GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

16. UTILIZATION OF INORGANIC HYDROGEN DONORS: AEROBIC CHEMOLITHOTROPHIC BACTERIA

Many groups of soil and aquatic Bacteria can utilize Inorganic Compounds (ammonium, nitrite, sulfide, thiosulfate, sulfite, and ferrous iron ions), as well as elemental sulfur, molecular hydrogen, and CO₂ as hydrogen or electron Donors. In other words, they are capable of deriving reducing equivalents and energy for biosynthetic processes through The oxidation of these compounds. Energy generation generally occurs via Respiration with O₂ acting as the terminal hydrogen acceptor. This mode of existence utilizing an inorganic hydrogen donor is referred to as chemolithotrophic. Most bacteria with this type of METABOLISM use CO₂ as their carbon source, making them chemolithoautotrophs. For some chemolithotrophic bacteria, this lifestyle is obligate, whereas others are facultative chemolithotrophs, meaning they are also capable of chemoorganoheterotrophic growth.

Reverse Electron Transport in the Respiratory Chain. Inorganic substrates (ammonia, nitrite, sulfide, etc.) possess a strongly positive redox potential (NH₄ /NH₂OH +899 mV; NО₃/NO₂ +420MB; Fe^3-/Fe^2- +770 mV), while the Redox Potential of the NAD/NADH pair is -320 mV. Consequently, due to thermodynamic constraints, their oxidation cannot be directly coupled to NADH formation (unlike the oxidation of organic substrates). At the same time, NADH is required by these bacteria for anabolic processes. Nevertheless, it has been demonstrated that electrons released during the oxidation of inorganic substrates enter the respiratory chain at the level of Cytochromes c or o. Therefore, ATP Synthesis in chemolithotrophic bacteria can occur at only a single oxidation step. A portion of this energy is expended on the reverse transfer of electrons along the respiratory chain (toward NADH formation). Thus, in these bacteria, NAD reduction takes place via reverse electron transport, which consumes ATP energy. For aerobic chemolithoautotrophs, this mechanism is obligatory and essential for generating reducing equivalents that are subsequently utilized in biosynthetic pathways.

Chemolithotrophic bacteria are characterized by low biomass yield. For instance, the synthesis of 1 g of cellular biomass by aerobic chemolithoautotrophs requires the expenditure of 30–150 g of the primary energy source (ammonia, sulfite), whereas chemoorganoheterotrophs require only about 1–2 g of substrate (glucose, crude oil, etc.). This is quite logical, given that chemolithoautotrophs, on the one hand, yield little energy and, on the other hand, carry out so-called "idle oxidation" — that is, the oxidation of substrates without the simultaneous synthesis of cellular components. Therefore, it is not surprising that various transformations in soil and Water are driven by relatively small populations of bacteria.