Tubificid worms reveal hidden pollutants in tropical upstream freshwater ecosystem

• First study on MP and HM bioaccumulation in riverine tubificids in Bangladesh • MPs in tubificids ranged from 86.33 to 214.17 items/g • PE (20.69 %), PET (14.94 %), and PMMA (14.94 %) were dominant polymers • Cd, Pb, Zn, and As posed a high ecological risk in the Surma River sediment • Tubificids showed strong HM and MP uptake from the sediment–water interface Tubificid worms, common aquatic benthic invertebrates, serve as a pathway for toxicants such as microplastics (MPs) and heavy metals (HMs) to enter the food chain in tropical freshwater ecosystems. This study provides the first detailed ecotoxicological assessment of MP and HM contamination in tubificids, along with water and sediment, from six sites along the Surma River in Bangladesh. The highest MP abundance were observed at Site 3, with 823.33±29.627 items/L in water, 39±5.29 items/g in sediment, and 214.17±25.73 items/g in tubificids. MP particles were mostly fragments and fibres, accounting for approximately 60% of the total. Nearly 70 % of MP particle sizes were below 500 µm, and PE, PET, and PMMA were identified as the dominant polymers in ATR-FTIR analysis. Conversely, ICP-MS revealed elevated HM concentration in tubificids, particularly Pb, As, and Zn. Risk assessment indices identified high-hazard polymer categories among MPs, while in case of HM, Cd individually exhibited significant ecological risk. MPs (BCF & BSAF > 1) and HM (BCF > 5000 for As; BSAF > 1 for Pb, Cd, Zn) bioaccumulation indices indicated potential contaminant uptake in worms. Multivariate analysis revealed co-contamination of MPs and HMs in tubificids, with ABS and PA positively correlating with Zn and Ni, suggesting the possibility of similar sources or uptake mechanisms. These results support that tubificids at the sediment-water interface accumulate MPs and HMs, with the potential ability to increase the risk of biomagnification in freshwater food webs. This underscores the need for integrated biomonitoring and pollution control in aquatic ecosystems.