Impact of dilution on microbial community structure and functional potential: Comparison of numerical simulations and batch culture experiments

A series of microcosm experiments was performed using serial dilutions of a sewage microbial community to inoculate a set of batch cultures in sterile sewage. After inoculation, the dilution-defined communities were allowed t o regrow for several days and a number of community attributes were measure d in the regrown assemblages. Based upon a set of numerical simulations, co mmunity structure was expected to differ along the dilution gradient;the gr eatest differences in structure were anticipated between the undiluted-low- dilution communities and the communities regrown from the very dilute (more than 10(-4)) inocula. Furthermore, some differences were expected among th e lower-dilution treatments (e.g., between undiluted and 10(-1)) depending upon the evenness of the original community. In general, each of the proced ures used to examine the experimental community structures separated the co mmunities into at least two, often three, distinct groups. The groupings we re consistent with the simulated dilution of a mixture of organisms with a very uneven distribution. Significant differences in community structure we re detected with genetic (amplified fragment length polymorphism and termin al restriction fragment length polymorphism), physiological (community leve l physiological profiling), and culture-based (colony morphology on R2A aga r) measurements. Along with differences in community structure, differences in community size (acridine orange direct counting), composition (ratio of sewage medium counts to R2A counts, monitoring of each colony morphology a cross the treatments), and metabolic redundancy (i.e., generalist versus sp ecialist) were also observed, suggesting that the differences in structure and diversity of communities maintained in the same environment can be mani fested as differences in community organization and function.