Micro- and nano-plastics and lead exposure influence growth, feeding, oxidative stress, and dimethyl sulfur compounds production in ciliate Euplotes vannus.

The increasing microplastics (MPs)/nanoplastics (NPs) pollution in the ocean has ecotoxicological effects on marine organisms, affecting their ecological function related to the sulfur cycle. Ciliated protozoa play an important role in dimethyl sulfide (DMS) emission in the microbial food chain. DMS is a negative greenhouse gas associated with the global climate. However, fewer studies were done on the influences of MPs or NPs on DMS and its related dimethyl sulfur compounds production caused by ciliates feeding. Here, we have investigated the ecotoxicological effects of 5 μm and 50 nm polystyrene (PS), alone and in combination with lead (Pb) on ciliates Euplotes vannus, and their impacts on DMS, dissolved and particulate dimethylsulfoniopropionate (DMSPd,p), and dissolved and particulate dimethyl sulfoxide (DMSOd,p) production. Our study indicated that MPs/NPs and Pb exposure slowed the growth rate of E. vannus, reduced its abundance and biomass, increased superoxide anion (O2·‒) generation rate, malondialdehyde (MDA) content, and superoxide dismutase (SOD) activity, decreased glutathione (GSH) content. The co-exposure to NPs and Pb exerted a synergistic effect on the O2·‒ generation rate and GSH content. In addition, exposure to MPs/NPs and Pb alone reduced the ingestion rate of ciliates, leading to inhibited production of DMS and DMSOd. These findings enhance our understanding of the ecotoxicological impacts of MPs/NPs and Pb, both alone and in combination, on protozoa and their importance on dimethyl sulfur compounds cycling.