Exploring the onset of B 12 -based mutualisms using a recently evolved Chlamydomonas auxotroph and B 12 -producing bacteria.

Cobalamin (vitamin B 12 ) is a cofactor for essential metabolic reactions in multiple eukaryotic taxa, including major primary producers such as algae, and yet only prokaryotes can produce it. Many bacteria can colonize the algal phycosphere, forming stable communities that gain preferential access to photosynthate and in return provide compounds such as B 12 . Extended coexistence can then drive gene loss, leading to greater algal-bacterial interdependence. In this study, we investigate how a recently evolved B 12 -dependent strain of Chlamydomonas reinhardtii, metE7, forms a mutualism with certain bacteria, including the rhizobium Mesorhizobium loti and even a strain of the gut bacterium E. coli engineered to produce cobalamin. Although metE7 was supported by B 12 producers, its growth in co-culture was slower than the B 12 -independent wild-type, suggesting that high bacterial B 12 provision may be necessary to favour B 12 auxotrophs and their evolution. Moreover, we found that an E. coli strain that releases more B 12 makes a better mutualistic partner, and although this trait may be more costly in isolation, greater B 12 release provided an advantage in co-cultures. We hypothesize that, given the right conditions, bacteria that release more B 12 may be selected for, particularly if they form close interactions with B 12 -dependent algae.