The corrinoid [B.sub.12] is synthesized only by prokaryotes yet is widely required by eukaryotes as an enzyme cofactor. Microalgae have evolved [B.sub.12] dependence on multiple occasions, and we previously demonstrated that experimental evolution of the non-[B.sub.12]requiring alga Chlamydomonas reinhardtii in media supplemented with [B.sub.12] generated a [B.sub.12]-dependent mutant (hereafter metE7). This clone provides a unique opportunity to study the physiology of a nascent [B.sub.12] auxotroph. Our analyses demonstrate that [B.sub.12] deprivation of metE7 disrupts C1 metabolism, causes an accumulation of starch and triacylglycerides, and leads to a decrease in photosynthetic pigments, proteins, and free amino acids. [B.sub.12] deprivation also caused a substantial increase in reactive oxygen species, which preceded rapid cell death. Survival could be improved without compromising growth by simultaneously depriving the cells of nitrogen, suggesting a type of cross protection. Significantly, we found further improvements in survival under [B.sub.12] limitation and an increase in [B.sub.12] use efficiency after metE7 underwent a further period of experimental evolution, this time in coculture with a [B.sub.12]-producing bacterium. Therefore, although an early [B.sub.12]-dependent alga would likely be poorly adapted to coping with [B.sub.12] deprivation, association with [B.sub.12]-producers can ensure long-term survival whilst also providing a suitable environment for evolving mechanisms to tolerate [B.sub.12] limitation better.