= Carbon source
A \b[carbon source] is a material from which an <organism> obtains carbon for its <metabolism> and growth. The carbon atoms can enter <biomass>, be transformed into other compounds, or leave through products such as <carbon dioxide>. Calling a material a carbon source therefore identifies its role in the carbon inventory; it does not mean every supplied carbon atom is retained in cells.
For an original inventory example,1millimole of <glucose>, $\mathrm{C_6H_{12}O_6}$, supplies6millimoles of carbon atoms, whereas1millimole of <methanol>, $\mathrm{CH_3OH}$, supplies1millimole. Equal concentrations measured in moles of molecules need not mean equal concentrations of carbon. Carbon supply alone also does not specify the amount of usable energy or the growth yield.
An experimental example is the2005 study of plant-associated <Methylobacterium>. Mutants unable to use <methanol> effectively were disadvantaged in competition, yet could colonize plants well when introduced alone. The researchers inferred that other carbon sources were available on the plants. A supplied substrate, the substrates an organism can use, and the substrate limiting growth are separate questions.<Methylotrophic metabolism is advantageous for Methylobacterium extorquens during colonization of Medicago truncatula under competitive conditions by Abdoulaye Sy and others>[Abstract and Figures2–3: competition and single-inoculation comparisons]{ref}
<Carbon assimilation> describes incorporation into cellular material, while <carbon fixation> concerns incorporation of inorganic carbon, such as <carbon dioxide>, into organic compounds. The carbon source and energy supply must be identified separately: for <photosynthesis>, light can supply energy while carbon comes from carbon dioxide.
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