Dilution/extinction of community phenotypic characters to estimate relative structural diversity in mixed communities

Recent interest in microbial diversity has led to increased emphasis on the development of appropriate techniques. Structural diversity encompasses the number and distribution of separate or interacting biological entities responsible for a given function within the overall set of functions of a community. This study evaluated an approach for estimating the relative degree of structural diversity in heterotrophic microbial communities by dilution to extinction of community phenotypic traits. Serial dilutions of environmental samples (rhizosphere, stream) were tested for community phenotypic traits (i.e. carbon source respiration). The non-linear relationship between the number of positive responses (i.e. functional richness or R) and inoculum density in each sample dilution (I) fit the simple rectangular hyperbola model, allowing estimation of the maximal richness (R(max)) and the inoculum density at half-maximal richness (K(I)). The later term appears to be useful in assessing relative structural diversity as evidenced by significantly higher values for communities with higher predicted species diversity. The examination of community functional characteristics across a series of dilutions, particularly in conjunction with other techniques, may be a useful approach for the study of microbial diversity and related ecological parameters such as niche width and metabolic redundancy.