Differential response of size-fractionated soil bacteria in BIOLOG® microtitre plates

The majority of bacterial cells in soil observed by fluorescence microscopy are less than 0.4 μm in diameter, yet these cells rarely are recovered on nutrient agar. Metabolically active bacteria that are not culturable on solid media might respond in BIOLOG® microtitre plates, which contain 95 different carbon substrates. In the present study, BIOLOG® GN and GP microtitre plates were used to compare functional diversity of large (>0.45 μm) and small (<0.45 μm) cells within A and B horizons of cultivated and forested site situated on a single soil taxon (Guernsey silt-loam; fine, mixed mesic, Aquic Hapludalfs). Overall, the cultivated site exhibited greater substrate richness and average well color development compared to the forested site. The small cell fraction was numerically greater at both sites, yet exhibited limited substrate utilization compared with large cells. Greater substrate utilization by small cells was evident in A compared with B horizons at both sites. Principal component analysis separated the bacterial community by size and horizon at each study site. Small cells primarily utilized carbohydrates and carboxylic acids, compared to a broad range of substrate utilization by large cells. These data suggest that small cells are metabolically distinct from large cells. Small cells in the B horizon may be metabolically dormant and/or physiologically distinct from those in A horizon soils.