Influence of Arsenate on PHB Degradation by Ralstonia sp. Strain C1: Implications for Co-Contaminated Soil Bioremediation

Polyhydroxyalkanoates (PHAs) are bio-based, soil-biodegradable alternatives to petroleum-derived plastics. Among PHAs, polyhydroxybutyrate (PHB) is particularly promising due to its thermoplastic properties and gas barrier performance, with applications in packaging and medicine. Although PHB biodegradation has been extensively studied, prior research has focused predominantly on single-contaminant systems. In practice, soil contamination frequently involves co-occurring pollutants; arsenic, in particular, remains a prevalent heavy metal contaminant at landfill sites in Japan, where concentrations exceeding 10 mg/kg have been reported, especially at facilities co-disposing incineration ash. Since most soil bacteria exhibit growth inhibition above this threshold, the influence of arsenate on PHB biodegradation is a critical yet unexplored question. Ralstonia sp. strain C1, a known PHB-degrading bacterium, was cultured in mineral salt medium with PHB (0.5% w/v) as the sole carbon source. Arsenate (H₃AsO₄) was added at 0 (control), 5, or 40 mg/L. Cultures were inoculated at 5% (w/v) initial cell density and incubated aerobically at 30°C, 150 rpm for 96 hours. Residual PHB was quantified by HPLC following alkaline hydrolysis. All conditions achieved approximately 60% PHB degradation within 24 hours, with no significant differences among treatments. After 96 hours, the control and 5 mg/L arsenate conditions both exceeded 90% degradation. Even at 40 mg/L arsenate, degradation reached 83%, demonstrating that strain C1 maintains robust PHB-degrading activity under arsenate stress concentrations typically inhibitory to most soil bacteria. This study provides the first evidence that arsenate does not substantially inhibit PHB biodegradation by Ralstonia sp. strain C1 in liquid culture, establishing a foundation for future bioremediation strategies targeting PHB–arsenic co-contaminated soils. Future investigations will evaluate degradation at higher arsenate concentrations, examine the effects of other heavy metals, and characterize the enzymatic mechanisms underlying PHB catabolism.