What is Chemolithotrophs / Chemolithotrophy?

Organisms that obtain their energy by oxidation of inorganic compounds are called chemolithotrophs. Chemolithotrophic organisms get all types of cellular carbon from CO2 and for this reason they are capable to grow in the absence of organic compounds and lights.

Forms of Chemolithotrophy – Hydrogen Bacteria

Many bacteria and archaea are involved in this group.

• The y are usually considered as autotroph when living on H2
• They fix CO2 by Calvin cycle
• They are called facultative chemolithoautotroph
• They prefer e glucose during metabolism  if available.

                                                                     Source: Brock Biology of Microorganisms, 13 ed.

                                                                                Source: Brock Biology of Microorganisms, 13 ed.

Forms of Chemolithotrophy – Sulfur Bacteria

• Oxidation of hydrogen sulfide (H2S), elementary sulfur (S) or thiosulfate (S2O32-) to sulfate (SO42-)
• Wide-spread among bacteria (e.g. Beggatonia) and archaea (e.g. Sulfolobus spp.)
• Occurs in stages:-

1. Either separate enzymatic steps
2. Or Sox complex (oxidises H2S to SO42- in one step)

                                                     Source: Brock Biology of Microorganisms, 13 ed.

                                                                                    Source: Brock Biology of Microorganisms, 13 ed.

Forms of Chemolithotrophy – Iron Oxidising Bacteria

Many bacteria of Acidophiles group are involved, such as

• Acidothiobacillus
• ferrooxidans or
• Leptospririllum ferrooxidans
• At the interphase of anaerobic and aerobic conditions, e.g. Gallionella ferruginea or Sphaerotilus natans
• Form brown deposits of ferric iron

                                                                               Source: Brock Biology of Microorganisms, 13 ed.

The Biochemistry of Chemolithotrophic iron oxidation

• Fe2+ oxidises spontaneously to Fe3+in the presence of oxygen at pH > 1
• Iron oxidisers live
• Either at very low pH (<1; extreme acidophiles)
• Or at the interphase between anaerobic and aerobic conditions

Forms of Chemolithotrophy – Nitrification

• Oxidation of ammonia (NH3) and nitrite (NO2-) to nitrate (NO3-)
• By specialised bacteria (rarely archaea)
• Two groups:

1. One group oxidises ammonia to nitrite (e.g. Nitrosomonas spp.)
2. The other group oxidises (e.g. Nitrobacter) nitrite to nitrate

• Autotrophs

1. Using Calvin cycle they do metabolism
2. Requires reverse electron flow to reduce NAD+ to form NADH for anabolism

                                                                                   Source: Brock Biology of Microorganisms, 13 ed.