepidodinium sp. made them less favourable for predators. Reduced top-down control would therefore enhance their competitivity to obligated autotrophs. Rising temperature and nutrient imbalance increased the ingestion rate of Lepidodinium sp., shifting Lepidodinium sp. towards more heterotrophic. Since acquiring limiting nutrients is the primary role of ingestion of Type II mixotrophs, the ratios and availability of macronutrients (TIN and phosphate) significantly regulated the ingestion rate of mixotrophs. Lepidodinium sp. performed pre-ingestive prey selection by selecting for more favourable prey when facing imbalanced environmental N:P ratios and low prey quality. They select prey with lower nitrogen content when inorganic nitrogen was in excess and select prey with higher nitrogen content when inorganic nitrogen was limited. As mixotrophs might become more heterotrophic and the mixed layer become more nutrient-limited in a warming ocean, the selective feeding behaviour of Lepidodinium sp. could be more pronounced. Compensatory feeding was also induced by culture medium with N:P ratios significantly deviated from their optimal growth N:P ratio. Global warming and nutrient imbalance promoted ingestion of mixotrophs, which might eventually shift their functional role from primary producers to consumers and affect the entire marine biogeochemical cycling.