Biodegradation and microbial colonization of different bioplastics at the seawater-sediment interface

Background information: The majority of biodegradable polymers, proposed as sustainable alternative to traditional plastics, are biodegraded under controlled conditions (i.e. composting and anaerobic digestion), whereas their mineralization in unmanaged, open environments, including marine settings, is not understood and is often controversial. The aim of this study was to assess the biodegradation of different biopolyesters and their progressive microbial colonization at the sediment/water interface under real conditions. Given the importance of projecting ecological processes in the forthcoming oceans, the experiment was set up in the proximity of the submarine CO2 vent off Panarea Island to explore the effect of different ocean acidification scenarios simulated by the natural pH gradient. Experimental design and main results: Bioplastic films (PBS, PLA, PBAT, and PHBH) and PLA item cut-outs were included into customized stainless-steel cages designed to contain triplicates of each film, and maintaining films separated from the others even in case of fragmentation. During summer 2023, cages were fixed at the sediment-water interface at three sites along a pH gradient in the proximity of a submarine CO2 vent off Panarea Island (pH 8, 7.8 and 7.6). After 6 months and 1 year of in situ incubation, one cage for each site was collected, along with sediment samples close to the cage placement site and few meters away, and a water sample close to the cage. Intact film samples (absence of macro and micro fragmentation) were recovered for all polymers after 6 and 12 months. Significant weight losses (in the range of 0.1 to 0.4 mg/cm2 after 12 months) were consistently observed over time and in all the three sites for PHBH > PBAT > PBSA > PBS. The biodegradation of PHBH took place at a similar rate during the warmer (first 6 months) and the colder (6-12 months) seasons, whereas decreased biodegradation rate was recorded during the colder season for PBS, PBSA, and PBAT. The upper (facing water column) and bottom (facing sediment) sides of the bioplastic films were colonized by different microbial communities, sharing taxonomical features with the water and sediment microbiomes, respectively. Microbial communities colonizing the two sides of the films varied in response to time of incubation (6 vs. 12 months), and pH. Conclusions: The study contributed in better understanding the extent and rate of degradation of biodegradable polymers in a real seafloor environment, in which biodegradation is influenced by both benthic and pelagic microbial communities, also offering a glimpse into possible future scenarios of ocean acidification. The degradation rate is sensibly lower than the rates obtained in controlled industrial conditions for the same polymers (Falzarano et al, 2022; Ahsan et al, 2023). The experiment also offered the possibility to explore the ecological assembly mechanism and dynamics of plastic microbial colonization on the seafloor.