5-4 - Chemolithotrophs and phototrophs

5-4: Chemolithotrophs & Phototrophs Lecture Overview

• Describes energy acquisition in chemolithotrophs and chemoorganotrophs.

• Overview and examples of chemolithotrophs, oxygenic and anoxygenic phototrophs.

• Discusses the source of reducing power for autotrophs' biosynthesis.

• Related textbook chapters: 3.11, 3.12, Chapter 14 (scattered).

Chemolithotrophs

• Definition: "Lithotroph" = rock-eater, organisms that gain energy from oxidizing inorganic molecules.

• Exclusively prokaryotic organisms.

• Habitat: Found in various environments with reduced inorganic compounds; many are extremophiles.

• Common electron donors (energy sources):

  • H2S (hydrogen sulfide)

  • H2 (hydrogen)

  • Fe2+ (ferrous iron)

  • NH4+ (ammonium)

• Electron acceptors can be aerobic (using O2) or anaerobic.

• Mostly autotrophs that fix CO2, requiring significant reducing power (NADH) for biosynthetic reactions.

Example of a Chemolithotroph: Ralstonia eutropha

• Type: Gram-negative bacterium found in soil and freshwater.

• Growth: Chemolithoautotroph on H2, CO2, and O2 under aerobic conditions (requires other nutrients).

• Enzymatic processes:

  • Produces two types of hydrogenase enzymes that split H2 into H+ and electrons.

  • Membrane-bound enzyme donates electrons to quinones in the electron transport chain (ETC), generating proton motive force (PMF) and ATP.

  • Soluble cytoplasmic enzyme reduces NAD+ to NADH, supplying reducing power for biosynthetic reactions.

Photosynthesis and Phototrophs

• Phototrophs: Use light energy to drive electron flow, generating PMF and producing ATP via photophosphorylation.

• Types of Phototrophs:

  • Oxygenic Phototrophs: Generate O2 as a byproduct (e.g., cyanobacteria, algae).

  • Anoxygenic Phototrophs: Do not produce O2 (evolved earlier); examples include green sulfur bacteria and phototrophic purple bacteria.

• Most are autotrophs, converting CO2 into organic molecules, though rare forms (photoheterotrophs) obtain carbon from organic sources.

Photosynthetic Reaction Centers

• Composed of proteins and pigments, facilitating electron excitation and transfer to the ETC.

• Light-sensitive pigments (e.g., chlorophylls for oxygenic phototrophs) absorb light energy and transfer it to reaction centers.

• Antenna pigments capture light and are embedded in the membrane, akin to heme groups of cytochromes, differing by containing magnesium (Mg) instead of iron (Fe).

Coexistence of Different Phototrophs

• Various bacteriochlorophylls allow phototrophs to occupy the same habitat, utilizing light wavelengths others cannot.

Purple Bacteria (Anoxygenic Phototrophs)

• Reaction center with bacteriochlorophyll (P870) capable of absorbing light energy, cycling electrons, and generating ATP.

• Utilizes cyclic photophosphorylation to maintain electron flow and energy production.

Reduction Potential and Electron Flow

• Different types of phototrophs use varied electron carriers; e.g., Q-type with quinones and FeS-type with Fe/S clusters.

• Not all anoxygenic phototrophs employ cyclic electron flow; some transfer electrons to external acceptors.

Reducing Power in Autotrophs

• Essential for producing NAD(P)H needed in biosynthesis reactions.

• Some autotrophs may lack adequate electron donors; they use reverse electron flow driven by PMF to generate NAD(P)H.

Oxygenic Phototrophs Features

• Comprise two photocenters: PSI (P700) and PSII (P680).

• Photosystems located in thylakoid membranes (cyanobacteria and eukaryotic chloroplasts).

• PSII extracts electrons from H2O, generates H+ and O2.

• PSI completes electron transfers to reduce NADP+ to NADPH, subsequently utilized for CO2 fixation in biosynthesis (Calvin cycle).

Electron Flow in Oxygenic Phototrophs

• Diagramming electron transitions through PSII and PSI, leading to PMF and ultimately reducing NADP+ to NADPH.