Many prokaryotes and single-celled eukaryotes have adapted to survive their entire life cycle in oxygen-less environments, such as tidal flat mud or water-logged soils. Although some multicellular organisms can tolerate anoxic conditions for a while, all of them require at least some time in an oxygenated environment to survive.

Recently, however, researchers have discovered three species of the small phylum Loricifera that appear to live their entire lives in permanent anoxic conditions within l’Atalante Basin brine pool in the Mediterranean sea.

A brine pool, as its name implies, is a large pit on the ocean floor filled with hypersaline water. The salt content of these basins is so great that the pool water does not readily mix with the surrounding seawater and thus produces a noticeable surface—in effect, creating an underwater lake. Since brine pools are deep pits on the current-less ocean floor, there is little movement of water in or out and, as a result, these pools are almost completely devoid of free oxygen.

Loriciferans are microscopic marine creatures that inhabit the spaces between sand grains on the ocean floor. Their small size and ability to cling tenaciously to the sediment prevented them from being discovered until 1983. They are believed to be related to Priapulids (a small phylum of fat worms with eversible proboscii—most popularly known from Cambrian fossils such as Ottoia). The loriciferan body consists of a mouth cone surrounded by hair-like scalids and a trunk enclosed in a chitinous shell called a lorica (hence the name “Lorica-bearer”). Little is known about their life habits since all specimens examined so far have been dead due to the fact that they adhere so tightly to the substrate that the only way to remove them is by fatally shocking them with fresh water.

The loriciferans found at the bottom of l’Atlante Basin exhibit several unique adaptations which indicate that they have evolved to live out their entire lives in an anoxic environment.

The most distinguishing of these is the presence of hydrogenosomes—hydrogen-producing organelles which have only previously been found in single-celled organisms living in oxygen-less environments. These loriciferan hydrogenosomes are furthermore found in close proximity to circular structures that closely resemble the endosymbiotic, methanogenic Archaea that often live next to the hydrogenosomes of strictly anoxic single-celled eukaryotes.

Additionally, the structural chemistry of the l’Atlante basin’s loriciferans is altered from chitin to a similar compound called chitosan. Although the exact purpose for this modification isn’t yet known, it suggests a specific adaptation for surviving in this environment.


Anoxic Metazoan ©2013 John Meszaros.  All Rights Reserved