Less toxic combined microplastics exposure towards attached Chlorella sorokiniana in the presence of sulfamethoxazole while massive microalgal nitrous oxide emission under multiple stresses

Abstract

Microalgae-based wastewater treatment could realize simultaneous nutrients recovery and CO₂ sequestration. However, impacts of environmental microplastics (MPs) and antibiotic co-exposure on microalgal growth, nutrients removal, intracellular nitric oxide (NO) accumulation and subsequent nitrous oxide (N₂O) emission are unclarified, which could greatly offset the CO₂ sequestration benefit. To reveal the potential impacts of environmental concentrations of MPs and antibiotic co-exposure on microalgal greenhouse gas mitigation, this study investigated the effects of representative MPs (PE, PVC, PA), antibiotic sulfamethoxazole (SMX), and nitrite (NO₂^--N) in various combinations on attached Chlorella sorokiniana growth, nutrients removal, anti-oxidative responses, and N₂O emission originated from intracellular NO build-up. Microalgal biofilm growth was more inhibited under 10 μg/L MPs than 100 μg/L SMX, and MPs+SMX co-exposure displayed toxicity antagonism while MPs+MPs co-exposure caused toxicity synergism (up to 66% growth inhibition). Extracellular polysaccharides content correlated well with microalgal biofilm density under various stresses, while SMX involved stresses displayed chlorophyll a content reduction. Microalgal assimilation and MPs adsorption contributed to nutrients removal, and phosphorus removal displayed less variance among different stresses (residual phosphorus <0.5 mg/L) than nitrogen. Intracellular NO conversion to N₂O almost doubled during the co-exposure processes, and N₂O emission under NO₂^--N+PE+PVC co-exposure could offset the contribution of microalgal CO₂ sequestration by as high as 176.2%. Results of this study appealed for urgent concern regarding environmental MPs and antibiotic co-exposure on primary producers’ growth characteristics and their greenhouse gas mitigation properties.