These determine whether a metabolism is possible, but they are not automatically primary energy sources.
Operational definition
Context category.
The externally imposed physical, chemical, and biological conditions that determine which energy-yielding reactions, transport processes, and biological activities are possible, favorable, or sufficiently rapid. These include temperature, pressure, pH, salinity, water activity, radiation, substrate availability, electron donors, electron acceptors, competitors, hosts, prey, and symbiotic partners.
A boundary condition affects energetic feasibility or cost but is not necessarily itself an energy source. High temperature, for example, changes reaction kinetics and protein stability without automatically providing metabolically usable energy.
1.1 Radiation conditions
- spectral composition
- intensity
- duration
- direction
- periodicity
- attenuation
1.2 Chemical conditions
- available electron donors
- available electron acceptors
- redox potential
- substrate concentration
- product concentration
- pH
1.3 Physical conditions
- temperature
- pressure
- salinity
- water activity
- viscosity
- flow
- mechanical disturbance
1.4 Biological conditions
- prey availability
- host availability
- detritus availability
- symbiotic partners
- competitors
- extracellular metabolites
- conductive biological structures
Temperature, pressure, salinity, and mechanical flow profoundly alter energetic feasibility and cost, but they should not normally be classified alongside light and chemical free energy as independent trophic sources.