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Chemotrophs & Phototrophs

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 (H 2 O, H 2 S, organic).

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Chemotrophs & Phototrophs

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  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: • 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.

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