2–3 — PRIMARY FREE-ENERGY SOURCE
The Primary External Free-Energy Source identifies the environmental disequilibrium from which the living system obtains the usable free energy that drives its metabolism.
Operational definition
Layer type: Fundamental source layer Governs: Items 2–3
Formal definition
In the present framework, the two firmly established primary source classes are:
- radiant free energy;
- chemical free energy.
Governing question
What external disequilibrium ultimately provides the energetic drive for this metabolism?
Radiant source
Radiant free energy is used when absorbed photons produce a photochemical event that contributes materially to:
- charge separation;
- electron transport;
- ion pumping;
- reducing-power generation;
- ATP production;
- biosynthesis.
Light used only for vision, phototaxis, circadian regulation, or signaling does not make an organism a phototroph.
Chemical source
Chemical free energy is used when an exergonic transformation supplies the energetic drive.
This commonly involves:
electron donor
+
electron acceptor
→
lower-free-energy products
The chemical substrates may be:
- organic and biogenic;
- inorganic and geochemical;
- generated by another organism;
- obtained from an electrode or conductive material;
- produced internally from previously acquired reserves.
Inclusion rule
A source belongs here only when there is a causal pathway from that source to a biologically conserved energetic state, such as:
- ion motive force;
- ATP;
- reducing power;
- activated chemical intermediates.
Mere exposure is insufficient.
Multiple-source rule
An organism may use more than one source:
- simultaneously;
- sequentially;
- conditionally;
- at different developmental stages.
The classification should therefore record:
- dominant source;
- supplementary source;
- switching conditions;
- approximate relative contribution where measurable.
Exclusion rule
This layer does not identify:
- how the source crosses the boundary;
- whether electrons come from organic or inorganic donors;
- where biomass carbon comes from;
- where electrons terminate;
- what conserved energetic product is formed.
Those questions belong to later layers.
2. Radiant free energy
2.1 Photonic input
hν → molecular excitation
Light is converted into a chemically or electrochemically conserved state.
2.2 Reaction-center phototrophy
- oxygenic photosynthesis
- anoxygenic photosynthesis
- cyclic photophosphorylation
- noncyclic electron flow
2.3 Retinal-based phototrophy
- proton-pumping rhodopsins
- sodium-pumping rhodopsins
- chloride-pumping systems
- supplementary light-driven energy capture
The light-driven system may supply ATP, reducing power, or merely reduce the organism’s chemical-energy demand. Not all phototrophs fix carbon.
3. Chemical free energy
reactants → lower-free-energy products
3.1 Organic or biogenic chemical substrates
Chemical free energy previously concentrated in biomass or biological products:
- carbohydrates
- lipids
- amino acids
- organic acids
- hydrocarbons
- methane
- alcohols
- dead organic matter
- host-derived compounds
- partner-derived metabolites
3.2 Inorganic or geochemical substrates
- molecular hydrogen
- reduced sulfur compounds
- ferrous iron
- ammonia
- nitrite
- carbon monoxide
- reduced minerals
- other inorganic electron donors
3.3 Mixed chemical substrate use
An organism uses combinations of organic and inorganic donors according to environmental conditions.
3.4 Chemical disproportionation
One compound or oxidation state is divided into both more oxidized and more reduced products, allowing the organism to obtain free energy without the simple donor–acceptor arrangement used in conventional respiration.
4. Direct extracellular electron access
This is a specialized access route into chemical/redox free energy, not a wholly separate physical form of energy.
4.1 Direct electron uptake
Electrons enter from:
- an electrode
- conductive mineral
- metallic surface
- another cell
- a conductive biological matrix
4.2 Mediated electron uptake
Electrons arrive through:
- redox mediators
- hydrogen generated at a surface
- formate
- soluble electron shuttles
- extracellular enzymes
4.3 Extracellular electron export
Electrons leave the organism toward:
- insoluble minerals
- anodes
- conductive partners
- extracellular electron acceptors
Microorganisms that take up extracellular electrons are commonly termed electrotrophs, while extracellular electron transfer can also operate in the opposite direction toward external acceptors. (PubMed Central (PMC))