Fig. 4: The effect of Indole-3-acetic acid on Symbiodiniaceae and its production by L. alexandrii and Marinobacter sp.

a The growth of both S. microadriaticum and (b) B. minutum was significantly enhanced by the presence of IAA, or the co-culture with each bacterial species, compared to untreated and extracellular bacteria removed (EBR) cultures (n = 5 per treatment). c Quantification of the absolute abundance of IAA per bacterial cell from the intracellular and exudate fractions (in attogram per cell and displayed as a logarithmic scale). Tryptophan significantly enhanced IAA concentration for both bacteria, suggesting the tryptophan-dependant synthesis of IAA by these bacteria (Two tailed T-test, L. alexandrii cell extracts t = −12.232, df = 6, p < 0.001, supernatant t = −13.607, df = 6, p < 0.001, Marinobacter sp. cell extracts t = −2.736, df = 6, p = 0.034, supernatant t = −2.799, df = 6, p = 0.031; Supplementary Data 16, n = 4 per treatment, per species). When Symbiodiniaceae were in co-culture with both bacteria, the abundance of IAA in the supernatant was reduced. Moreover, there was significantly more IAA in the supernatant when S. microadriaticum was in co-culture with L. alexandrii than Marinobacter sp. (Two tailed T-test, t = 4.361, df = 6, p = 0.005; Supplementary Data 16, n = 4 per treatment), while the inverse (although not statistically significant) was true for B. minutum (Two tailed T-test, t = −1.431, df = 6, p = 0.201; Supplementary Data 16, n = 4 per treatment). Data are presented as mean values with standard error bars. Source data are provided as a Source Data file.