Effects of Chlorella pyrenoidosa Amendment on Tetracycline–Copper Dissipation, Soil Microbial Communities, and Tetracycline Resistance Genes

Tetracycline (TC)–copper (Cu) co-contamination can impair soil functioning and intensify selection for antibiotic resistance. In a 49-day microcosm experiment, we evaluated the effects of Chlorella pyrenoidosa supplementation on soils treated with TC (0, 30, or 100 mg·kg−1) and Cu (0, 100, or 500 mg·kg−1). TC removal, extractable Cu, soil physicochemical properties, enzyme activities, tetracycline resistance genes (TRGs), and microbial community composition were assessed. In unamended contaminated microcosms, TC dissipation ranged from 22.7% to 43.5%, whereas C. pyrenoidosa amendment increased TC dissipation to 49.9–73.5%. The addition also reduced extractable Cu by 35.6–50.6%, partially restored dehydrogenase and catalase activities, and altered bacterial community structure. Metagenomic analysis showed that tetX was the predominant TRG detected and that algal supplementation was associated with lower total TRG signals and reduced relative abundances of several pollution-associated taxa, including Rhodanobacter. Correlation analyses revealed associations among TRGs, microbial taxa, and treatment conditions but did not establish direct host–gene relationships or horizontal gene transfer. Overall, C. pyrenoidosa application enhanced TC removal and Cu immobilization and was associated with reduced enrichment of the tetracycline resistome.