Department of Biotechnology, Delft University of Technology, Delft, Netherlands
Soehngen Institute of Anaerobic Microbiology, Radboud University Nijmegen, Nijmegen, Netherlands.
Appl Environ Microbiol. 2018 Aug 31;84(18). doi: 10.1128/AEM.01370-18. Print 2018 Sep 15.
Agricultural residues such as sugar beet pulp and citrus peel are rich in pectin, which contains galacturonic acid as a main monomer. Pectin-rich residues are underexploited as feedstocks for production of bulk chemicals or biofuels. The anaerobic, fermentative conversion of d-galacturonate in anaerobic chemostat enrichment cultures provides valuable information toward valorization of these pectin-rich feedstocks. Replicate anaerobic chemostat enrichments, with d-galacturonate as the sole limiting carbon source and inoculum from cow rumen content and rotting orange peels, yielded stable microbial communities, which were dominated by a novel species, for which the name " Galacturonibacter soehngenii" was proposed. Acetate was the dominant catabolic product, with formate and H as coproducts. The observed molar ratio of acetate and the combined amounts of H and formate deviated significantly from 1, which suggested that some of the hydrogen and CO formed during d-galacturonate fermentation was converted into acetate via the Wood-Ljungdahl acetogenesis pathway. Indeed, metagenomic analysis of the enrichment cultures indicated that the genome of " G. soehngenii" encoded enzymes of the adapted Entner-Doudoroff pathway for d-galacturonate metabolism as well as enzymes of the Wood-Ljungdahl pathway. The simultaneous operation of these pathways may provide a selective advantage under d-galacturonate-limited conditions by enabling a higher specific ATP production rate and lower residual d-galacturonate concentration than would be possible with a strictly fermentative metabolism of this carbon and energy source. This study on d-galacturonate metabolism by open, mixed-culture enrichments under anaerobic, d-galacturonate-limited chemostat conditions shows a stable and efficient fermentation of d-galacturonate into acetate as the dominant organic fermentation product. This fermentation stoichiometry and population analyses provide a valuable baseline for interpretation of the conversion of pectin-rich agricultural feedstocks by mixed microbial cultures. Moreover, the results of this study provide a reference for studies on the microbial metabolism of d-galacturonate under different cultivation regimes.
农业废弃物如糖甜菜渣和柑橘皮富含果胶，其主要单体为半乳糖醛酸。富含果胶的残留物未得到充分利用，作为生产大宗化学品或生物燃料的原料。在厌氧恒化器富集培养物中，对 D-半乳糖醛酸的厌氧发酵转化为这些富含果胶的原料的增值提供了有价值的信息。以 D-半乳糖醛酸为唯一限制碳源，接种物来自牛瘤胃液和腐烂的橙皮，进行重复的厌氧恒化器富集，产生了稳定的微生物群落，该群落由一种新型的种主导，该种被提议命名为“Galacturonibacter soehngenii”。乙酸盐是主要的代谢产物，而甲酸盐和 H 是副产物。观察到的乙酸盐摩尔比和 H 和甲酸盐的总和与 1 显著偏离，这表明在 D-半乳糖醛酸发酵过程中形成的一些氢气和 CO 通过 Wood-Ljungdahl 乙酰生成途径转化为乙酸盐。事实上，富集培养物的宏基因组分析表明，“G. soehngenii”的基因组编码了适应 D-半乳糖醛酸代谢的 Entner-Doudoroff 途径的酶，以及 Wood-Ljungdahl 途径的酶。在 D-半乳糖醛酸限制条件下，这些途径的同时运作可能通过提供更高的特定 ATP 产生率和更低的残留 D-半乳糖醛酸浓度，为适应 D-半乳糖醛酸限制条件提供了选择性优势，而不是严格发酵这种碳和能源来源。本研究通过开放、混合培养物在厌氧、D-半乳糖醛酸限制恒化器条件下对 D-半乳糖醛酸代谢的研究表明，D-半乳糖醛酸稳定而有效地发酵为乙酸盐，作为主要的有机发酵产物。这种发酵化学计量和种群分析为解释混合微生物培养物对富含果胶的农业饲料的转化提供了有价值的基线。此外，本研究结果为不同培养条件下 D-半乳糖醛酸微生物代谢的研究提供了参考。
