Cyanobacteria and Eukaryotic Algae Use Different Chemical Variants of Vitamin B12

Abstract

SummaryEukaryotic microalgae and prokaryotic cyanobacteria are the major components of the phytoplankton. Determining factors that govern growth of these primary producers, and how they interact, is therefore essential to understanding aquatic ecosystem productivity. Over half of microalgal species representing marine and freshwater habitats require for growth the corrinoid cofactor B12, which is synthesized de novo only by certain prokaryotes, including the majority of cyanobacteria. There are several chemical variants of B12, which are not necessarily functionally interchangeable. Cobalamin, the form bioavailable to humans, has as its lower axial ligand 5,6-dimethylbenzimidazole (DMB). Here, we show that the abundant marine cyanobacterium Synechococcus synthesizes only pseudocobalamin, in which the lower axial ligand is adenine. Moreover, bioinformatic searches of over 100 sequenced cyanobacterial genomes for B12 biosynthesis genes, including those involved in nucleotide loop assembly, suggest this is the form synthesized by cyanobacteria more broadly. We further demonstrate that pseudocobalamin is several orders of magnitude less bioavailable than cobalamin to several B12-dependent microalgae representing diverse lineages. This indicates that the two major phytoplankton groups use a different B12 currency. However, in an intriguing twist, some microalgal species can use pseudocobalamin if DMB is provided, suggesting that they are able to remodel the cofactor, whereas Synechococcus cannot. This species-specific attribute implicates algal remodelers as novel and keystone players of the B12 cycle, transforming our perception of the dynamics and complexity of the flux of this nutrient in aquatic ecosystems.

Two distinct pools of B12analogs reveal community interdependencies in the ocean

“…SCM1, HCE1, HCA1, and PS0) also produced cobalamin (Table S1), confirming earlier suggestions based on the presence of cobalamin biosynthesis genes in Thaumarchaeota genomes (10). Like other Cyanobacteria (11,13,14), four axenic strains of marine Cyanobacteria (Prochlorococcus MED4 and MIT9313 and Synechococcus WH8102 and WH7803) produced pseudocobalamin (Table S1). In all of the cobalamin or pseudocobalamin producers, we detected compounds with β ligands Me-, Ado-, and OH-but not CN- (Table S1).…”

“…Of the 255 cyanobacterial whole genomes, 247 possessed genes for the synthesis of the corrin ring, but only one genome possessed an annotated bluB or cobT gene (Fig. 3), suggesting the vast majority of Cyanobacteria are unable to produce DMB, in agreement with a recent study that examined a subset of the available Cyanobacteria genomes (11). (21), has maintained these genes to synthesize pseudocobalamin.…”

“…The exchange of cobalamin in return for organic compounds is hypothesized to underpin mutualistic interactions between heterotrophic bacteria and autotrophic algae (3,6,8,9). The apparent pervasiveness of cobalamin biosynthesis genes in chemoautotrophic Thaumarchaeota and photoautotrophic Cyanobacteria genomes (1,10,11) raises the question of whether cobalamin production by these autotrophs may underlie additional, unexplored microbial interactions.…”

“…1). Previous studies have shown that instead of producing cobalamin, Cyanobacteria produce pseudocobalamin (11,13,14), an analog of cobalamin in which adenine substitutes for DMB as the α ligand (12) (Fig. 1).…”

“…In some bacteria, pseudocobalamin can be produced if DMB synthesis is impaired; this is due to the natural presence of adenine in cells and enzyme substrate promiscuity that allows adenine to substitute for DMB in some organisms' CobT (22)(23)(24)(25)(26)(27). Cyanobacteria use the O 2 -independent pathway for corrin ring synthesis and neither pathway for DMB synthesis (11,18) (Fig. 3).…”