Transcript Slide 1

Chemotrophs &
Phototrophs
• Chemoorganotrophs: reduced
inorganic electron donor for
energy and electrons.
• Chemolithotrophs: reduced
inorganic electron donor for
energy and electrons.
• Phototrophs: use light energy
and an electron donor molecule
(H2O, H2S, organic).
• Both may be autotrophs: fix
CO2 into organic carbon via the
Calvin Cycle.
Chemolithotrophs
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Electron donor molecule often unique to species.
Electron acceptor is usually O2.
Most are autotrophs using the Calvin Cycle to fix CO2.
Some can also grow heterotrophically.
Energy yield from electron donors varies, yet is always
lower than that for a glucose molecule.
Chemolithotrophs
• ΔGo’ = -686 kcal/mol glucose to CO2.
• In most cases, electrons from the donors can enter ETC
directly and yield ATP by oxidative phosphorylation.
• P/O Ratios are ≤ 1 for most (H2 the exception).
• Huge amounts of inorganics are oxidized for growth, which
can make a major impact on ecosystems.
Chemolithotrophs
• Hydrogen Oxidizers:
– Most efficient (P/O > 1); E’o H2 < E’o NADH
– Hydrogenase may donate electrons to NAD+
• Sulfur Oxidizers:
– ATP by SLP in addition to oxidative phosphorylation
– SLP is via adenosine 5’-phosphosulfate (APS)
• Iron Oxidizers
– Acidophilic Thiobacillus ferrooxidans Fe+2 → Fe+3
– Acid Mine Drainage if pyrite is exposed to O2 and H2O!
– Circumneutral Gallionella ferruginea Fe+2 → Fe+3
• Nitrifying Bacteria:
– Ammonium Oxidizers (NH4+ → NO2-)
– Nitrate Oxidizers (NO2- → NO3-)
– Process of “Nitrification” (NH4+ → NO3-)
Chemolithotrophs
• Most can’t directly reduce NAD+ to NADH (only H2 oxidizers).
• NADH is needed to convert to NADPH for anabolic reactions.
• ETC must reverse electron flow from donors with more positive E’o than
NADH; an energy source is needed for this “up-hill” reverse e- transfer.
• PMF is used for reversed electron flow, instead of making ATP.