ORIGIN AND SIGNIFICANCE OF CHEMOLITHOAUTOTROPHY: THE WINOGRADSKY CONCEPT OF CHEMOLITHOTROPHY

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An organism that obtains its energy from the oxidation of inorganic compounds is known as chemolithotroph. The concept of chemolithotrophic autotrophy was first conceived by Sergei Winogradsky, a Russian Microbiologist who contributed so much in the development of environmental microbiology. He was the first to provide evidence and show that an organism could oxidize inorganic substance as energy source. Winogradsky studied sulphur bacteria because certain colourless sulphur bacteria (Beggiatoa, Thiothrix) are very large and easy to investigate, even in the absence of pure cultures. Springs with waters rich in hydrogen sulphide (H2S) are fairly common around the world. In the Bernese Oberland district of Switzerland, vast populations of Beggiatoa and Thriothrix develop in the outflow channels of sulphur springs in that region.

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It was here that Winogradsky found suitable material for microscopic and physiological studies by merely lifting up the white filamentous masses of cells and performing experiments directly in the field. Winogradsky first showed that colourless sulphur bacteria were only present in water containing H2S. As the water flowed away from the source, the H2S gradually dissipated, and the sulphur bacteria disappeared. This suggested to him that their development depended on the presence of H2S. Sergei Winogradsky then showed that when Beggiatoa filaments were starved for sulphide, they lose they granules. He also found that the granules were rapidly restored if a small amount of H2S was added.

Sergei Winogradsky then concluded that H2S was being oxidized to elemental sulphur. But what happened to the sulphur granules when filaments were starved of H2S? Winogradsky showed by some clever microchemical tests that when the sulphur granules disappeared, sulphate appeared in the medium. He concluded that Beggiatoa (and by inference other colourless sulphur bacteria) oxidized H2S to elemental sulphur and subsequently to sulphate (H2S – So – SO42-). Because this organism seemed to require H2S for development in the springs, he further postulated that this oxidation was the principal source of energy for these organisms.

Winogradsky’s studies on Beggiatoa thus provided the first evidence that an organism could oxidize an inorganic substance as energy source. This was the origin of the concept of chemolithotrophy. From these beginnings, Winogradsky turned to a study of the nitrifying bacteria, and it was with this group that he first showed that autotrophic fixation of CO2 was coupled to the oxidation of an inorganic compound. The process of nitrification had been known before Winogradsky’s work from studies on the fate of sewage added to soil. For example, ammonia-rich sewage that passed through a soil column was converted to nitrate.

Winogradsky proceeded to isolate nitrifying bacteria using completely mineral media in which CO2 was the sole electron donor. Because ammonia is chemically stable, it was easy to show that the oxidation of ammonia to nitrite, and subsequently to nitrate, was a strictly bacterial process. Winogradsky further showed that nitrification was a two-step process, with one group of organisms converting NH4+ to NO2 and a second converting NO2 to NO3. Because no organic materials were present in the medium, it was also possible to show that organic matter (the bacterial cell material) was formed from CO2. When the ammonia or nitrite was left out of the medium, the cells did not grow. Careful chemical analyses showed that the amount of organic formed by the bacteria was proportional to the amount of ammonia or nitrite they oxidized.

Winogradsky concluded, “This [process] is contradictory to that fundamental doctrine of physiology which states that a complete synthesis of organic matter cannot take place in nature except through chlorophyll-containing plants by the action of light.” The concept of chemolithoautotrophy was born. We now know that at least in one way autotrophy in most chemolithotrophs and phototrophs is similar. In both groups, the pathway of CO2 fixation follows the same biochemical steps (the Calvin cycle), which require the enzyme ribulose bisphosphate carboxylase.

Some other chemolithotrophs use alternative autotrophic pathways, such as the reverse critic acid cycle or the hydroxypropionate cycle, mechanisms first discovered in anoxygenic phototrophs. Many bacteria and Archaea are chemolithotrophs, but all employ the same basic metabolic strategy: oxidize the inorganic compound by an electron transport chain that generates a proton motive force (PMF). The various forms of chemolithotrophy are simply variations on a common metabolic theme first discovered by Winogradsky with the sulphur bacteria. In summary, hydrogen(H2) and reduced sulphur compounds such as H2S, S2O32-, and So are excellent electron donors for energy metabolism in chemolithotrophs. Electrons from these substances enter electron transport chains, yielding a PMF. Sulphur and hydrogen chemolithotrophs are also autotrophs and fix CO2 by the Calvin cycle as aforesaid.      

Further reading

Madigan M.T., Martinko J.M., Dunlap P.V and Clark D.P (2009). Brock Biology of Microorganisms, 12th edition. Pearson Benjamin Cummings Inc, USA.

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