This paper presents a synthetic review of studies conducted recently and consolidated by other literature insights that substantiate a multilayer structured model of the population organization in the anaerobic granule. Syntrophic bacteria associations would be located between an external, predominantly acidogenic, layer including also H2-consuming bacteria, and an aceticlastic core. This was evidenced conjointly by electron microscopic observations of cross-cleaved granules, by the specific activity analysis of stepwise abraded granules and by the changes in the specific metabolic activities with granule size distribution and enlargement. Such a structure was deduced from observations of propionate degradation in spite of endergonic conditions in the bulk liquid, of the two-step transfer of CH4 and H2 from liquid to the gas bubbles and from a substrate and product diffusion model which differentiates active zones for trophic groups along the granule radius. Such a structured aggregate is a stable metabolic arrangement that creates optimal nutritional conditions for all its members.

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