SMX reshapes algal communities in blooms by altering algal metabolism and the potential functions of phycosphere bacteria

The increasing global incidence of cyanobacterial blooms coincides with the widespread detection of sulfamethoxazole (SMX) in aquatic environments, posing a potential threat to the integrity of freshwater ecosystems. However, the impact of SMX on algal communities under bloom conditions remains unclear. In this study, multi-omics approaches were employed under laboratory conditions to investigate the reshaping of algal communities under different concentrations of SMX. The results showed that SMX exposure significantly promoted the growth of Cyanobium sp. PCC 7009, which reached a relative abundance of 47.69%–62.08% in the SMX-treated groups (E1–E4), markedly affected algal community density and co-occurrence networks, and also reduced community diversity. Interestingly, algal community alpha diversity under high-concentration SMX exposure (2000 μg/L) (2.10) was higher than under medium concentrations (20–200 μg/L), indicating a reshaping of community structure, while low concentrations stimulated overall algal growth. This is likely because the metabolic functions of species within the community underwent varying degrees of responses under high-concentration SMX stress. The metabolic functions of sensitive species such as Chlorococcum sp. KSF0227 were significantly inhibited, while the downregulation of glycerolipid metabolism and glycerophospholipid metabolism in tolerant species may have released niche space. The behavioral network analysis focused on the impacts of the phycosphere bacteria (e.g., Pseudomonadota and Bacteroidota) on algal communities, with these interactions predominantly characterized by altruistic and antagonistic interactions. These impacts may be linked to the upregulation of signaling and nutrient metabolic pathways within the phycosphere bacteria consortia. This study advances beyond traditional single-species toxicity testing and provides a basis for assessing the ecological risks of antibiotics in eutrophic waters.