Biofilm mitigation in hybrid chemical-biological upcycling of waste polymers. Biofilm mitigation for waste polymer upcycling

Accumulation of plastic waste in the environment is a serious global issue. To deal with this, there is a need for improved and more efficient methods for plastic waste recycling. One approach is to depolymerize plastic using pyrolysis or chemical deconstruction followed by microbial-upcycling of the monomers into more valuable products. Microbial consortia may be able to increase stability in response to process perturbations and adapt to diverse carbon sources, but may be more likely to form biofilms that foul process equipment, increasing the challenge of harvesting the cell biomass. To better understand the relationship between bioprocess conditions, biofilm formation, and ecology within the bioreactor, in this study a previously-enriched microbial consortium (LS1_Calumet) was grown on (1) ammonium hydroxide-depolymerized polyethylene terephthalate (PET) monomers and (2) the pyrolysis products of polyethylene (PE) and polypropylene (PP). Bioreactor temperature, pH, agitation speed, and aeration were varied to determine the conditions that led to the highest production of planktonic biomass and minimal formation of biofilm. The community makeup and diversity in the planktonic and biofilm states were evaluated using 16S rRNA gene amplicon sequencing. Results showed that there was very little microbial growth on the liquid product from pyrolysis under all fermentation conditions. When grown on the chemically-deconstructed PET the highest cell density (0.69 g/L) with minimal biofilm formation was produced at 30 °C, pH 7, 100 rpm agitation, and 10 sL/hr airflow. Results from 16S rRNAsequencing showed that the planktonic phase had higher observed diversity than the biofilm, and that Rhodococcus, Paracoccus, and Chelatococcus were the most abundant genera for all process conditions. Biofilm formation by Rhodococcus sp. and Paracoccus sp. isolates was typically lower than the full microbial community and varied based on the carbon source. Ultimately, the results indicate that biofilm formation within the bioreactor can be significantly reduced by optimizing process conditions and using pure cultures or a less diverse community, while maintaining high biomass productivity. The results of this study provide insight into methods for upcycling plastic waste and how process conditions can be used to control the formation of biofilm in bioreactors.

环境中塑料废弃物的累积是一项严峻的全球性问题。为此，亟需开发更完善、高效的塑料废弃物回收处理方法。其中一种可行路径是通过热解或化学解构实现塑料解聚，随后利用微生物将解聚得到的单体升级循环为高附加值产物。微生物群落（microbial consortia）可提升体系对工艺扰动的稳定性，并适配多种碳源，但也更易形成生物膜，污染工艺设备，增加细胞生物质收集的难度。为深入解析生物反应器内生物工艺条件、生物膜形成与群落生态间的相互关系，本研究以预先富集的微生物群落（LS1_Calumet）为对象，分别以（1）氢氧化铵解聚聚对苯二甲酸乙二酯（PET）得到的单体，以及（2）聚乙烯（PE）与聚丙烯（PP）的热解产物作为碳源进行培养。通过调控生物反应器的温度、pH值、搅拌速率与通气量，筛选可实现最高浮游生物质产量与最低生物膜形成量的工艺条件。采用16S rRNA基因扩增子测序技术，对浮游态与生物膜态的群落组成及多样性进行分析。结果显示，在所有发酵条件下，微生物在热解液相产物上的生长量均极低。以化学解构PET为碳源时，在30℃、pH7、搅拌速率100转/分钟、通气量10标准升/小时的条件下，可获得最高细胞密度（0.69 g/L）且生物膜形成量最低。16S rRNA测序结果表明，浮游相的群落观测多样性高于生物膜相；且在所有工艺条件下，红球菌属（Rhodococcus）、副球菌属（Paracoccus）与螯球菌属（Chelatococcus）均为优势菌属。红球菌属（Rhodococcus）与副球菌属（Paracoccus）分离菌株的生物膜形成量通常低于完整微生物群落，且其形成量随碳源不同而存在差异。综上，本研究结果表明，通过优化工艺条件并采用纯培养或低多样性群落，可在维持高生物质产率的同时，显著降低生物反应器内的生物膜形成量。本研究结果可为塑料废弃物升级循环方法提供参考，并揭示了如何通过调控工艺条件来控制生物反应器内的生物膜形成。