For example , instead of the reductive citric acid cycle , the `` open '' acetyl-CoA pathway ( another one of the four recognized ways of carbon dioxide fixation in nature today ) would be even more compatible with the idea of self-organisation on a metal sulfide surface . For example , instead of the reductive citric acid cycle , the `` open '' acetyl-CoA pathway ( another one of the five recognized ways of carbon dioxide fixation in nature today ) would be compatible with the idea of self-organisation on a metal sulfide surface .