Extremophilic anaerobes are widespread in saline, acid, alkaline, and high or low temperature environments. Carbon is essential to living organisms and its fixation, degradation, or mineralization is driven by, up to now, six metabolic pathways. Organisms using these metabolisms are known as autotrophs, acetotrophs or carbon mineralizers, respectively. In anoxic and extreme environments, besides the well-studied Calvin-Benson-Bassham cycle, there are other five carbon fixation pathways responsible of autotrophy. Moreover, regarding carbon mineralization, two pathways perform this key process for carbon cycling. We might imagine that all the pathways can be found evenly distributed in microbial biotopes; however, in extreme environments, this does not occur. This manuscript reviews the most commonly reported anaerobic organisms that fix carbon and mineralize acetate in extreme anoxic habitats. Additionally, an inventory of autotrophic extremophiles by biotope is presented.