FIG 3 .

Phytoplankton growth rates on thiamin or pyrimidine compounds. (A) Acclimated mid-exponential growth rates of three haptophyte species, as well as the stramenopile P. calceolata and prasinophytes M. commoda and M. pusilla in response to 1 µM substrate additions. The negative control and the HET-amended treatment did not support growth. Pie charts illustrate the presence or absence of the three primary synthase enzymes in the thiamin biosynthesis pathway. (B and C) Exponential-phase growth rates of (B) E. huxleyi and (C) Pavlova sp. in grown-out cultures amended with 100 to 500 pM thiamin, HMP, or AmMP. Controls and treatments were performed in biological triplicates, and error bars represent the standard deviation (SD). Treatments that are not significantly different from one another share the same letter. nd, not determined.