Evaluating the impact of redox potential on the growth capacity of anaerobic gut fungi.

Anaerobic gut fungi (AGF, Neocallimastigomycota) inhabit the alimentary tract of herbivores. Although strict anaerobes, studies have suggested their capacity to retain viability after various durations of air exposure. It is currently unclear whether AGF can actively grow, and not merely survive, in redox potentials (E_h) higher than those encountered in the herbivorous gut. We evaluated the growth of two AGF strains (Orpinomyces joyonii and Testudinimyces gracilis) at various E_h levels, achieved by manipulating the concentrations of reductant (cysteine hydrochloride) in culture media. Both strains exhibited robust and sustainable growth at negative E_h (-50 mV or below). However, growth in the absence of cysteine hydrochloride (E_h value around +50 mV) was possible only for O. joyonii and only for one subcultivation. The capacity to grow at +50 mV was further confirmed in four additional taxa (Pecoramyces ruminatium, Anaeromyces mucronatus, Aklioshbmyces papillarum, and Piromyces communis), while two (Aestipascuomyces dupliciliberans and Capellomyces foraminis) failed to grow under these conditions. Our results establish the ability of AGF to grow at redox potential values higher than those encountered in their natural habitats. Such capability could contribute to efficient AGF dispersal and horizontal transmission between hosts, and could have important implications for industrial applications of AGF.