Microbial Metabolism: Electron Transport, Respiration, and Photosynthesis

Introduction

This study guide covers key aspects of microbial metabolism, focusing on the electron transport chain (ETC), types of respiration (aerobic and anaerobic), fermentation, and the mechanisms and diversity of photosynthesis in microbes. It also introduces unique metabolic pathways such as acetogenesis and methanogenesis. Understanding these processes is fundamental to microbiology, as they underpin microbial energy generation, ecological roles, and evolutionary history.

Electron Transport Chain (ETC) in Microbes

Overview of the Electron Transport Chain

• The electron transport chain (ETC) is a series of protein and non-protein carriers embedded in the cytoplasmic membrane (in prokaryotes) or mitochondrial/chloroplast membranes (in eukaryotes).

• Electrons are transferred from electron donors (e.g., NADH, FADH2, or inorganic molecules) through a series of carriers, ultimately reducing a terminal electron acceptor.

• As electrons move through the chain, some carriers pump protons across the membrane, creating a proton motive force (PMF).

• The PMF drives ATP synthase to produce ATP from ADP and inorganic phosphate (Pi) by oxidative phosphorylation:

Types of Electron Donors and Acceptors

• Chemo-organotrophs: Use organic compounds (e.g., glucose, NADH) as electron donors. Can perform respiration (aerobic or anaerobic) or fermentation.

• Chemolithotrophs: Use inorganic compounds (e.g., H2, Fe2+, NH3) as electron donors. Only perform respiration (aerobic or anaerobic), not fermentation.

• Aerobic respiration: Oxygen (O2) is the terminal electron acceptor.

• Anaerobic respiration: Other molecules (e.g., nitrate, sulfate) serve as terminal electron acceptors.

Key Electron Carriers

• Protein carriers: Flavoproteins (e.g., FMN), iron-sulfur proteins, cytochromes.

• Non-protein carriers: Quinones (ubiquinone, menaquinone), plastoquinones (in photosynthetic organisms).

Examples of Microbial ETCs

• Paracoccus denitrificans: Chemo-organotroph capable of both aerobic and anaerobic respiration. Uses NADH as electron donor; O2 (aerobic) or NO3- (anaerobic) as acceptor.

• Escherichia coli: Similar to Paracoccus; can use nitrate reductase for anaerobic respiration.

• Pseudomonas stutzeri: Capable of denitrification, reducing nitrate all the way to dinitrogen gas (N2).

• Iron-oxidizing bacteria: Use Fe2+ as electron donor; O2 as acceptor.

ATP Synthesis and Proton Motive Force

• Protons are pumped out of the cytoplasm (or into the periplasm in Gram-negative bacteria), creating a gradient.

• ATP synthase allows protons to flow back, coupling this movement to ATP synthesis.

Reverse Electron Flow

• Some chemolithotrophs use reverse electron flow to generate NADH for biosynthetic reactions (e.g., Calvin-Benson cycle).

Fermentation vs. Respiration

Fermentation

• Occurs when no suitable terminal electron acceptor is available for respiration.

• ATP is produced by substrate-level phosphorylation during glycolysis or similar pathways.

• Yields much less ATP per glucose (typically 1–2 ATP) compared to respiration.

• End products vary: e.g., ethanol and CO2 (alcoholic fermentation by Saccharomyces), lactic acid (homolactic fermentation).

Respiration

• More efficient; yields up to ~38 ATP per glucose (in prokaryotes).

• Requires an electron transport chain and a terminal electron acceptor (O2 or alternatives).

Comparison Table: Fermentation vs. Respiration

Photosynthesis in Microbes

Introduction to Microbial Photosynthesis

• Photosynthesis is the process by which light energy is converted into chemical energy (ATP, NADPH), enabling CO2 fixation into organic molecules.

• Microbes perform more photosynthesis globally than plants, especially in aquatic environments.

Photosynthetic Pigments

• Chlorophylls: Main pigments in plants, algae, and cyanobacteria; absorb mainly red and blue light.

• Bacteriochlorophylls: Found in photosynthetic bacteria; absorb different wavelengths than plant chlorophylls.

• Carotenoids: Accessory pigments; absorb in the blue-green region, provide photoprotection (antioxidant function).

• Phycobilins: Found in cyanobacteria and red algae; absorb in the green to orange region, giving cells a blue-green color.

Location of Pigments

• In eukaryotes: Located in chloroplast thylakoid membranes.

• In prokaryotes: Located in cytoplasmic membrane, thylakoid-like membranes (e.g., cyanobacteria), or chlorosomes (specialized structures in some bacteria).

Endosymbiotic Origin of Chloroplasts

• Chloroplasts are believed to have evolved from ancestral cyanobacteria via endosymbiosis.

• Evidence: Similarities in DNA, ribosomes, and internal membrane structures (thylakoids).

Types of Photosynthesis

• Oxygenic photosynthesis: Produces O2 as a byproduct; uses water as the electron donor. Performed by plants, algae, and cyanobacteria.

• Anoxygenic photosynthesis: Does not produce O2; uses other inorganic molecules (e.g., H2S, Fe2+) as electron donors. Performed by various photosynthetic bacteria.

Photosynthetic Apparatus

• Reaction centers: Specialized pigment-protein complexes where light energy is converted to electron flow.

• Antenna pigments: Collect and transfer light energy to reaction centers (also called light-harvesting complexes).

Absorption Spectra

• Different pigments absorb different wavelengths, allowing microbes to utilize a broad range of light energy.

• Having multiple pigments increases the efficiency of light capture.

Mechanisms of Photosynthetic Electron Transport

Anoxygenic Photosynthesis

• Light excites the reaction center (e.g., P870 in purple bacteria), initiating electron flow through carriers (bacteriopheophytin, quinones, cytochromes).

• Electron flow is typically cyclic: electrons return to the reaction center after passing through the chain.

• Protons are pumped across the membrane, generating a proton motive force for ATP synthesis (photo-oxidative phosphorylation).

• Electron donors are inorganic molecules other than water (e.g., H2S, Fe2+).

• Occasionally, electrons are diverted to reduce NAD+ to NADH for biosynthesis (requires reverse electron flow).

Oxygenic Photosynthesis

• Involves two photosystems (Photosystem II [P680] and Photosystem I [P700]).

• Light excites P680, which extracts electrons from water, releasing O2 as a byproduct.

• Electrons pass through a chain (including plastoquinones), generating a proton gradient in the thylakoid lumen.

• Light also excites P700, further energizing electrons, which are ultimately used to reduce NADP+ to NADPH.

• ATP is synthesized as protons flow back into the stroma via ATP synthase.

• Electron flow is non-cyclic, but can become cyclic under certain conditions (no O2 produced).

Comparison Table: Anoxygenic vs. Oxygenic Photosynthesis

Specialized Microbial Metabolisms: Acetogenesis and Methanogenesis

Acetogenesis

• Performed by acetogens, mainly bacteria.

• Fix CO2 using H2 as the electron donor to produce acetate:

• ATP is generated by substrate-level phosphorylation and sometimes by sodium motive force (Na+ gradient) instead of proton motive force.

Methanogenesis

• Performed by methanogens (archaea), strictly anaerobic.

• Reduce CO2 with H2 to produce methane (CH4):

• ATP is generated via proton or sodium motive force.

• Ecological significance: Major source of methane in animal guts (e.g., cows, termites, humans).

Summary of Key Terms

• Chemo-organotroph: Organism that uses organic compounds as energy and electron sources.

• Chemolithotroph: Organism that uses inorganic compounds as energy and electron sources.

• Phototroph: Organism that uses light as an energy source.

• Autotroph: Organism that uses CO2 as its carbon source.

• Heterotroph: Organism that uses organic compounds as its carbon source.

• Fermentation: Energy-yielding metabolism without an external electron acceptor; ATP by substrate-level phosphorylation.

• Respiration: Energy-yielding metabolism with an external electron acceptor; ATP by oxidative phosphorylation.

• Denitrification: Anaerobic respiration using nitrate as the terminal electron acceptor, producing N2 gas.

• Photosystem: Protein-pigment complex that initiates electron flow in photosynthesis.

• Calvin-Benson cycle: Main pathway for CO2 fixation in autotrophs, using ATP and NADPH from photosynthesis.

Additional Info

• Magnesium is a macronutrient essential for chlorophyll and bacteriochlorophyll structure.

• Reverse electron flow is required in some chemolithotrophs and anoxygenic phototrophs to generate reducing power (NADH/NADPH) for biosynthesis.

• Endosymbiotic theory explains the origin of chloroplasts from cyanobacteria-like ancestors.

• Fermentation is a last-resort energy strategy for facultative organisms when no external electron acceptor is available.