Photosynthesis in Bacteria

Page 1: Phototrophy Fundamentals

• Phototrophy

  • Involves light-dependent reactions where photoexcited electrons (e-) are harnessed for cell growth.

  • Electrons are transferred through the Electron Transport System (ETS) to pump protons.

  • Photoreceptors absorb light, exciting e- to a higher orbital, which return to the ground state, releasing energy.

  • High concentrations of photoreceptors are located in cellular membranes.

  • The process involves energy storage from the absorption and relaxation of light-absorbing molecules, creating a Proton Gradient.

Page 2: Reaction Center and Electron Carriers

• The Reaction Center delivers electrons through various carriers to the ETS.

• Resulting electrons generate significant energy carriers, namely NADH and NADPH.

• An electrochemical gradient is established across the photosynthetic membrane.

• ATP is produced through a process called photophosphorylation.

Page 3: Trapping Sunlight in Bacteria

• Thylakoids: Membranes in purple bacteria and cyanobacteria folded into oval pockets to enhance photon trapping.

• ATP synthase F1 knob faces outward in photosynthetic organelles, creating a more negative proton potential in the cytoplasm (stroma).

• This arrangement drives protons through ATP synthase, resulting in ATP generation.

Page 4: Phototrophy Components

• Antenna System:

  • A complex of chlorophylls captures photons and transfers energy among photopigments.

  • Key pigments include:

    • Bacteriochlorophyll (Green, Purple, Red)

    • Carotenoids (Orange, Red, Yellow)

    • Chlorophylls (Green)

    • Phycocyanins (Blue)

    • Phycoerythrins (Red)

• Energy from these pigments is transferred to the Reaction Center Complex, consisting of Photosystem I and II.

• Upon photon absorption, an electron is separated from chlorophyll, replaced by H2S (Photosystem I) or from the ETS (Photosystem II).

Page 5: Cyanobacteria Characteristics

• Cyanobacteria:

  • Oxygen-producing, green bacteria due to chlorophyll presence.

  • Classified as Phototrophic Autotrophs with varied sizes.

  • Involved in light reactions of photosynthesis where photoexcitation leads to water splitting and electron release.

  • Electrons enter ETS, generating proton potential that drives ATP Synthase functionality.

  • Purple Sulfur Bacteria use H2S for electron acquisition.

Page 6: Light Absorption by Chlorophyll

• Chromophore:

  • Functions as a light-absorbing electron carrier.

  • Chlorophyll absorbs red and blue light while reflecting green.

  • Types of Bacteria:

    • Cyanobacteria

    • Rhodobacter (Purple-Sulfur Bacteria): absorbs far red to UV light, ensuring efficiency in light capturing missed by other organisms.

Page 7: Electron Transport Systems (ETS)

• Three Different ETS Systems:

  • Anaerobic Photosystem I:

    • Receives electrons from H2S, HS-, H2, or reduced iron (Chlorobia sp).

  • Anaerobic Photosystem II:

    • Returns electrons from ETS to bacteriochlorophyll.

  • Oxygenic Z Pathway:

    • Harnesses two pairs of electrons from water to produce O2. Found in cyanobacteria and chloroplasts.

Page 8: Detailed Electron Transport Process

• Involves H2O photolysis, where electrons flow from PSII to PSI, releasing O2 from water.

• Oxygenic Photosynthesis:

  • H2S and thiosulfate can act as electron donors during anoxygenic photosynthesis where oxygen isn't a byproduct.

• Each photoexcited electron enters ETS;

  • PSI: Electrons transferred to NADP+;

  • PSII: Drives H+ pump activation for ATP synthesis.

• The oxygenic Z pathway synthesizes both NADPH and ATP, vital for carbon fixation.

Page 9: Calvin Benson Cycle

• Represents light-independent reactions occurring in the cytoplasm of bacteria.

• Three Stages of the Cycle:

  • Carbon Fixation:

    • Ribulose bisphosphate carboxylase (Rubisco) fixes CO2 with Ribulose Bisphosphate (RuBP).

  • Carbon Reduction:

    • Six ATP and NADPH convert 3-PGA to G3P, ultimately leading to glucose production.

  • Regeneration:

  • Remaining G3P regenerates RuBP, consuming three more ATP.

Page 10: Rhodopsin-Based Phototrophy

• Used by various bacteria and archaea, this phototrophy type involves rhodopsin, a membrane protein acting as a light-driven proton pump.

• Generates a proton motive force without involving an electron transport chain.

Page 11: Photophosphorylation in Purple Bacteria

• Key components and processes:

  • P870: The reaction center for electron transfer.

  • Photophosphorylation:

    • Involves ATP generation from ADP + P through suitable electron flow in cellular components (e.g., Q, bc, Fe-S)

    • Facilitates reverse electron flow during photoautotrophic growth.

Page 12: Classification of Photosynthetic Bacteria

• Oxygenic Photosynthetic Bacteria:

  • Cyanobacteria using H₂O as an electron donor.

• Anoxygenic Photosynthetic Bacteria:

  • Use organic and inorganic electron donors.

  • Includes various types (Purple, Green, etc.), showcasing their adaptability to different environmental conditions.

Page 13: Winogradsky Column Overview

• A classic demonstration of prokaryotic metabolic diversity.

• Organisms categorized by carbon and energy source:

  • Phototrophs obtain energy from light;

  • Chemotrophs from chemical oxidations.

  • Carbon sources include CO2 for autotrophs or organic compounds for heterotrophs.

• Illustrates microbial microsite occupation based on environmental tolerances and requirements, plus nutrient cycling.

Page 14: Winogradsky Column Zonation

• Key Gradients:

  • Varying levels of sulfide and oxygen from aerobic to anaerobic conditions.

• Represents different bacteria types seen:

  • Cyanobacteria,

  • Purple and Green Non-Sulfur Bacteria,

  • Purple Sulfur Bacteria,

  • Green Sulfur Bacteria,

  • Various iron oxidizing bacteria and sulfur-reducing bacteria.