Figure 1. A synthetic mutualism between S. cerevisiae and C. reinhardtii.

(A) A metabolic circuit for mutualism based on carbon and nitrogen exchange. S. cerevisiae (orange, left) metabolizes glucose (C₆H₁₂O₆) and releases carbon dioxide (CO₂), which is assimilated photosynthetically by C. reinhardtii (green, right) to release oxygen (O₂); C. reinhardtii metabolizes nitrite (NO₂⁻) and releases ammonia (NH₃) as a nitrogen source for S. cerevisiae. An intrinsic, near-neutral pH balance between 6.8–7.4 is maintained by a metabolic exchange of protons between yeast and alga (15). (B) Proliferation of S. cerevisiae and C. reinhardtii under different co-culture conditions demonstrates that obligate mutualism can arise without any genetic engineering of metabolic pathways. Top: cartoons of the different conditions tested; middle: cell density of yeast and alga over the course of the experiment (mean ± 95% confidence interval; N=4); bottom: images of the cell populations from four representative examples of each culture condition (after 10 days). The dark green hue of the pellets is due to C. reinhardtii cells; S. cerevisiae cells are off-white and are interspersed throughout the pellet. See (15) for further details.