Phototrophy: Principles, Physiology, and Diversity in Microorganisms

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Phototrophy

Introduction

Phototrophy is the process by which organisms capture light energy and convert it into chemical energy for cellular activities. This process is fundamental to many microorganisms and is a key driver of global biogeochemical cycles.

Learning Objectives

Understand parallels and differences between respiration and phototrophy.
Learn about oxygenic and anoxygenic phototrophy and their environmental impacts.
Explore the general physiology of phototrophs, including cyclic and non-cyclic electron flow.
Identify major lineages of phototrophic microorganisms and their special attributes.
Understand rhodopsin-based light-harvesting pigments.

Physiological Groups of Life

Energy Sources and Classification

Organisms are classified by their energy sources:

Chemotrophs: Use chemicals (organic or inorganic) for energy.
Phototrophs: Use light as an energy source.

Further classification by electron donor:

Organotrophs: Organic electron donors.
Lithotrophs: Inorganic electron donors (e.g., H2, H2S, Fe2+).

Phototrophs use light to drive ATP synthesis and carbon fixation.

Common Features of Phototrophs

General Characteristics

Phototrophs are pigmented and contain chlorophyll or related pigments.
They possess specialized membrane structures for photosynthesis.
Light-harvesting complexes are associated with membrane structures.
Membrane invaginations or vesicles provide increased surface area for energy harvest.
Photosynthetic membranes differ among groups (e.g., thylakoids, chromatophores, chlorosomes).

Mechanisms of Energy Conversion

Light energy is used to generate ATP via electron transport chains (ETC).
Electron flow can be cyclic or non-cyclic, depending on the type of phototrophy.
ATP synthesis is driven by a proton motive force (PMF) and ATPase.

Phototrophy Terms & Concepts

Key Definitions

Phototrophy: The use of light as an energy source.
Photosynthesis (photoautotrophy): Biochemical reactions where ATP is synthesized by light-driven reactions and CO2 is fixed into cell material.
Photophosphorylation: ATP production from light energy.
CO2 fixation: Reduction of CO2 to cell material for growth.

Chloroplasts and Endosymbiosis

Chloroplasts are organelles in plants and algae responsible for photosynthesis. Cyanobacteria are close relatives of chloroplasts, and endosymbiotic theory suggests that chloroplasts and mitochondria originated from such organisms.

Overview of Phototrophic Physiology

Electron Donors and Acceptors

Electron donors can be organic (e.g., glucose) or inorganic (e.g., H2S, Fe2+).
Electron acceptors include O2 (in respiration) or other molecules in phototrophy.

General Reaction

Example of aerobic respiration:

Types of Phototrophy

Oxygenic Phototrophy

Occurs in cyanobacteria and chloroplasts; produces molecular oxygen (O2).

Uses water as electron donor.
Involves two photosystems (PSI and PSII).
Major process for global oxygen production and carbon fixation (Calvin Cycle).

Anoxygenic Phototrophy

Occurs in various bacteria; does not produce O2.

Uses electron donors like H2S, Fe2+, or organic compounds.
Involves a single photosystem.
Found in environments where O2 is absent.

Electron Flow in Phototrophy

Cyclic Electron Flow

Electrons cycle back to the reaction center.
Generates ATP but not reducing power (NAD(P)H).

Non-Cyclic Electron Flow

Electrons flow from donor to acceptor, generating both ATP and reducing power.
Required for carbon fixation.

Major Lineages of Phototrophic Microorganisms

Cyanobacteria

Oxygenic photoautotrophs; some can also be photoheterotrophs.
Dominant in marine and freshwater environments.
Important genera: Synechococcus, Prochlorococcus, Oscillatoria, Anabaena.

Purple Sulfur Bacteria

Use H2S as electron donor; found in anoxic, illuminated environments.
Example genera: Chromatium.

Purple Non-Sulfur Bacteria

Use organic compounds or H2 as electron donors.
Example genera: Rhodospirillum, Rhodopseudomonas.

Green Sulfur Bacteria

Strictly anaerobic; use H2S as electron donor.
Example genera: Chlorobium, Pelodictyon.

Green Non-Sulfur Bacteria

Filamentous; use organic compounds as electron donors.
Example genera: Chloroflexus, Roseiflexus.

Light-Harvesting Pigments

Chlorophylls and Bacteriochlorophylls

Tetrapyrrole molecules coordinating Mg2+.
Absorb light at specific wavelengths; diversity in R groups leads to different absorption spectra.
Found in reaction centers and antenna complexes.

Carotenoids

Conjugated double bond systems; absorb light and transfer energy to reaction centers.
Protect against reactive oxygen species (antioxidants).

Phycobilins

Main antenna pigments in cyanobacteria and red algae.
Protein-bound, form aggregates near reaction centers.
Increase sensitivity to low light.

Rhodopsin-Based Light Harvesting

Bacteriorhodopsins are light-driven proton pumps found in some halophilic archaea and bacteria.
Enable light energy capture without chlorophyll-based photosystems.

Light-Harvesting Membrane Structures

Thylakoids in cyanobacteria.
Chromatophores in purple bacteria.
Chlorosomes in green bacteria.
Lamellae in Chloroflexus.

Summary Table: Types of Phototrophy

Type	Electron Donor	Oxygen Production	Representative Organisms
Oxygenic	H2O	Yes	Cyanobacteria, chloroplasts
Anoxygenic (Purple Sulfur)	H2S	No	Chromatium
Anoxygenic (Purple Non-Sulfur)	Organic compounds, H2	No	Rhodospirillum
Anoxygenic (Green Sulfur)	H2S	No	Chlorobium
Anoxygenic (Green Non-Sulfur)	Organic compounds	No	Chloroflexus
Rhodopsin-based	None (light-driven proton pump)	No	Halobacterium

Key Equations

General photosynthesis reaction:

ATP synthesis via photophosphorylation:

Additional info:

Phototrophy is a major driver of primary production in aquatic and terrestrial ecosystems.
Different phototrophic lineages have adapted to specific light environments and electron donors, contributing to microbial diversity.