Isolation of PHB-Degrading Bacteria from Marine Plastic Waste

Plastic mismanagement has become a major environmental concern, particularly in marine environments where plastic debris threatens ecosystem integrity and may affect marine organisms. Polyhydroxybutyrate (PHB), a biodegradable plastic produced and degraded by microorganisms, is regarded as a promising environmentally compatible alternative to conventional petroleum-based plastics. However, PHB degradation is strongly influenced by environmental conditions and is often limited in seawater because of factors such as salinity, temperature, nutrient availability, and microbial abundance. In addition, compared with the extensive literature on PHB-degrading bacteria from soil or sludge, information on strains from seawater or marine plastic waste remains limited. Therefore, this study aimed to isolate PHB-degrading bacteria from marine plastic waste and evaluate their degradation activity under seawater conditions. Marine plastic waste samples were collected from the coast of Muroran, Hokkaido, Japan. PHB-degrading bacteria were isolated using artificial seawater agar medium containing 1 g/L PHB as the sole carbon source, and bacterial strains that formed clear zones around colonies were selected as PHB-degrading candidates. As a result, isolates belonging to the genera Bacillus were obtained. The isolated strains were further examined by scanning electron microscopy (SEM) to observe their morphological characteristics. In addition, their PHB-degrading ability was evaluated in artificial seawater liquid medium containing 1 g/L PHB, and the residual PHB was quantified by HPLC analysis. Among the isolates, Bacillus sp. CTA-1 showed the highest degradation activity, degrading 95.6% of the initial 1 g/L PHB after 96 h of incubation at 25°C and 150 rpm. These results demonstrate that marine plastic waste harbors bacteria capable of degrading PHB and that some isolates retain high degradation activity under seawater conditions. This study provides novel information on PHB-degrading bacteria derived from marine plastic waste and contributes to a better understanding of the degradation behavior of biodegradable plastics in marine environments.