Biodegradability of commercial biopolyesters in marine sediments under anoxic conditions

While the sea bottom is the major final sink for plastic items in the oceans, where they end up buried in sediments, the biodegradation of biopolymers has been studied so far under oxic conditions in the water column or the water-sediment interface. Here, the biodegradability of thin films of commercial biopolyesters, i.e., poly(butylene succinate) (PBS), poly(butylene succinate-co-adipate) (PBSA), poly(lactic acid) (PLA), poly(butylene adipate-co-terephthalate) (PBAT), and poly(hydroxybutyrate-co-hydroxyhexanoate) (PHBH), was investigated in microcosms of an anoxic marine sediment. PHBH rapidly biodegraded within 1 month both under sulfate-reducing and sulfate-depleted conditions, showing evidences of mineralization into carbon dioxide and methane. Hydrolytic and fermentative bacteria likely acting in the first PHBH degradation steps, along with sulfate-reducing bacteria and methanogens completing its mineralization, enriched in the sediment microbial community. All other biopolyesters did not biodegrade within 8 months of incubation. No biodegradation was also observed over 4 months when incubated concomitantly with PHBH, pointing out that PHBH biodegradation does not promote the contextual biodegradation of other biopolyesters. This work indicates that current commercial biopolyesters, except polyhydroxyalkanoates, would persist after burying in anoxic marine sediments and highlights the importance to expand the current standard tests for determining bioplastic biodegradability in marine settings to anoxic sediments.