Regulation of Diel Size Spectrum Variation by Dissolved Inorganic Nutrients in Starved Mixotroph Mesodinium rubrum

The obligate mixotroph Mesodinium rubrum significantly impacts coastal ecosystems, yet its population control in oligotrophic waters remains unclear. Integrating field observations from Coast of Sanya (South China Sea) with laboratory nutrient manipulation, we investigated how dissolved inorganic nutrients and prey availability regulate cell cycle progression, using biovolume as a proxy for cycle transitions. Nutrient starvation arrested cells at the small (newly divided) stage. Inorganic replenishment triggered rapid somatic growth and consistent diel biovolume oscillations, expanding in light and shrinking in darkness. However, without cryptophyte prey, cells failed to progress beyond the medium (actively growing) stage and could not accumulate into the large (pre-division) size class, revealing a decoupled regulatory mechanism. Dissolved inorganic nutrients drive cell size expansion (somatic growth), whereas prey-derived organelles serve as a critical prerequisite for division. Field data confirmed that the virtual absence of cryptophytes in Sanya waters restricts M. rubrum to consistently low levels. Our findings demonstrate that population dynamics of this specialist mixotroph transcend traditional nutrient-driven paradigms, underscoring the irreplaceable role of prey in sustaining photosynthetic metabolism and triggering population expansion in oligotrophic systems.