Chlamydomonas and Vitamin B12

Nutrient amendment experiments suggest that B₁₂ limits phytoplankton growth in many aquatic ecosystems.  Eukaryotic algae cannot synthesize B₁₂ and must instead obtain it from certain B₁₂-producing prokaryotes. In many algae, B₁₂ is required as a cofactor for the B₁₂-dependent Met synthase enzyme (METH), although some algae encode a B₁₂-independent isoform of this enzyme (METE) and thus do not require B₁₂ for growth. The chlorophyte Chlamydomonas reinhardtii possesses a functional copy of METE and so is B₁₂-independent. Previously, researchers associated with Bunbury et al. (10.1104/pp.19.01375) have reported that they had been able, over the course of ~500 cell generations, to generate a METE mutant of C. reinhardtii (metE7) by experimental evolution under  conditions of high vitamin B₁₂ concentration, demonstrating that sustained levels of B₁₂ in the environment can drive METE gene loss. metE7 provides a unique opportunity to study the physiology of a nascent B₁₂ auxotroph. In the present work, the authors characterized the responses of metE7 to different vitamin B₁₂ regimes and compared them with the responses of its ancestral B₁₂-independent strain as well as those of a closely related, naturally B₁₂-dependent alga Lobomonas rostrata. The authors demonstrate that B₁₂ 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. B₁₂ deprivation also caused a substantial increase in ROS, which preceded rapid cell death. Further improvements in survival under B₁₂ limitation and an increase in B₁₂ use efficiency were found after metE7 underwent a further period of experimental evolution, this time in coculture with a B₁₂-producing bacterium. Therefore, although an early B₁₂-dependent alga would likely be poorly adapted to coping with B₁₂ deprivation, association with B₁₂-producers can ensure long-term survival while also providing a suitable environment for evolving mechanisms to tolerate B₁₂ limitation better.