Dissolved Organic Carbon Regulates Bacterial Ingestion by Tetraselmis sp.

Tetraselmis sp. is a widespread marine green alga capable of both dissolved organic carbon (DOC) and bacterial ingestion, making it a potentially important contributor to carbon fluxes in microbial food webs. Mixotrophy can enhance survival under nutrient or light limitation, yet how DOC availability influences phagotrophic activity in marine green algae remains poorly understood. Here, we experimentally examined the interaction between osmotrophy and phagotrophy in Tetraselmis sp. under low-light and dark conditions across different dissolved organic carbon levels. Bacterial ingestion rates were quantified using live fluorescently labeled bacteria (FLB), enabling direct assessment of feeding responses. Carbon utilization assays showed that Tetraselmis sp. can assimilate a broad range of organic substrates, with glucose preferentially utilized under low-light conditions. Increasing glucose concentrations (0.1, 1, and 10 g L⁻¹) significantly suppressed bacterial ingestion, with ingestion rates declining from 2.34 to 0.17 bacteria cell⁻¹ h⁻¹ under low-light conditions and from 1.48 to 0.16 bacteria cell⁻¹ h⁻¹ in darkness. Together, these results demonstrate that DOC availability directly regulates phagocytosis, suggesting a trade-off between osmotrophic uptake and phagotrophic feeding. Our findings refine current frameworks of algal mixotrophy by highlighting the interaction among multiple trophic strategies and underscore the trophic plasticity of marine green algae in shaping carbon transfer within microbial food webs.