Biodegradation of Polystyrene and Polyethylene Terephthalate microplastics by native freshwater bacterial isolates

Microplastic (MP) pollution threatens aquatic ecosystems and food webs. This study evaluated two indigenous freshwater isolates—Bacillus subtilis BI8 (Pavana River) and Bacillus aryabhattai BI3 (CME Lake)—for their ability to colonize and degrade polystyrene (PS) and polyethylene terephthalate (PET) MPs under controlled laboratory conditions. Biofilm-coated polymer pieces were incubated in mineral salt medium (MSM) with each isolate as the sole carbon source for 30 days. Biodegradation was assessed through microbial growth (OD₆₀₀), gravimetric weight loss, first-order kinetics, Fourier-transform infrared spectroscopy (FTIR) and Scanning electron microscopy (SEM). BI8 showed substantially higher growth and degradation than BI3 on both polymers (PS: 38.94 ± 1.52% weight loss, OD₆₀₀ = 1.16; PET: 21.75 ± 1.10%, OD₆₀₀ = 0.97). In contrast, BI3 achieved lower mass loss (PS: 15.33 ± 0.88%, OD₆₀₀ = 0.88; PET: 8.33 ± 0.44%, OD₆₀₀ = 0.09). Kinetic modelling indicated faster degradation by BI8 (PS k = 0.03 day⁻¹, t½ = 21.08 days; PET k = 0.02 day⁻¹, t½ = 42.38 days) than BI3. FTIR revealed oxidation and ester-hydrolysis signatures, while SEM showed dense biofilms, surface pitting and erosion, particularly with BI8. Collectively, the results identify Bacillus subtilis BI8 as an efficient PS degrader with measurable PET activity, whereas BI3 demonstrated limited early-stage modification, supporting further investigation for bioremediation applications.