Basic Energy Yielding Mechanism of Chemoautotrophic & Photoautotrophic Bacteria
• Use energy from inorganic chemicals
• Energy is used in the Calvin-Benson cycle to fix CO2
• Thiobacillus ferrooxidans
ADP + P ATP
Unlike those of heterotrophs, the fueling reactions of autotrophs, which obtain all or nearly all
of their cellular carbon from CO2, occur in two biochemically distinct phases1) that leading to
the synthesis of precursor metabolites, and (2) that leading to the synthesis of ATP and reduced
pyridine nucleotides. One path way of synthesis of precursor metabolites, the Calvin-Benson
cycle, is shared by most phototrophs and chemoautotrophs.
•The Calvin – Benson cycle: Synthesis of precursor metabolites.
Most autotrophs capture (Fix) CO2 by a reaction catalysed by ribulose diphosphate
carboxylase. The phosphorylated pentose ribulose – 1-5 diphosphate accepts one molecule of
CO2 and is simultaneously cleaved yielding two molecules of glyceric acid – 3 – phosphate;
the carboxyl group of one glyceric acid – 3 – phosphate molecule is thus derived from CO2.
The Primary product of CO2 fixation via the Calvin –Benson cycle is glyceric acid – 3-
phosphate from which all precursor metabolites are synthesized. However CO2 fixation is
depended on a supply of ribulose diphosphate for which most of the glyceric acid is utilized.
For simplicity of analysis the Calvin – Benson cycle can be divided in to three phases: CO2
fixation, reduction of fixed CO2and regeneration of the CO2 acceptor.
• Generation of ATP and reduced pyridine nucleotide by chemoautotrophs.
Chemoautotrophs obtain ATP and reducing power by the oxidation of inorganic compounds. The
substrate that can serve as energy sources are H2, CO, NH3, NO2-
, and reduced sulphur
compounds (H2S, S, S2O32-
). In this mode of respiratory metabolism, electron from these compounds
are passed through an electron transport chain that generates ATP by the mechanism that
functioning heterotrophs: Generation of proton motive forces that drives ATP synthesis by a
membrane - located ATP phosphorylase. The terminal electron acceptor of the chain of
chemoautotrophs is usually O2.
Electrons derived from the oxidation of nitrite (NO2-
) to Nitrate(NO3-
) flow in to a branched electron transport chain
composed of various cytochromes (Cyta1, Cyta3, Cytb and CytC) and a flavoprotein(FP). Those that flow in the
thermodynamically favorable (forward) direction generate a proton motive force that is utilized in part to drive other electrons
in the thermodynamically unfavorable (reverse) direction to reduce NAD+
Phototrophs use sunlight to produce ATP through phosphorylation, referred as
photophosphorylation. The phototrophs convert the light energy to chemical
energy (ATP) through the process called photosynthesis.
Photosynthesis is the process by which plants and certain bacteria, termed
phototrophs, convert radiant energy in the form of light into metabolic energy and
For conversion of light energy to ATP, the bacteria possess light harvesting
pigments. They are chlorophyll a, carotenoids, phycobiliproteins (which are
present in cyanobacteria) and bacteriochlorophyll (which are present in purple
sulphur bacteria). In bacteria, there are two types of light reactions (conversion of
light to ATP) and two types of CO2 fixation occur.
For photophosphorylation, light harvesting pigments, a membrane electron transport chain, source of electron (electron
donor) and ATPase enzymes are required. Two types of photophosphorylations occur during photosynthesis. They are
cyclic photophosphorylation and non-cyclic photophosphorylation.
•In plant and cyanobacteria, both cyclic and non-cyclic photophosphorylation occurs whereas in purple bacteria, the
cyclic photophosphorylation only occurs.
•In plant and cyanobacteria, the electron source is water, by photolysis, H2O split into H+ and O2 and during the process, O2
is evolved and referred as oxygenic photosynthesis
•Since, the sulphur bacteria is an anaerobic bacterium, they use H2S instead of H2O as electron donor. Since, there won’t
be any O2 evolution during photosynthesis, referred as anoxygenic photosynthesis.
1. The oxygenic photophosphorylation
The end product of the light reaction is ATP, NADPH and O2. The ATP and NADPH, the energy and electron sources
thus produced were used for dark reaction.
2. The anoxygenic photo phosphorylation
The anoxygenic photo phosphorylation will take palce as in the image and the end product of the light reaction is ATP,
NADPH and Sulphur. The ATP and NADPH, the energy and electron sources thus produced were used for dark