Fig. 3

Uptake and distribution of ¹³C and ¹⁵N by host cells and sponge-associated microbes of the HMA species P. angulospiculatus (a–h) and LMA species H. caerulea (i–p) after a pulse of isotopically labelled DOM. NanoSIMS images show the distribution of ¹³C/¹²C (a–d, i–l) and ¹⁵N/¹⁴N (e–h, m–p) ratios after 0.25 h, 0.5 h, 3 h and 48 h. The colour scale represents enrichment relative to natural abundance ratios (in blue, 2 × 0.011 for ¹³C/¹²C and 0.0037 for ¹⁵N/¹⁴N). Incorporation of DOM can be traced into host choanocyte cells and sponge-associated microbes (white arrow heads) over time. Rapid uptake of DOM is evident in the apical tip of choanocytes after 0.25 and 0.5 h (white arrows). After 3 h, intracellular hotspots of ¹³C and ¹⁵N appear in choanocytes (yellow arrow heads), and during the chase period, the assimilated ¹³C and ¹⁵N is dispersed throughout these cells. c, choanocyte; m, mesohyl cell; sc, spherulous cell. Dashed lines delineate choanocyte chambers. Scale bars are 5 μm. For a summary of extracted values, see Additional file 2: Table S1