  The ubiquity of modules in biological networks may result from an
evolutionary benefit of a modular organization. For instance, modularity may
increase the rate of adaptive evolution, because modules can be easily combined
into new arrangements that may benefit their carrier. Conversely, modularity
may emerge as a by-product of some trait. We here ask whether this last
scenario may play a role in genome-scale metabolic networks that need to
sustain life in one or more chemical environments. For such networks, we define
a network module as a maximal set of reactions that are fully coupled, i.e.,
whose fluxes can only vary in fixed proportions. This definition overcomes
limitations of purely graph based analyses of metabolism by exploiting the
functional links between reactions. We call a metabolic network viable in a
given chemical environment if it can synthesize all of an organism's biomass
compounds from nutrients in this environment. An organism's metabolism is
highly versatile if it can sustain life in many different chemical
environments. We here ask whether versatility affects the modularity of
metabolic networks.
