a type of nutrition characteristic of certain bacteria whose only carbon source is CO2 obtained from the energy of oxidation of inorganic compounds.
The discovery of chemosynthesis by S. N. Vinogradskii in 1887 significantly altered prevailing views on the main types of metabolism in living organisms. Unlike photosynthesis, chemosynthesis does not involve the use of light energy but energy derived from oxidation-reductions that must be adequate for the synthesis of adenosine triphosphate (ATP) and that must exceed 10 kcal/mole. Chemosynthetic bacteria are not strictly a taxonomic group; they are classified according to the inorganic substrate they oxidize. They include microorganisms that oxidize hydrogen, carbon monoxide, reduced sulfur compounds, iron, ammonia, nitrites, and antimony.
Hydrogen bacteria are the most numerous and varied group of chemosynthetic organisms. They perform the reaction 6H2 + 2O2 + CO2 = (CH2O) + 5H2O, in which (CH2O) is the conventional designation of the organic substances formed. Compared with other autotrophic microorganisms, hydrogen bacteria have a high growth rate and may produce a large biomass; they are also able to grow on mediums containing organic matter, that is, they are mixotrophic or facultatively chemoautotrophic.
Carboxydation bacteria are similar to hydrogen bacteria, but they oxidize CO by the reaction 25CO + 12O2 + H2O + 24CO2+ (CH2O). Sulfur bacteria oxidize hydrogen sulfide, thiosulfate, and molecular sulfur to sulfuric acid. Some, for example, Thiobacillus ferrooxidans, oxidize sulfide minerals and ferrous oxide. The capacity of various aquatic sulfur bacteria for chemosynthesis has not yet been demonstrated. Nitrifying bacteria oxidize ammonia to nitrite (first stage of nitrification) and nitrite to nitrate (second stage). Chemosynthesis occurs under anaerobic conditions in some denitrifying bacteria, which oxidize hydrogen or sulfur but often require organic matter for biosynthesis. Chemosynthesis has been described in some strictly anaerobic meth-anogenic bacteria according to the reaction 4H2 + CO2 = CH4 + 2H2O.
The biosynthesis of organic compounds in chemosynthesis is achieved by autotrophic assimilation of CO2 (Calvin carbon reduction cycle) just as in photosynthesis. Energy is obtained in the form of ATP from electron transfer via the chain of respiratory enzymes incorporated into the bacterial cell membrane. Some oxidized substances give off electrons to the chain at the cytochrome c level, thereby creating additional energy to synthesize the reducing agent. Owing to the fairly large consumption of energy, chemosynthetic bacteria—with the exception of hydrogen bacteria—produce a small biomass but oxidize a substantial amount of inorganic matter. Since chemosynthetic bacteria control the oxidative parts of the cycle of major elements in the biosphere, they are vitally important in biogeochemistry. Hydrogen bacteria can be used to obtain protein and to free the atmosphere from CO2 in closed ecological systems. Chemosynthetic bacteria are highly varied morphologically. Although most of them are pseudomonades, they are also found among budding and filamentous bacteria, spirilla, leptospira, and corynebacteria.
G. A. ZAVARZIN