Comparative, polymer-specific degradation of polyethylene, polypropylene and polystyrene microplastics by floc-forming bacteria from mud crab aquaculture systems

Abstract

• Biofloc-producing bacteria has the ability to degrade microplastics (MPs). • Bacillus tropicus SHBF1 shows higher microplastics degradation, 11.08% for PS. • Microplastics degradation is strain and polymer -specific. • SHBF1 strain showed higher cell surface hydrophobicity. • Cytobacillus firmus SHBF3 shows lowest MPs degradation for PP (1.23%). This study demonstrates the ability of two floc-forming bacteria, Bacillus tropicus SHBF1 and Cytobacillus firmus SHBF3, to degrade polyethylene (PE), polypropylene (PP), and polystyrene (PS) microplastics over a 60-day incubation period. A multi-analytical approach confirmed degradation through polymer and strain-specific mechanisms. SHBF1 exhibited higher cell surface hydrophobicity (up to 79.16%) compared to SHBF3 (54.50%), correlating with stronger biofilm-forming potential. Growth kinetic revealed time-dependent MP utilization, with significant increases in viable cell count from Day 6 onwards (p < 0.001), peaking at 9.01 log CFU/mL for SHBF1 on PE. Hydrolase secretion was polymer-specific where SHBF1 showed highest activity with PE (2.09 ± 0.05 µg/mL), while SHBF3 was most active with PS (1.74 ± 0.03 µg/mL). Weight loss, influenced significantly by both strain (H = 8.24, p = 0.004) and polymer type (H = 6.88, p = 0.032), was highest for PS degraded by SHBF1 (11.08 ± 0.37%). Media pH shifted from neutral to alkaline (up to pH 8.49), indicating active metabolism of breakdown products. FTIR and SEM analyses confirmed polymers’ chemical and physical alterations, including pitting and cracking. This comparison identifies Bacillus tropicus SHBF1 as a potent microplastic degrader, linking adhesion, enzymatic specificity, and growth dynamics.