MEPS 678:37-49 (2021)  -  DOI: https://doi.org/10.3354/meps13865

ABSTRACT: Mixotrophs are widely distributed in aquatic ecosystems and play critical roles in the planktonic food web. However, how mixotrophs respond to projected ocean warming remains a debatable topic. To close the knowledge gap, we investigated the thermal responses of growth rate and functional traits of a mixotrophic dinoflagellate (Lepidodinium sp.) isolated from subtropical coastal waters. We found that Lepidodinium sp. is a facultative mixotroph with an obligate phototrophic lifestyle that adjusts its phagocytotic feeding according to inorganic nutrient concentrations. The thermal sensitivity in terms of activation energy (E_a, eV) of Lepidodinium sp. grown in mixotrophic mode (with sufficient prey, 0.69-0.89 eV) was significantly higher than in autotrophic mode (without prey, 0.30-0.37 eV). This finding is consistent with the results of predominantly heterotrophic mixotrophs, providing experimental evidence for the hypothesis that mixotrophs shift towards more heterotrophy with rising temperatures. Warming stimulated a higher growth rate of Lepidodinium sp. grown in mixotrophic conditions than in autotrophic conditions, indicating that mixotrophic dinoflagellates may benefit substantially from mixotrophy when temperature increases and prey is sufficient. Moreover, the cell size of both autotrophic and mixotrophic Lepidodinium sp. decreased with increasing temperature. The N:P and C:P ratios of Lepidodinium sp. did not vary with temperature, while the C:N ratio slightly increased. Our results suggest that mixotrophs like Lepidodinium sp. would become more heterotrophic with higher growth rates in warming oceans. The subsequent changes in their functional role from primary producers to consumers may affect food web dynamics and carbon and nutrient cycling.

KEY WORDS: Mixotrophs · Nutrition · Temperature · Stoichiometric ratio