A systematic review of biodegradable microplastics-induced toxicity in fish: Toxic effects, mechanisms, and ecological implications

Despite their green image, biodegradable plastics incompletely degrade in water to BMPs, whose toxicity to fish has not been systematically reviewed. This review consolidates evidence from 27 studies examining BMP-induced toxicity in fish, focusing on exposure pathways, underlying mechanisms, and broader ecological implications. The available data indicate that BMPs—mainly polylactic acid (PLA), polybutylene adipate-co-terephthalate (PBAT), polyglycolic acid (PGA), polybutylene succinate (PBS), and polyhydroxyalkanoate (PHA)—enter fish through waterborne and dietary routes and accumulate preferentially in the intestine, liver and gills. Multi-organ toxicities include intestinal inflammation and villus damage, hepatotoxicity, neurobehavioral deficits, reproductive and developmental abnormalities, cardiotoxicity, immunotoxicity and haematological disruptions. Mechanistically, BMPs trigger oxidative stress, mitochondrial dysfunction, apoptosis, inflammatory cascades (NF-κB pathway), metabolic disorders (PPARγ pathway), and gut microbiota dysbiosis, with consequent involvement of the gut–brain axis and the gut–liver–brain axis. Notably, mitochondrial dysfunction is regulated by the upstream Sesn2/Nrf2 signaling pathway, which serves as a critical checkpoint linking oxidative stress to downstream inflammatory and apoptotic responses. Neurobehavioral toxicity is mediated in part through BDNF/TrkB signaling disruption, with transcriptomic evidence revealing dose-dependent recruitment of immune-related pathways. Toxicity is modulated by particle aging, polymer composition, exposure route and the presence of co-occurring pollutants. Critically, the assumption that biodegradable plastics are inherently safer than conventional counterparts is not supported by current evidence: aged BMPs often exhibit enhanced toxicity compared with pristine forms, and under certain conditions biodegradable MPs pose risks comparable to or even greater than those of conventional microplastics. Ecological implications include potential population-level effects through reduced fitness, reproductive impairment and transgenerational toxicity, as well as altered trophic transfer of associated contaminants. This review identifies critical knowledge gaps and proposes a framework for assessing the long-term ecological risks of biodegradable plastics in aquatic ecosystems.