Biodegradation of biopolyesters in an anoxic marine sediment and effects on microbial activities and biodiversity

Background information: Biodegradability of bioplastics implies their ability to undergo complete mineralization by microbial activity. However, the biodegradation timescale and extent varies in different environments compared to controlled industrial conditions (e.g., composting, anaerobic digestion) typically used to assess biodegradability (Lavagnolo et al, 2024). While the biodegradation of bioplastics has been extensively studied in the seawater column, little is known about their fate in anoxic marine sediments, which act as final sink of plastic items released into the marine ecosystem. In this study we investigated the biodegradability of the main commercial biopolyesters, and to what extent their biodegradation potentially affects the surrounding environment, in anoxic sediment microcosms mimicking the biogeochemical conditions facing plastic items buried in the seabed. Experimental design and main results: Thin films of poly(hydroxybutyrate-co-hydroxyhexanoate) (PHBH), poly(lactic acid) (PLA), poly(butylene succinate) (PBS), poly(butylene succinate-co-adipate) (PBSA), poly(butylene adipate terephthalate) (PBAT) and low density poly(ethylene) (LDPE) as not biodegradable control were incubated buried in the sediment within sacrificial anoxic microcosms of sediment and overlaying seawater collected from the Pialassa Baiona lagoon (Ravenna Italy). Incubation took place statically in the dark at 20°C, with periodic monitoring of biogas production and composition (CO2 and CH4), concentration of sulfate in the water phase, redox potential, gravimetric weight loss of polymer films and sediment microbial community composition. None of the tested polymers lost weight over a 4-month incubation period with the exception of PHBH, which was completely degraded in 4 weeks. A remarkable production of CO2 was observed concomitantly with PHBH biodegradation, indicating mineralization. Simultaneously, sulfate reduction took place, leading to the complete consumption of sulfates and a remarkable decrease in redox potential. Changes in the sediment microbial community reflected those in metabolic activities, with the enrichment of putative hydrolytic/fermentative taxa and sulfate reducers. Additional PHBH films added to the same microcosms after complete sulfates depletion degraded at the same rate, leading to CO2 and CH4 production and the concomitant enrichment of known methanogenic taxa. Finally, at month 4, PHBH powder was supplemented to microcosms containing the other undegraded polymers, to explore the occurrence of possible co-metabolic degradation of other biopolyesters with PHBH. Evolution of CO2 and CH4, along with sulfate reduction were stimulated as expected by PHBH biodegradation, but no degradation of any other biopolyester was observed over an additional 4-month incubation period. Conclusions: PHBH degraded rapidly in an anoxic marine sediment, both under sulfate-reducing and methanogenic (sulfate-depleted) conditions, affecting remarkably the structure of the sediment microbial community. Conversely, none of the other tested biopolyesters underwent biodegradation within a timeframe of 8 months, neither alone, nor in concomitance with PHBH biodegradation.