The network of chemical reactions that maintains a living system, supplies energy and builds or processes its material. Gas emissions can arise as primary metabolic byproducts or secondary metabolic byproducts, with different implications for an atmospheric biosignature gas.
Conversion of light energy into chemical energy used to build biological material. Oxygenic photosynthesis uses water as an electron donor and releases molecular oxygen; other forms do not necessarily produce oxygen. Its atmospheric oxygen source is a major component of the modern terrestrial atmospheric biosignature gas interpretation.
A compound released through specialized functions such as signaling, defense or stress adaptation rather than the core reactions needed for energy and biomass. Dimethyl sulfide and chloromethane are examples discussed as atmospheric biosignatures. Their specialized origins may improve specificity while lowering the global emission flux.
A product released by reactions needed for energy acquisition, growth or biomass construction. Methane from energy-yielding microbial reactions and molecular oxygen from oxygenic photosynthesis are examples. Abiotic chemistry can make some of the same molecules, so production mechanism and environmental abundance matter.
