Single and combined exposure to micro(nano)plastics and azithromycin disturbing the photosynthetic carbon fixation of Synechocystis sp.

Abstract

Revealing the ecotoxicity of micro(nano)plastics (MNPs) and azithromycin (AZM) is of great importance regarding their wide occurrence in aquatic environments. Herein, we investigated the toxic effects of MNPs and AZM, alone and in combination, on a cyanobacterium Synechocystis sp. by monitoring physiological and proteomic responses, especially changes of photosynthetic carbon fixation. Although the inhibitory effects of MNPs on cell viability was slight and particle size independent, the antagonistic effects between MNPs and AZM decreased with increasing plastic particle size. Microplastics (MPs) inhibited the electron transport and photosynthetic efficiency, and induced inactivation of carbon-fixing enzymes. Proteomic analysis revealed that MPs disturbed the photosynthetic carbon fixation by inhibiting CO₂ uptake, CO₂ fixation, and photosynthetic reactions. Compared to MPs, nanoplastics (NPs) interfered less with photosynthetic carbon fixation at both cellular and molecular levels, but they caused more severe membrane destruction. Unlike MNPs, AZM exerted minimal effects on the photosynthetic carbon fixation. The inhibition of photosynthetic carbon fixation decreased upon co-exposure to AZM and NPs, but increased upon co-exposure to AZM and MPs compared to the corresponding single MNP exposure. These findings provide novel insights into the ecotoxicity of MNPs or/and AZM, and highlight the potential for MNPs to disturb the global carbon cycle.