(A) B. subtilis cells grown with dead M. fragilis in Nafion TS microcosm undergo a dry-wet cycle, mimicking the dry-down and wet-up of soils. Cells were exposed to D2O for 16 hr during the wet-up phase. Because M. fragilis spores were trapped within the Nafion matrix on the side where they were inoculated, after they germinated, one side of the microcosm filled with hyphae, while the other side remains empty. Confocal microscopy in GFP channel shows autofluorescent M. fragilis hyphae on one side of microcosm (above, and left inset), and no growth on the other side (right inset). Scale bar is 400 µm in both top micrograph and insets. B. subtilis cells measured were classified as either ‘on’ (cells attached to M. fragilis hyphae), ‘near’ (cells attached to Nafion on M. fragilis-inoculated side of microcosm, within 20 µm radius of nearest hypha), or ‘far’ (cells attached to Nafion on side of microcosm without M. fragilis). ‘Far’ cells are 3 mm or more away from nearest M. fragilis hyphae. (B) Most cells (~60–70 percent) have no detectable activity after a dry-wet cycle, regardless of distance from M. fragilis (left panel, all cells with CD area less than 0.5, indicating no activity detectable by D2O uptake). However, cells that did take up D2O took up more of the label when on M. fragilis than cells far from M. fragilis (right panel, one-way ANOVA of all three categories F-statistic 4.7194, p-value=0.0160; Tukey-Kramer HSD p-value<0.0131 for cells on M. fragilis vs far from M. fragilis, Welch’s t-test p-value=0.002398). Results pooled from three separate biological replicate experiments in three separate TS microcosms, indicated by color.