Enhanced Phototoxicity toward Chlorella Pyrenoidosa via Nanocolloid/g-C3N4 Heterostructure-Mediated Singlet Oxygen Generation

Graphite-phase carbon nitride (CN) is widely used but may leak into the environment, where its interactions with widely present nanocolloids (Ncs) are under-researched, and its ecological effects remain unclear. This study shows that CN and Ncs interact through charge exchange, with electron transfer from Ncs to CN, leading to edge group bonding. Additionally, the humic acid component in Ncs forms heterostructure with CN via interfacial adhesion (C–O bonds). Toxicity to phytoplankton (Chlorella pyrenoidosa) was assessed based on CN and Ncs concentrations in natural aquatic ecosystems. The results showed that Ncs+CN severely damaged the photosynthetic system of algal, resulting in a 16.35% inhibition of algal growth (compared to the control group). Mechanistically, (1) Ncs+CN generates more ¹O₂ (69.75 vs 57.36 μmol L^–1), resulting in more damage to the algal. (2) Compared to the control group, light-induced phototoxicity led to inhibition of chlorophyll a (chl-a) and rubisco enzymes (3.50–22.23%, 1.73–21.76%); (3) Metabolomics and transcriptomics analyses revealed disruptions in energy metabolism and carbon/nitrogen assimilation within the algal cells. Notably, genes associated with photosynthesis and carbon sequestration (such as psbR, psbB, and rbcS) showed significant downregulation, consistent with reduced photosynthetic capacity (e.g., chl-a and rubisco enzyme activity). These findings highlight Ncs’ role in modulating CN’s environmental behavior (particularly toxicity), and underscore the importance of long-term ecological experiments conducted at subeffect concentrations (EC10, EC20).