Impact of extracellular polymeric substances from Skeletonema costatum on the combined toxicity of microplastics and antibiotics in estuarine environment

Estuarine ecosystems act as major sinks for land-derived pollutants entering the marine environment, and the co-occurrence of microplastics and antibiotics, along with their potential ecological effects on estuarine microalgae, has become an increasingly prominent marine environmental issue. However, little is known about the role of extracellular polymeric substances (EPS) from estuarine microalgae on the joint toxicity of microplastics and antibiotics. This study investigated the protective role of EPS in the typical estuarine diatom Skeletonema costatum by comparing physiological responses with and without EPS under combined stress from polypropylene (PP) and polyethylene (PE) microplastics and the antibiotic sulfamethazine (SM). To further elucidate the regulatory mechanisms through which microalgal EPS responds to co-stress, changes in EPS composition under combined stress were investigated. The results showed that microalgal with EPS exhibited varying degrees of oxidative stress under combined exposure, whereas high-concentration treatments may inhibit SOD production due to cell death. EPS removal compromised the algal resistance, leading to severe chlorophyll a damage, a significant reduction in the DNA synthesis preliminary phase (G0/G1), and a notable increase in the apoptotic cells (Sub-G1). The protein and polysaccharide contents in tightly bound EPS were considerably reduced by combined stress. This study reveals that EPS plays a crucial protective role in microalgae under combined pollution stress by regulating its composition and physicochemical properties. These findings provide a theoretical basis for accurately assessing the ecological risks of emerging contaminants in estuarine environments and offer important guidance for developing pollution control strategies based on biological processes in estuarine environments.