Phago‐mixotrophic activity within nanophytoplankton community in a subtropical marginal sea

Abstract An increasing number of studies have documented the ecological importance of phago‐mixotrophy within phytoplankton communities, especially in open ocean environments. We know less about the distribution and function of such phytoplankton within marginal seas. This study was an investigation of phago‐mixotrophy among nanoeukaryotic phytoplankton along a shelf‐to‐off‐shelf transect in the South China Sea with a focus on prasinophytes (Mamiellophyceae) and haptophytes (Prymnesiophyceae). We measured group‐specific grazing rates using tyramide signal amplified fluorescent in situ hybridization and assessed community‐level inorganic nutrient (including carbon and nitrogen) uptake rates to demonstrate the heterotrophic and autotrophic growth capabilities of the phytoplankton. We also used correlation analysis, principal component analysis, generalized additive models, and structural equation modeling to evaluate the interrelationship between phago‐mixotrophic activity and key environmental parameters, including abiotic factors (e.g., temperature, salinity, nutrients) and biotic factors (e.g., bacterial abundance). Our results revealed that phago‐mixotrophic Mamiellophyceae were more abundant at lower salinities and higher temperatures. Grazing rates were positively correlated with the abundance of ambient bacteria. Grazing rates of Prymnesiophyceae on bacteria were highest at stations where nutrient concentrations were low and light intensity was high and were found to be positively correlated with phytoplankton nitrate uptake rates. These findings highlighted the dynamic interplay between autotrophic and heterotrophic nutrition of phago‐mixotrophic phytoplankton and demonstrated how environmental conditions regulate their ability to balance photosynthesis and predation on bacteria. The results provided valuable insights into the ecological roles of phago‐mixotrophy and its contributions to biogeochemical cycling in an under‐investigated subtropical marginal sea.