Oxygen, light and food. Many living beings need these three things to live. Some only need two of them, and there are rare cases that only require one.
Among the organisms that can live without food , understood as the organic remains of other living beings, are autotrophs: plants, algae and cyanobacteria. Among those that do require food, the heterotrophs, there are many that can live without light , such as the animals of the abyssal bottom. And there are even living beings that do not need oxygen ; they do not breathe, but rather ferment food; They are called anaerobic organisms.
However, there are some living beings that are totally exceptional, and can live without any of these three components. They are the organisms called chemoautotrophs . There are no multicellular organisms with this kind of ability, at least as far as we know: all known chemoautotrophs are either archaea or bacteria.
Mainly, living beings need two things: a source of energy that allows us to carry out our metabolic activities and a source of carbon that provides us with the raw material from which to produce our own structures. Any other needs, without being less important, are of lesser magnitude or are specific to certain organisms. But no living being can maintain itself and evolve if it does not have these two main resources.
Plants and other autotrophic organisms obtain energy from sunlight, through the process known as photosynthesis, and carbon is obtained from CO2 present in the air. We heterotrophs get both resources from food; we assimilate carbon from organic molecules through digestion, and we acquire the energy stored in the complex bonds of said molecules, thanks to the process of cellular respiration or through fermentation.
Chemoautotrophic organisms, however, do not have organic matter that they can use as a carbon source, nor oxygen to obtain energy by respiration. Carbon is usually obtained from CO2, like plants; but unlike them, they cannot obtain energy from sunlight. But even in such inhospitable environments, life finds its way , and chemoautotrophic organisms have entirely different mechanisms for obtaining their energy.
In principle, any oxidation reaction, even from inorganic sources, produces energy, and a suitably adapted organism could obtain and harness that energy, without the need for available organic matter or sunlight. This is what chemotrophic organisms do, by oxidizing hydrogen, ammonia, nitrite, sulfur, or the ferrous or cuprous ions. Organisms that obtain energy from the oxidation of elements present in rocks or soil are called chemolithotrophs. Although they use oxygen.
However, oxygen need not actually be necessary in an oxidation reaction . In fact, oxidation is a reaction by which a substance loses electrons, which are acquired by another, called an electron acceptor. In combustion reactions, such as when something is burned, or in respiration, the electron acceptor for oxidation is oxygen, which usually combines with carbon to form CO2. But chemoautotrophic organisms, inhabitants of inhospitable environments without oxygen, use other molecules as electron acceptors.
For example, bacteria such as Thiobacillus ferrooxidans or T. thiooxidans are capable of oxidizing sulfur at the expense of reducing ferric iron , which acts as an electron acceptor.
Chemoautotrophic organisms inhabit really harsh environments. They do not usually live on land or on the surface of seas and oceans, where there is light, oxygen and food in relative abundance. They inhabit mid-ocean ridges, and more specifically, underwater hydrothermal vents , one of the most inhospitable environments on the planet. These are cracks through which seawater comes out that has previously infiltrated and, passing through the rock and approaching the mantle, re-emerges, very hot, and loaded with chemical compounds that it has been dissolving in its path.
Since in their environment they play a role similar to that of plants and algae on the surface, acting as primary producers , chemoautotrophic organisms are also the basis of an ecosystem. One much simpler than the complex ecosystems we find on the surface of the sea or on land, but much stranger.
Riftia pachyptila is a species of polychaete worm that lives in symbiosis with chemoautotrophic bacteria of the genera Epsilonproteobacteria and Sulfurovum. In fact, in 2016 a new species was discovered in this worm, which received the name of Sulfurovum riftiae, and which carried out the oxidation of thiosulfates using nitrates as an electron acceptor. The symbiosis between chemoautotrophic bacteria and the worm is, as far as we know, a unique association in the world, and they alone form one of the simplest and most fascinating communities of living beings in the world.
Referencias:
Boden, R. et al. 2017. An evaluation of Thiomicrospira, Hydrogenovibrio and Thioalkalimicrobium: reclassification of four species of Thiomicrospira to each Thiomicrorhabdus gen. nov. and Hydrogenovibrio, and reclassification of all four species of Thioalkalimicrobium to Thiomicrospira. International Journal of Systematic and Evolutionary Microbiology, 67(5), 1140-1151. DOI: 10.1099/ijsem.0.001855
Dobrinski, K. P. et al. 2005. The Carbon-Concentrating Mechanism of the Hydrothermal Vent Chemolithoautotroph Thiomicrospira crunogena. Journal of Bacteriology, 187(16), 5761-5766. DOI: 10.1128/JB.187.16.5761-5766.2005
Giovannelli, D. et al. 2016. Sulfurovum riftiae sp. nov., a mesophilic, thiosulfate-oxidizing, nitrate-reducing chemolithoautotrophic epsilonproteobacterium isolated from the tube of the deep-sea hydrothermal vent polychaete Riftia pachyptila. International Journal of Systematic and Evolutionary Microbiology, 66(7), 2697-2701. DOI: 10.1099/ijsem.0.001106
Kelly, D. P. et al. 2006. The Chemolithotrophic Prokaryotes. En M. Dworkin et al. (Eds.), The Prokaryotes (pp. 441-456). Springer New York. DOI: 10.1007/0-387-30742-7_15
Zhang, Y. et al. 2019. Complete Genome Sequence of Acidithiobacillus ferrooxidans YNTRS-40, a Strain of the Ferrous Iron- and Sulfur-Oxidizing Acidophile. Microorganisms, 8(1), 2. DOI: 10.3390/microorganisms8010002
