Isolation and characterization of polyhydroxyalkanoate-degrading bacteria in seawater at two different depths from Suruga Bay.

Polyhydroxyalkanoate (PHA) is a biopolymer used as a plastic material. PHA can be a panacea for global plastic pollution as being susceptible to biodegradation in seawater besides soil and freshwater. However, the current knowledge of species that degrade PHA in marine environments is insufficient. This study first demonstrated the biodegradation of PHA in the surface and deep seawater from Suruga Bay (Shizuoka, Japan). We isolated 11 and 80 strains of PHA-degrading bacteria from the seawater and from the seawater after a biochemical oxygen demand test, respectively. Subsequent 16S rRNA analysis suggested that these strains belong to the genus Alloalcanivorax , Alteromonas , Arenicella , Microbacterium , or Pseudoalteromonas. Arenicella spp. and Microbacterium spp. have not been previously identified as marine PHA-degrading bacteria. The selected Arenicella sp. showed no growth at 10°C and displayed weak PHA degradation ability. This result is consistent with the fact that the strain was not isolated from the cold deep seawater but only from the surface seawater. We also predicted the domain composition of the extracellular poly(3-hydroxybutyrate) depolymerase in the type strains closest to the isolated strains. Depolymerases in the genus Microbacterium showed a different pattern of domain composition from those in previously identified marine bacteria, fitting a terrestrial type. Subsequently, we performed an in vitro assay of a depolymerase from Microbacterium schleiferi (DSM 20489), isolated from soil and closest to the isolated Microbacterium spp. in this study. The depolymerase showed substantial activity in the presence of NaCl, whereas it showed reduced activity without NaCl. IMPORTANCE Polyhydroxyalkanoate (PHA) is a highly biodegradable microbial polyester, even in marine environments. In this study, we incorporated an enrichment culture-like approach in the process of isolating marine PHA-degrading bacteria. The resulting 91 isolates were suggested to fall into five genera ( Alloalcanivorax , Alteromonas , Arenicella , Microbacterium , and Pseudoalteromonas ) based on 16S rRNA analysis, including two novel genera ( Arenicella and Microbacterium ) as marine PHA-degrading bacteria. Microbacterium schleiferi (DSM 20489) and Alteromonas macleodii (NBRC 102226), the type strains closest to the several isolates, have an extracellular poly(3-hydroxybutyrate) [P(3HB)] depolymerase homolog that does not fit a marine-type domain composition. However, A. macleodii exhibited no PHA degradation ability, unlike M. schleiferi . This result demonstrates that the isolated Alteromonas spp. are different species from A. macleodii . P(3HB) depolymerase homologs in the genus Alteromonas should be scrutinized in the future, particularly about which ones work as the depolymerase.