Are there anaerobic animals?

An anaerobic organism is one that does not require oxygen to live. Some, 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 eukaryotes.

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

In that year, a group of researchers led by Roberto Danovaro reported the existence of metazoans (Kingdom Animalia) that live more than 3,000 meters deep, in the sediments at the bottom of the L’Atalante Basin in the Mediterranean Sea, and that complete their entire life cycle. life without the presence of light or oxygen.

These animals were classified in the genera Spinoloricus, rugiloricus S Pliciloricusos three within the edge Loriciferaum phylum of marine animals discovered in 1983 by Reinhardt Kristensen.

deep hypersaline basins

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

The density of water in hypersaline basins is greater than 1.2 g/cm3o which prevents 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 sulfur concentration to increase. sulphide above 2.9 mM, toxic to animals.

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How to survive without oxygen

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

Until now, it has been observed that mitochondria 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 studied mitochondria with facultative anaerobic capacity in some species, for example in molluscs of the genus Mytilus or in the Fasciola worm.

But this mitochondrial anaerobic capacity is limited at some point, making it impossible for the animal to complete its entire life cycle under anoxic conditions.

The anaerobic animals of Conta L’Atalante do not have mitochondria. Instead, they feature never-before-seen organelles that look more like hydrogenosomes—some 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 (H2).

Hydrogenosomes have been observed in some protozoa, such as the human parasite Trichomonase in some fungi, and are believed to be organelles that evolved from mitochondria. The energy organelles of anaerobic animals, while different from hydrogenosomes, are more similar to them than to an aerobic mitochondria and may also have evolved from them.

In addition to not using oxygen, these organelles lack cytochrome c oxidaseenzyme that reduces O2 to water in mitochondria and whose deactivation is the cause of sulfide toxicity. As ATP production in these animals does not depend on cytochrome c oxidase, it can also survive high concentration of sulfides in deep hypersaline basins.

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