My two great thesis project loves are hydrogen and symbiosis, and as such, the recent news of a multicellular organism that lives in a completely oxygen free environment and gets its energy from hydrogenosomes instead of mitochondria is totally fascinating.
Hydrogenosomes are organelles that are evolutionarily related to mitochondria. Mitochondria generate energy for the cell by transferring electrons pulled off of sugars molecules to oxygen (this is why we breathe oxygen). The energy from this electron motion is transferred to the production of ATP, the energy currency of the cell, through the function of many complex molecular machines. Hydrogenosomes are present in organisms that live without oxygen, so they generate energy in a similar fashion by instead adding the electrons to protons, generating hydrogen gas. Hydrogenosomes are essentially just anaerobic mitochondria that produce hydrogen.
For the few people interested in electron transfer and the molecular details of hydrogen production (like me), hydrogenosomes are awesome. For people interested in symbiosis, hydrogenosomes might be even more awesome. Hydrogenosomes evolved from mitochondria, which evolved from free-living bacteria that became endosymbiotic (inside-symbionts) with other, larger cells sometime billions of years ago. Hydrogenosomes are typically found inside anaerobic protozoa, large single-celled organisms, that can be parasitic or commensal, living inside the guts of insects and helping them digest.
One hydrogenosome-containing species in particular, Nyctotherus ovalis, lives inside the guts of cockroaches in a very special matryoshka doll organization. The hydrogenosomes of these organisms, themselves ancient endosymbionts, have a symbiotic relationship with a species of tiny bacteria. These bacteria can eat hydrogen as a source of energy and are called methanogens because they produce methane as a waste product. Scientists measuring the gas content of cockroach farts discovered this fascinating symbiotic chain.
Almost all animals can’t get enough energy from hydrogenosomes to support their multicellular lifestyles, which makes the newly discovered anaerobic animals totally unexpected and amazing. But perhaps as we learn more about the complex relationships between animals and the single-celled species that live inside them, the matryoshka doll chains of cooperation seen in cockroaches will seem commonplace and totally natural.