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Methylotrophy is the ability to grow using reduced carbon compounds with no carbon–carbon bonds, such as methanol, as carbon and energy sources. It requires both energy-yielding transformations and carbon assimilation into biomass. Facultative methylotrophs can also use other carbon sources; the historical description of Methylobacterium explicitly distinguished this metabolic flexibility.^[ref]

The two carbon fates are different. Complete methanol oxidation can be represented byThis balanced equation accounts for carbon oxidized to carbon dioxide, not for net cell production. A growing methylotroph must retain carbon in cellular material as well. In the studied Methylobacterium pathway, formaldehyde is an intermediate at the junction of oxidation and assimilation routes.^[ref]

A2005 plant-colonization experiment by Abdoulaye Sy and colleagues compared wild-type bacteria with mutants defective in methylotrophy. The mutants were disadvantaged during competition, but could reach wild-type colonization levels when introduced alone. This contrast showed that the metabolic ability mattered under the tested competitive conditions, while other usable carbon sources remained available on the plants. Growth in isolation and fitness in a microbial community therefore answered different questions.^[ref]

Microbial ecology studies how microorganisms interact with one another and their surroundings, and how those interactions affect their abundance, distribution and activities. A microbial community can be investigated as part of a soil, an aquatic ecosystem or a living host. Identifying its members, counting them and measuring what they do are related but distinct tasks.

An invented counting example makes the distinction concrete. A population increases from one hundred to two hundred cells while all other cells in the same sample increase from nine hundred to eighteen hundred. Its abundance doubles, but its fraction of the total remains $100/1000=200/2000=0.10$. A report of unchanged relative abundance could therefore conceal a large change in absolute abundance. Sampling volume and the measurement denominator must be explicit.

Molecular measurements add further interpretation. A 16S ribosomal RNA gene is a useful marker, but genomes can contain different numbers of copies and different sequences among those copies. Marker counts and sequence variants cannot automatically be equated with cell counts and distinct species.^[ref]

Experiments can move from association to intervention. In a controlled Arabidopsis thaliana system, adding particular Sphingomonas strains changed the outcome of a subsequent bacterial challenge. Comparing defined microbial conditions tested an interaction that a list of organisms on healthy leaves alone could not establish.^[ref]

The questions range from local interactions to biogeochemical cycles. A convincing ecological explanation connects the organisms present, the environmental conditions and an explicitly measured process; abundance of a genetic marker alone is not a direct rate measurement.