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.

2. Chemolithotrophs 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.

3. 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.

4. Chemolithotrophs Hydrogen Oxidizers: Sulfur Oxidizers: Iron 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-)

5. 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.