Bacterial community colonization with minimal alteration of plastics in suboxic sediments from a marine methane seep

Although marine plastics fragment and sink into suboxic seafloor sediments, little is known regarding microbiological and material outcomes. Here, open glass microcosms containing hydrocarbon-venting marine sediments were amended with either polymer film (three types of ethylene vinyl alcohol or EVOH, a polyethylene terephthalate or PET, and cellulose acetate or CA) or glass control substrates, submerged in a natural seawater flow-through tank, and sampled immediately after deployment (T0) and at approximately 0.5, 1, 3, and 6 months of incubation (T1-T4). Recovered polymer substrates were analyzed for chemical composition, surface morphology, hydrophobicity, crystallinity, and glass transition, melting, and crystallization temperatures. All substrates were colonized, as per extractable DNA. By sequencing genes encoding 16S rRNA, substrate bacterial communities differed substantially from those in seawater and sediments, even at T0, and changed over time, particularly from T3 to T4 (ANOSIM test, all P = 0.001). Gammaproteobacteria were initially dominant, and Bacteroidota increased over time. No bacterial genera were relatively abundant on the EVOHs, and only one genus, Clostridium, was more abundant on PET. However, several taxa were more abundant on CA, including those from the classes Bacillota and Clostridia, and sulfate-reducing bacteria. According to changes in cellulose backbone spectral abundances of CA polymers, biodegradation was inferred. Evidence for EVOH biodegradation was scant, although abiotic hydrolysis was apparent. Altogether, the polymer substrates were readily colonized, but only CA-according to material alteration and changes to surrounding sediment bacterial communities-appeared to biodegrade, suggesting otherwise slow degradation or broad recalcitrance of the studied plastics in suboxic marine sediments.IMPORTANCEPlastic debris entering the ocean is a worldwide problem. Most plastics sink to the seafloor and become buried in marine sediments, yet ecosystem-associated fates of such accumulations are unknown. Additionally, plastics studied for environmental influence and biodegradation in marine ecosystems rarely include laminate polymers such as the ethylene vinyl alcohol copolymers (EVOH) in addition to core polymers such as polyethylene terephthalate (PET). The results of this study indicated CA biodegradation, weak evidence for EVOH biodegradation, and none for PET over the study time frames. Thus, many plastics are likely to persist in suboxic marine sediments.