Effects of temperature on a mixotrophic dinoflagellate (Lepidodinium sp.) under different nutritional strategies

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 (Ea, 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.

Selective Feeding of a Mixotrophic Dinoflagellate (Lepidodinium sp.) in Response to Experimental Warming and Inorganic Nutrient Imbalance

“…Lepidodinium sp. is a facultative phagotrophic dinoflagellate, of which photosynthesis is obligate and phagotrophy is facultative ( Liu et al, 2021 ). Different from many mixotrophic algae that consume prey when inorganic nutrients are limited, Lepidodinium sp.…”

“…Different from many mixotrophic algae that consume prey when inorganic nutrients are limited, Lepidodinium sp. can ingest prey even when nutrients are sufficient for their photosynthesis ( Liu et al, 2021 ). Our study also observed the ingestion behaviours under N Redfiled conditions in which the nutrient concentrations (~20% f/2 medium) did not limit the growth of Lepidodinium sp.…”

“…Our study also observed the ingestion behaviours under N Redfiled conditions in which the nutrient concentrations (~20% f/2 medium) did not limit the growth of Lepidodinium sp. ( Liu et al, 2021 ). This result suggests that the ingestion is advantageous to Lepidodinium sp.…”

“…It has the innate ability to photosynthesize but can ingest prey even under nutrient replete conditions. Their ingestion behaviour can be regulated according to the changes in inorganic nutrient concentrations ( Liu et al, 2021 ). Our previous study also found that Lepidodinium sp.…”

“…Our previous study also found that Lepidodinium sp. enhanced their ingestion rate and shifted towards more heterotrophy under warming conditions ( Liu et al, 2021 ). If warming drives mixotrophs to behave more like heterotrophic protists, the effect of both inorganic nutrient concentrations and prey quality on the ingestion activities of mixotrophs could change accordingly.…”