Predicted growth rate of the symbiosis, (d-1), vs. ratio of the effective spherical radii of the haptophyte and UCYN-A, RH/RD. The solid blue line indicates the predicted growth rate for the symbiotic pair when nothing limits the growth but the C provided through the haptophyte photosynthesis across different host-symbiont size ratios, and the blue shading indicates the range inferred by 1 standard deviation in the C-specific photosynthesis rate of haptophytes in this size range.21The red line is the modeled maximum growth rate when nothing limits the growth but the N provided by UCYN-A through N2fixation, calibrated by the maximum observed N-specific N2fixation rate in UCYN-A.18The lowest of these two growth rates is the achievable growth rate (black dash dotted line). The highest synchronized growth rate occurs at the radius ratio where the blue and red lines cross (RH/RD)opt(black circle).
