Differential photosynthetic and metabolic susceptibility of Microcystis aeruginosa and Raphidocelis subcapitata to environmentally relevant levels of Diuron

The sensitivity of phytoplankton growth to chemical stressors influences the composition and function of phytoplankton communities. In this study, we compared the growth, photosynthetic, and metabolic responses of the cyanobacterium Microcystis aeruginosa FACHB-469 and the green alga Raphidocelis subcapitata FACHB-271 to herbicide diuron. The 5-day growth inhibition test showed the M. aeruginosa FACHB-469 is more sensitive to diuron. Meanwhile, the photosynthetic performance, quantum yield and efficiencies, and structural indicators of the photosynthetic apparatus were influenced by environmentally relevant levels of diuron differently between the two phytoplankton. These photosynthetic alterations were more significant in the M. aeruginosa FACHB-469 than in the R. subcapitata FACHB-271 under the three levels of diuron (5, 10, and 20 µg/L). Cellular metabolism of the two phytoplankton was considerably disturbed after diuron exposure. The metabolism of amino acids, such as L-arginine, L-homoserine, and N-Acetyl-L-alanine, was significantly altered in the R. subcapitata FACHB-271, while only L-valine level was significantly altered in the M. aeruginosa FACHB-469. The levels of allantoic acid, epicatechin, and taurine were significantly downregulated in the R. subcapitata FACHB-271, while pyrimidine metabolism and γ-linolenic acid metabolism were altered in the M. aeruginosa FACHB-469. Overall, distinct responses in photosynthetic physiology and cellular metabolism are suggestive of invisible differences under environmentally relevant level of diuron, contributing to the differences in sensitivity of growth between the two phytoplankton under diuron.