In college, I learned to think about fermentation as a problem of getting rid of protons, and I carried a shorthand for it: H⁺ → H₂ → CH₄. It took me years to see the other half of that shorthand. The cell needs to unload electrons. In this pathway, protons are where it puts them. Inside a cow's rumen, whole communities of organisms are linked by the need to keep those disposal routes open.
As microbes break down carbohydrates, they load electrons onto carriers, producing NADH and reduced ferredoxin. Those carriers have to unload before they can work again. Hydrogen-producing microbes use enzymes called hydrogenases to hand the electrons to protons: 2 H⁺ + 2 e⁻ → H₂. Hydrogen leaves the cell carrying reducing power with it. In the rumen bacterium Ruminococcus albus, researchers characterized two kinds of hydrogenase, including one that lets reduced ferredoxin help drive hydrogen production from NADH when that reaction would otherwise be energetically unfavorable. Zheng et al., 2014
That outlet has a limit. As hydrogen accumulates, producing more becomes energetically harder. Methanogenic archaea consume hydrogen to reduce carbon dioxide: CO₂ + 4 H₂ → CH₄ + 2 H₂O. Carbon dioxide supplies the carbon in methane. Consuming hydrogen keeps its concentration low and helps keep the outlet open for their neighbors. When researchers grew a rumen fungus with methanogens, hydrogen did not accumulate and the fungus made more acetate and less lactate and ethanol. Changing where the electrons could go changed what the community produced. Bauchop and Mountfort, 1981
For anyone designing a process, the habit worth keeping is to track two things: the product you want and the reducing power the process has to get rid of. What has to leave before the next reaction can happen? Could another organism use it? Methanogens are one group of hydrogen-consuming partners; acetogens and microbes that reduce fumarate or nitrate offer other routes. Greening et al., 2019 The proton shorthand has a limit too. Consuming protons in one step does not necessarily make a ferment less acidic. In the fungus experiment, both cultures dropped from pH 6.9 to 6.0, even though the partnership changed what they produced. Bauchop and Mountfort, 1981
From 1998 onward, that question shaped most of my work: where do the electrons go, and what chemistry can take them? It eventually became the design brief for AGATE, a fermentation platform built by choosing partner organisms that make the main reaction possible.