Pengbin Wang

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.

The sol-gel method was employed to prepare the pure TiO ₂ (T), Nitrogen-doped TiO ₂ (NT), and carbon-nitrogen co-doped TiO ₂ (CNT) nanoparticles (NPs). The T, NT and CNT NPs are fully characterized. With average particle sizes of 36, 38, and 40 nm for T, NT, and CNT , respectively, all samples crystallized in the anatase phase. The measured band gaps (Eg) were 3.16, 2.90, and 2.80 eV for T, NT and CNT NPs respectively, suggesting dopant-dependent morphological and surface functional alterations as well as a red shift of optical absorption into the visible region. In 2 h visible-light tests at pH 1, CNT completely (100%) degraded both Reactive Red 195 (RR195) and Reactive Blue 4 (RB4) dyes, whereas NT degradation activity reached 27.7% and 21% of RR195 and RB4 respectively, while T reached 18.7% and 16.6% of RR195 and RB4 respectively. With CNT NPs , dye elimination increased with contact time, reaching 96.07% of RB4 and 100% of RR195 after 45 min and total degradation after 1 h CNT NPs eliminated 92% of RR195 and 88.6% of RB4 in 15 min when raw wastewater was diluted. For at least five reuse cycles, CNT’s better activity which is ascribed to increased charge separation and transfer, enhanced visible light absorption, and the action of electrons and hydroxyl radicals remained stable. CNT showed intracellular ROS and showed considerable cytotoxicity against HepG-2 and HCT-116 cell lines (IC ₅₀ = 18.05 μM and 28.23 μM, respectively), indicating potential for anticancer uses and further investigation as therapeutic or antioxidant possibilities.