Phenylomicrobe

The progressively improved exploitation of organic substrates for biogas production increased both, methane yields and the risk of undesired by-product formation. Especially the improved disintegration of lignocellulosic material leads not only to an effective degradation of these materials but also to an increased release of aromatic compounds like phenylacetate, phenylpropionate and/or phenylbutyrate. These compounds can inhibit methane formation and thus lead to severe and long- lasting process failures. However, detailed information of the inhibitory effects of aromatic compounds on methanogenic consortia is still missing. The project Phenylomicrobe aims at studying the effects of phenyl acids on microorganisms engaged in methane formation. Microorganisms of the acetogenesis and methanogenesis phase are of special interest as they often live in complex microbial consortia and dependent on specific, microbial partners to effectively convert substances. These microbial associations promote all involved partners and are thus syntrophic interactions. However, these microorganisms are especially susceptible towards phenyl acids, which - in further consequence - can stop the entire methane producing process. Therefore, the objectives of the project are the evaluation of the effects of phenyl acids on microorganisms engaged in the acetogenesis and methanogenesis phase in pure and defined mixed cultures, development of strategies to counteract accumulation of phenyl acids in methanogenic biogas systems, and enrichment and study of phenyl acid degrading microbial consortia. One approach will be to isolate strains capable of degrading phenyl acids in pure culture thus without a microbial partner. For these project, a mix of analytical, microbiological (cultural), and highly modern molecular biological techniques will be applied.

Anaerobic digestion (AD) of organic waste materials is an environmentally friendly, sustainable, and resource-efficient energy production method that has gained importance over the past decade due to continuous technical improvements and the expansion of the substrate spectrum. The use of locally collected organic materials that do not represent potential food sources is highly desirable in terms of effective, sustainable, and ethical energy management. The improved decomposition of these substrates, especially lignocellulosic biomass often contained in green waste, lawn clippings, tree and shrub cuttings, as well as the use of protein-rich waste, also carries risks such as the formation of undesirable by-products. The latter can disrupt the multi-stage process of methane formation and cause economically relevant losses in biogas plants. In particular, the pretreatment of lignocellulosic materials not only leads to improved methane yield, but also to increased release of aromatic compounds, which include phenyl acids phenyl acetate, phenyl propionate, and phenyl butyrate. The objectives of "Phenylomicrobe" were to investigate i. the effects of phenyl acids on pure, co- and mixed cultures during anaerobic digestion ii. the degradability of phenyl acids accumulated during AD via various electron acceptors, and iii. the isolation and cultivation of PA-degrading microorganisms In the course of the project, which used cultural, chemical-analytical, and molecular biological methods and resulted in the completion of one dissertation and nine master's theses, it was shown that phenyl acids have an influence on microbial substrate utilization in pure and co-cultures as well as in mixed cultures. However, this effect was organism- or population-specific and dependent on the concentration of phenyl acids. With regard to methanogenic pure cultures, 12 of which were tested for their resilience to phenyl acids in the project, it was shown that acetoclastic methanogens in particular were more sensitive to these aromatic compounds than hydrogenotrophic ones. Through the use of mixed culture systems, it was possible to demonstrate a dependence of the negative effect of phenyl acids on pH and buffer capacity, as well as a connection to the prevailing degradation stage. This identified the necessity of microbial redundancies in the microbiomes of AD-systems as a very important factor. In contrast, the use of alternative electron acceptors to improve the degradation of accumulated phenyl acids proved to be of only limited use, similar to how changing the temperature regime hardly led to increased deconstruction of phenyl acids by the anaerobic microorganisms. In the course of the project, enrichment cultures capable of phenyl acid degradation were obtained and described using molecular methods (MAGs). The isolated and ultimately pure cultures obtained from this approach were also investigated. Some of these were microorganisms that had not yet been described and are to be described and further investigated in follow-up projects.