Cobalamin-Independent Methionine Synthase Distribution and its Influence on Vitamin B12 Growth Requirements in Marine Diatoms

The requirement for cobalamin (vitamin B₁₂) in microalgae is primarily a function of the type of methionine synthase present within their gene repertoires. This study validates this concept through analysis of the distribution of B₁₂-independent methionine synthase in ecologically relevant diatom genera, including the closely related bloom-forming diatoms Pseudo-nitzschia and Fragilariopsis. Growth and gene expression analysis of the vitamin B₁₂-requiring version of the methionine synthase enzyme, MetH, and the B₁₂-independent version, MetE, demonstrate that it is the presence of the MetE gene which allows Fragilariopsis cylindrus to grow in the absence of B_12, while P. granii’s lack of a functional MetE gene means that it cannot survive without the vitamin. Through phylogenetic analysis, we further substantiate a lack of obvious grouping in MetE presence among diatom clades. In addition, we also show how this trend may have a biogeographical basis, particularly in High-Nutrient, Low-Chlorophyll (HNLC) regions such as the Southern Ocean where B₁₂ concentrations may be consistently low. These results are paired with field experiments showing patterns of MetE and MetH gene expression in natural phytoplankton communities under a matrix of iron and B₁₂ limitations in the HNLC NE Pacific. Our findings demonstrate the important role vitamins can play in diatom community dynamics within areas where vitamin supply may be variable and limiting.