Tentative map of major pathways of energy metabolism in hydrogen-producing mitochondria of the anaerobic ciliate Nyctotherus ovalis. Tentative map of major pathways of energy metabolism in hydrogen-producing mitochondria of the anaerobic ciliate Nyctotherus ovalis. The map is redrawn based on data reported previously (49). The ciliate lives in the hindgut of cockroaches and harbors organelles that provided a link between mitochondria and hydrogenosomes (49). The Krebs cycle is incomplete and is likely used in the reductive direction (177). A proton gradient is generated, probably by a functional respiratory complex I, which passes the electrons from the NADH pool through rhodoquinone to complex II, acting as fumarate reductase synthesizing succinate (101). Redox balance is also achieved with the help of hydrogenase, releasing molecular hydrogen, hence the term hydrogen-producing anaerobic mitochondria (49). ATP can be synthesized by substrate-level phosphorylation, producing acetate. Abbreviations: CI, respiratory complex I; RQ, rhodoquinone; CII, fumarate reductase/succinate dehydrogenase; [2A], acetate:succinate CoA-transferase subfamily 1A (503); [3], succinyl-CoA synthetase; [4], hydrogenase; [5], malic enzyme; [6], pyruvate dehydrogenase complex; [8], fumarase (predicted); [9], pyruvate kinase; [12], lactate dehydrogenase; [18], alanine aminotransferase; [32], pyruvate decarboxylase; [33], alcohol dehydrogenase (NADH dependent); [42], α-ketoglutarate dehydrogenase (predicted); [41], glutamate dehydrogenase. The photograph on the left shows Nyctotherus ovalis, with a length of ca. 80 μm. The hydrogenosomes (H) are surrounded by endosymbiotic methane-producing archaebacteria (dark spots). N, macronucleus; n, micronucleus; V, vacuole. (Reprinted from reference 7 with permission of Macmillan Publishers Ltd.) The right photograph shows a closeup view of a Nyctotherus hydrogenosome (H) and an associated methanogen (M). (Reprinted from reference 49 with permission of Macmillan Publishers Ltd.)‏ Miklós Müller et al. Microbiol. Mol. Biol. Rev. 2012; doi:10.1128/MMBR.05024-11