Hydrogenosomes are energy generating organelles which are found in a v
ariety of anaerobic microbial eukaryotes. Recent data suggest that som
e of these hydrogenosomes may be biochemically modified mitochondria,
subverting the conventional role of this compartment from one of oxida
tive phosphorylation, to one of hydrogen-evolving fermentation. The or
igins of the key enzymes which are responsible for hydrogen production
are currently obscure but they are not considered to be typical of mo
st eukaryotes.