Structural and Functional Characteristics of the Anaerobic Microbial Community Emerging upon Its Contact with Extruded Polystyrene Waste

Increasing plastic pollution is a serious environmental problem as widespread production and inadequate disposal of plastic materials lead to adverse impacts on ecosystems. The structural and functional characteristics of the anaerobic microbial community contacting with extruded polystyrene (XPS) waste were investigated under methanogenic (MG) and nitrate-(NR) and sulfate-reducing (SR) conditions. The presence of XPS in the microbial community was shown to have no negative effect on the processes of biogas formation and, on the contrary, resulted in an increase in the yield of methane and volatile fatty acids and a change in their ratio. Microparticles of different sizes were found in the culture liquid of the variants with XPS: 2.4 × 106/mL (NR), 1.2 × 106/mL (SR), and 0.4 × 106/mL (MG), while no microparticles were found in the control variants. Scanning electron microscopy revealed that in all experimental variants the surface of the polymer became looser, more textured, with formation of irregularities, cracks, and holes. Increased diversity in the microbial community associated with an increase in the number of microbial morphotypes, correlated with the results of high-throughput sequencing of the 16S rRNA gene. When XPS was introduced into an anaerobic community incubated in different donor-acceptor conditions, the number of microbial groups in it increased, as well as the proportion of hydrolytic and acidogenic bacteria (Sedimentibacter, Lentimicrobium), acetogenic syntrophs (Syntrophomonas, Desulfovibrio, Geobacter), and methanogenic archaea (Methanosarcina, Methanobacterium). Our study showed that the XPS waste was not inert for the microbial community, the contact with it resulting in significant changes in its structure and functioning. Changes in the sample surface, emergence of microparticles, and differences in the composition of the intermediate and terminal metabolic products may indicate slow partial degradation of the plastic via fragmentation. However, since the experiments were carried out using household XPS containing various fillers apart from the main polymer, it is probable that, along with polystyrene, additional substances included in its composition (plasticizers, dyes, etc.) are also subject to degradation. The ability of microorganisms to degrade the polymer itself requires further research.