A Lake Under the Sea: Anoxic Animals and Anoxic Protists

A couple of posts by Jelena and myself have looked at the deep hypersaline anoxic basins (DHABs) of the Mediterranean and the prokaryotic and eukaryotic life found within them. This paper, although not purely microbiology, does make some interesting comparisons between the first obligate anaerobic animals and single cellular anaerobic protists. These animals were not just the first anoxic animals to be discovered in the DHABs, but also in the world in general.

In a number of expeditions to a DHAB in the Mediterranean Sea called L’Atalante, Danovaro et al (2010) discovered and examined three new species of animal that belonged to the phylum Loricfera. These three metazoans were found living in the sediments of the brine lake and were less than a millimetre in size. Two other phyla were found but staining methods suggested that they had been dead for some time, possible victims of the DHAB’s unforgiving environment. This study concluded that this was the first example of purely oxygen independent animals to have been discovered. Other metazoans have been found living in anoxic environments before but only at certain stages of their lifecycle, these three species however appear to live their entire lives in 0% oxygen.

Mentel and Martin (2010) examined the results of the previous study in more detail and made many cellular comparisons with unicellular protists. Transmission electron microscopy (TEM) revealed a complete lack of traditional eukaryotic mitochondria, instead there were organelles which resembled hydrogenosomes. Instead of using oxygen as a terminal electron acceptor, hydrogenosomes pass electrons from pyruvate to 2H+ to produce H2. These organelles have never been seen in animals before but are found in members of the unrelated unicellular phyla Amoebozoa and Metamonada. Surrounding the hydrogenosomes were what appeared to be rod shaped cells. The authors suggested that they might be methanogen achaea in a endosymbiotic relationship with the cells as this is common with protists that contain hydrogenosomes. Sadly, most of the research has so far been carried out with TEM and so the details about the hydrogenosomes and the rod shaped cells remain a mystery.

Mental and Martin (2010) discuss the fact that recent evidence shows that deep sea environments were anoxic until about 580 million years ago, about the same time that large animals start to appear in the fossil record. They passionately suggest that anaerobic animals might have evolved from anaerobic protests similar to the ones seen today but when oxygen levels increased it gave way to more productive aerobic animals that rapidly increased in size during the Cambrian explosion. They concluded from this idea that the animals discovered in L’Atalante have evolved from anaerobic unicellular eukaryotes millions of years ago and are not descended from aerobic animals that adapted to the DHAB after its formation.

I think it will be fascinating to see what research into the metabolism and genomes of these organisms will uncover. I imagine such studies will indeed take place as it is a very significant discovery but I cannot find any articles on it just yet.

A Review of:
Mentel, M and Martin, W (2010) Anaerobic animals from an ancient, anoxic ecological niche. BMC Biology. 8, 32.

Additional Reference:
Danovaro, R. Dell’Anno, A. Pusceddu, A. Gambi, C. Heiner, I and Kristensen, R (2010) The first metazoan living in permanently anoxic conditions. BMC Biology. 8, 30.