Original PaperCobalamin-independent Methionine Synthase Distribution and 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. Our 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 genera 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 B12. Pseudo-nitzschia 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 regions such as the Southern Ocean where B12 concentrations may be consistently low. Our findings demonstrate the important role vitamins can play in diatom community dynamics within areas where vitamin supply may be variable and limiting.