Up until the late 1970’s people were unaware of a radically different ecosystem that existed deep in the ocean at the site of hydrothermal vents. The depth, pressure, lack of light and oxygen associated with these areas was an indication that the species that lived there must have a way of surviving which was not dependant on plants or algae for primary production. Other than living conditions the high biomasses and sheer size of the dominant species present, such as tube worms, molluscs, indicated something was different about the metabolism of these organisms. In the early 1980’s the secret of these organisms success was discovered; an endosymbiotic relationship with chemoautotrophic microorganisms. The primary production of this metabolism was fueled by the oxidation of hydrogen sulphide. Another type of symbiosis has also been uncovered, methanotrophic symbiosis; however there is a very little literature on this type of metabolism as much is still unknown.
The authors compare photoautotrophic, chemoautrophic and methanotrophic metabolism throughout the paper and although there are various similarities such as the placement of symbionts in the host, near the surface, there are a massive number of differences. The uptake of inorganic carbon, the use of nitrites and the elimination of proton equivalents are just a few examples along with the toxicity problems caused by sulphide; which must be oxidised to thiosulphate in order to reduce toxicity. For chemoautrophy one of the most important functions is the host’s ability to support high oxygen demands, as it is critical for the oxidation of sulphide and carbon fixation.
After years of studying the physiological functioning of vent species studies showed that haemoglobins play a key role in binding sulphide and oxygen and allowing their transport to symbionts. The presence of these high affinity, tissue or free flowing haemoglobins seems a necessary and efficient way to support the high oxygen demand required to support symbiont metabolism (thiotrophic endosymbiosis), and is a crucial adaptation for chemoautotropic species. The characteristics required metabolically, clearly restrict the ability of certain taxa to evolve this kind of endosymbiosis, and is a proposed theory as to why the most diverse phylum for phototrophic endosymbiosis, Cnidarian, has been unable to develop this type of relationship.
A review of: Childress. J, Girguis. P (2011) The metabolic demands of endosymbiotic chemoautotrophic metabolism on host physiological capacities. The Journal of Experimental Biology. Vol. 214(2) 312-325
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