An anaerobic organism is one that does not require oxygen to live. Some, the so-called strict anaerobes, can even die in the presence of oxygen. Anaerobes are very common among bacteria, which are prokaryotes, and also among protists and fungi, both of which are eukaryotes.

Plants are characterized by carrying out oxygenic photosynthesis, and therefore they necessarily need oxygen to live. Among animals, many are known that can withstand anoxic conditions for long periods of time, but until 2010 no animal capable of carrying out its entire life cycle without oxygen was known.

That year, a group of researchers led by Roberto Danovaroreported the existence of metazoans (Kingdom Animalia) that live more than 3000 meters deep, in the sediments from the bottom of the L’Atalante Basinin the Mediterranean Sea, and that complete their entire life cycle without the presence of light or oxygen.

These animals were classified in the genera Spinoloricus, rugiloricus Y Pliciloricusthe three inside the edge Loriciferaa phylum of marine animals discovered in 1983 by Reinhardt Kristensen.

deep hypersaline basins

The L’Atalante Basin is one of the known deep, anoxic and hypersaline basins of the Mediterranean Sea. The salinity of the water in these basins is very high, almost at saturation point, with a salt concentration about eight times higher than the average seawater.

The density of water in hypersaline basins is greater than 1.2 g/cm3what prevents the water in these areas from mixing with higher waters that contain dissolved oxygen. Consequently, it creates a completely anoxic environment. In these extreme conditions live chemoautotrophic sulfur-reducing bacteria and archaea that also cause the concentration of sulfur to rise. sulfide above 2.9 mM, toxic to animals.

How to survive without oxygen

The animals discovered in the L’Atalante Basin are the first known anaerobic animals. To survive without oxygen they have had to adapt, and they have done so in a unique way.

Until now, mitochondria have been observed to were not strictly dependent on oxygen as a terminal electron acceptor in the respiratory chain, but could use another terminal acceptor, usually fumarate. In animals, they have studied mitochondria with facultative anaerobic capacity in some species, for example in molluscs of the genus Mytilus or in the flatworm Fasciola.

But this mitochondrial anaerobic capacity is limited at some pointmaking it impossible for the animal to complete its entire life cycle in anoxic conditions.

The anaerobic animals of the L’Atalante Account do not have mitochondria. Instead, they feature organelles never before observed that look more like hydrogenosomessome organelles that produce ATP without using oxygen as an electron acceptor and that, instead of water among the end products of energy metabolism, produce molecular hydrogen (Htwo).

Hydrogenosomes have been observed in some protozoa, such as the human parasite Trichomonasand in some fungi, and are thought to be organelles evolved from the mitochondria. The energy organelles of anaerobic animals, although different from hydrogenosomes, are more similar to these than to an aerobic mitochondria and could also have evolved from it.

In addition to not using oxygen, these organelles have no cytochrome c oxidaseenzyme that reduces Otwo to water in the mitochondria and whose deactivation is the cause of sulfide toxicity. Since ATP production in these animals is not dependent on cytochrome c oxidase, can also survive high concentration of sulfides in deep hypersaline basins.