Isolation and genomic characterization of three PHA degrading bacteria from the marine environment

The broader goal of this project was to investigate what kind of effects the accumulation of polyhydroxyalkanoates (PHA) in the marine environment might have on the microbial world. Bacteria were isolated from biofilms fouling PHA pellets placed in the sediment-water interface; the goal was to isolate PHA degraders that were also sulphate reducers. Eleven strains were isolated and for three of them genome sequencing and genomic analysis were performed. Two of them (named SRB1LM and SRB3LM) turned out to be Bacillus strains and genes involved both in PHA degradation pathways and dissimilatory sulphate reduction pathway were detected in their genomes. These two strains also grew on agar plates where the only carbon source present was PHB (poly-(3-hydroxybutyrate)); they are facultative anaerobes and their growth at 15, 30 and 37°C was studied in laboratory conditions. In a complex medium, they both reached a higher growth rate (µmax) at 37°C; in particular, SRB1LM had the highest µ at 37°C in anaerobic conditions (0.407 h-1), while SRB3LM had the highest µ at 37°C in aerobic conditions (0.849 h-1). The third bacterial genome sequenced belonged to the Exiguobacterium genus, whose known species and strains have been isolated from a variety of environments. This isolate (named SRB7LM) can also grow on PHB agar plates, it is a facultative anaerobe but does not have genes of the dissimilatory pathway of sulphate reduction. SRB7LM had a similar µ at the three tested temperatures in anaerobic conditions (0.2 h-1), which were higher compared to the ones in aerobic conditions. The two Bacillus strains could be interesting for the future since the increase in PHA production will cause a higher accumulation in the marine environment; this, coupled with the already high concentration of sulphate, could arise favoring conditions for their growth.