Growth of Coniochaeta species on acetate in biomass sugars

Authors: Nichols, Nancy; Mertens, Jeffrey; Frazer, Sarah; Hector, Ronald - Ron

Submitted to: Fermentation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/6/2022
Publication Date: 12/8/2022
Citation: Nichols, N.N., Mertens, J.A., Frazer, S.E., Hector, R.E. 2022. Growth of Coniochaeta species on acetate in biomass sugars. Fermentation. 8(12). Article 721. https://doi.org/10.3390/fermentation8120721.
DOI: https://doi.org/10.3390/fermentation8120721

Interpretive Summary: A technical roadblock to commercializing cellulosic-based fermentation is that the unrefined biomass contains numerous chemicals that inhibit yeast growth. One of the most problematic is acetic acid (found in vinegar) because it requires the yeast to waste energy maintaining their pH and persists throughout the fermentations. Acetate is also a component of the plant cell wall and, therefore, an intrinsic component of cellulosic sugars. Even at modest concentrations, it lowers production yields, and it is too expensive to separate from the sugars. Here an alternate technology was investigated based on biological abatement. A set of 21 fungi that are intrinsically tolerant to fermentation inhibitors were screened and several strains were discovered to also tolerate acetic acid. The best strains were next tested on acetate containing hydrolysate generated from rice hulls. Rice hulls are notoriously difficult to ferment due to their high acetate content. The process has been successfully demonstrated at lab scale where the selected fungus was used to remove acetate and the bio-refined sugars were subsequently fermented to ethanol using Brewers’ yeast. This result benefits agricultural processors interested in biofuel production and is of special interest to rice producers seeking a new market for hulls.

Technical Abstract: Degradation products from sugars and lignin are commonly generated as byproducts during pretreatment of biomass being processed for production of renewable fuels and chemicals. Many of the degradation products act as microbial inhibitors, including furanic and phenolic compounds and acetate, which is solubilized from hemicellulose. We previously identified a group of fungi, Coniochaeta species, that are intrinsically tolerant to and capable of mineralizing furans present in biomass hydrolysates. Here, we challenged 20 C. ligniaria and phylogenetically related isolates with acetate to test if the robustness phenotype extended to this important inhibitor as well, and all strains grew at concentrations up to 2.5% (w/v) sodium acetate. At the highest concentrations tested (5.0–7.5% w/v), some variation in growth on solid medium containing glucose plus acetate was apparent among the strains. The hardiness of four promising strains was further evaluated by challenging them (0.5% w/v sodium acetate) in mineral medium containing 10 or 15 mM furfural. The strains grew and consumed all of the acetate and furfural. At a higher (2.5% w/v) concentration, consumption of acetate varied among the strains: only one consumed any acetate in the presence of furfural, but all four strains consumed acetate provided that a small amount (0.2% w/v) of glucose was added. Finally, the four strains were evaluated for biological abatement of rice hull hydrolysates having elevated acetate content. The hardiest strains were also able to consume furfural and 5-hydroxymethylfurfural (HMF) within 24 h, followed by acetate within 40 h when grown in dilute acid pretreated rice hulls containing 0.55% acetate, 15 mM furfural, and 1.7 mM HMF. As such, these strains are expected to be helpful for abating non-desirable compounds from unrefined hydrolysates so as to enable their conversion to bioproducts.