Molecular Characterization of the Bacterial Community in Biofilms for Degradation of Poly(3-Hydroxybutyrate-<i>co</i>-3-Hydroxyhexanoate) Films in Seawater

Abstract

Microplastics are fragmented pieces of plastic in marine environments, and have become a serious environmental issue. However, the dynamics of the biodegradation of plastic in marine environments have not yet been elucidated in detail. Polyhydroxyalkanoates (PHAs) are biodegradable polymers that are synthesized by a wide range of microorganisms. One of the PHA derivatives, poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH) has flexible material properties and a low melting temperature. After an incubation in seawater samples, a significant amount of biofilms were observed on the surfaces of PHBH films, and some PHBH films were mostly or partially degraded. In the biofilms that formed on the surfaces of unbroken PHBH films, the most dominant operational taxonomic units (OTUs) showed high similarity with the genus Glaciecola in the family Alteromonadaceae. On the other hand, the dominant OTUs in the biofilms that formed on the surfaces of broken PHBH films were assigned to the families Rhodobacteraceae, Rhodospirillaceae, and Oceanospirillaceae, and the genus Glaciecola mostly disappeared. The bacterial community in the biofilms on PHBH films was assumed to have dynamically changed according to the progression of degradation. Approximately 50 colonies were isolated from the biofilm samples that formed on the PHBH films and their PHBH-degrading activities were assessed. Two out of three PHBH-degrading isolates showed high similarities to Glaciecola lipolytica and Aestuariibacter halophilus in the family Alteromonadaceae. These results suggest that bacterial strains belonging to the family Alteromonadaceae function as the principal PHBH-degrading bacteria in these biofilms.

Biofilm Formation and Degradation of Commercially Available Biodegradable Plastic Films by Bacterial Consortiums in Freshwater Environments

“…Few studies have investigated the biodegradation of plastics through biofilm formation in aqueous environments. We previously demonstrated that one of the PHA derivatives, poly(3-hydroxybutyrate- co -3-hydroxyhexanoate) (PHBH), was degraded by the formation of a biofilm on its surface in seawater samples ( 9 ). Microplastic pollution has become a serious global issue not only in seawater, but also in freshwater environments ( 17 ).…”

“…The biodegradability in freshwater of six types of general biodegradable plastic films: poly(lactic acid) (PLA), poly(butylene adipate- co -terephthalate) (PBAT), polybutylene succinate (PBS), poly(butylene succinate- co- butylene adipate) (PBSA), poly(ɛ-caprolactone) (PCL), and PHBH, was assessed ( 9 ). Biofilms were formed on plastic surfaces using a previously described method with slight modifications ( 9 ). Briefly, all plastic films had a thickness of approximately 100 μm, and were prepared by T die cast extrusion.…”

“…The entire surfaces of the PHBH and PCL films were covered by biofilms in all of the freshwater samples tested. We previously demonstrated that biofilms were not detectable on the surfaces of PLA, PBAT, PBS, or PBSA films after an incubation in seawater samples ( 9 ). Biofilm formation and the biodegradation of plastics have been suggested to proceed more easily in freshwater environments than in seawater environments.…”

“…These findings suggest that PHBH films are degraded by PHA-degrading exoenzymes, which are secreted by the order Burkholderiales in freshwater as well as soil environments. We previously demonstrated that the family Alteromonadaceae was the most dominant bacteria in the biofilms that formed on PHBH films in seawater, whereas the order Burkholderiales was not detected ( 9 ). The difference in the dominant bacteria between seawater and freshwater samples appears to be due to the indigenous bacterial compositions in each sample.…”

Recent Biofilm Studies Open a New Door in Microbial Ecology

“…Acidovorax and Undibacterium were isolated from the biofilm as PHBH degraders. In contrast to seawater experiments (Morohoshi et al, 2018a), they found biofilm formation on the PBS, PBSA, and PBAT films (but not on PLA). Although the degradation of these polymers was not observed, these findings strongly indicate that bioplastics are more easily deteriorated in freshwater environments than in seawater environments.…”