Identification of a polyethene-interacting gut-associated bacterium from microplastic-exposed freshwater Labeo rohita

Microplastic (MP) contamination poses a growing threat to freshwater ecosystems, particularly in South Asian inland waters. This study presents a field-based comparative investigation of MP distribution and associated biological indicators in urban and rural freshwater ponds of Odisha, India, using Labeo rohita as a representative freshwater species. MPs isolated from water and fish gastrointestinal tracts were characterised using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy, Fourier-transform infrared spectroscopy, and Raman spectroscopy. MP concentrations were markedly higher at the urban site relative to the rural site, both in water (9330 ± 2520 vs. 1330 ± 580 particles m^-3) and in fish (4.00 ± 1.00 vs. 1.67 ± 0.58 MPs per fish), reflecting the influence of anthropogenic inputs. Fibres were the dominant morphotype across all samples, with a high proportion of particles smaller than 30 μm. Polyamide (PA), polyethylene (PE), and polypropylene (PP) were the predominant polymers identified. Despite differences in abundance, MP morphotype, colour, and size distributions were statistically comparable between sites (p > 0.05), suggesting shared sources and pervasive dispersal mechanisms. Histopathological examination of intestinal tissues revealed comparatively greater structural irregularities in fish from the urban site, including irregular mucosal folds, localised epithelial thinning, and vascular congestion, features suggestive of early-stage responses to environmental stressors. Additionally, a gut-associated bacterial isolate, Sphingobacterium alimentarium (UR-1), identified by 16S rRNA gene sequencing (99.07% similarity; GenBank FN908504.1), demonstrated measurable surface-associated interaction with PE, resulting in a statistically significant weight reduction, alongside surface-associated morphological alterations and elemental compositional changes. Collectively, these findings integrate environmental distribution, biological response, and bacterial interaction within a single field-based framework, providing region-specific baseline data for Odisha and contributing to the understanding of MP dynamics and ecological implications in freshwater systems.