Single and combined effects of amino polystyrene and perfluorooctane sulfonate on hydrogen-producing thermophilic bacteria and the interaction mechanisms

As a carrier of perfluorooctane sulfonate, nano-plastics are ubiquitous and finally enriched in the sludge, which is widely used as a raw material for the production of bioenergy (hydrogen or methane) by anaerobic digestion. However, there are still many unknowns about their metabolic toxicity to functional microbes (e.g. hydrogen-producing thermophilic bacteria). Therefore, single and combined effects of amino polystyrene (NPS: 70 nm; 0.2 mg/L) and perfluorooctane sulfonate (PFOS: 0.1, 1 and 5 mg/L) on hydrogen-producing thermophilic bacteria were investigated after exposure for 7 days at 55 °C and pH = 5.7. Single NPS exhibited obvious interference to the metabolism of thermophilic bacteria, resulting in a 53.9% reduction in hydrogen production. However, the combined NPS + PFOS produced an antagonistic effect, leading to a 31.6% reduction in hydrogen production. Nonetheless, the single and combined exposure did not alter the type of hydrogen production (acetic acid-type hydrogen fermentation). Moreover, single NPS and combined NPS + PFOS not only induced the changes of the composition of extracellular Polymers (EPSs) and π bond structure of the protein in EPSs, but also decreased the activity of hydrolase in EPSs and surface charge of EPSs. Compared to single NPS exposure, NPS + PFOS-exposed thermophilic bacteria was less permeable to a semi-membrane permeable dye and produced less Reactive Oxygen Species, but were still significantly higher than control group. In short, the main mechanisms of single NPS and combined NPS + PFOS were both to increase cell permeability and to induce oxidative stress. The addition of PFOS alleviated the toxic effect of NPS, but did not change its mechanism of toxicity.