The influence of microbial community structure and function on community-level physiological profiles
Patterns of carbon source utilization, or community-level physiological profiles (CLPP), produced from direct incubation of environmental samples in BIOLOG microplates can consistently discriminate spatial and temporal gradients within microbial communities. While the resolving power of the assay appears significant, the basis for the differences in the patterns of sole carbon source utilization among communities remains unclear. Carbon source utilization as measured in this assay is a measure of functional potential, rather than in situ activity, since enrichment occurs over the course of incubation, which can range from 24 to 72 hours (or even longer) depending on inoculum density. The functional profile of a community could be an indicator of carbon source availability and concomitant selection for specific functional types of organisms. A more limited view of the profile is as a composite descriptor of the microbial community composition without any ecologically relevant functional information. We manipulated microbial community structure and function in laboratory microcosms to evaluate their influence on CLPP. The structure of rhizosphere communities was controlled by inoculating axenic plants (wheat and potato) with different mixed species (non-gnotobiotic) inocula. Inoculum source influenced CLPP more strongly than plant type, indicating that CLPP primarily reflected differences in microbial community structure than function. In order to more specifically examine the influence of microbial function on CLPP, specific carbon sources in the BIOLOG plates (asparagine and acetate) were added to a continuously stirred tank reactor (CSTR) containing a mixed community of microorganisms degrading plant material. Daily additions of these carbon sources at levels up to 50% of the total respired carbon in the bioreactor caused significant changes in overall CLPP, but caused no, or minor, increases in the specific response of these substrates in the plates. These studies indicate that the functional relevance of CLPP should be interpreted with caution.