Variations in vitamin B12 requirements among bloom-forming marine diatoms (2013)

Undergraduates: Kelsey Ellis, Natalie Cohen

Faculty Advisor: Adrian Marchetti
Department: Environmental Science

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The need for cobalamin (vitamin B12) among diatoms is primarily a function of the forms of methionine synthase in their gene repertoires. Diatoms with an obligate requirement for vitamin B12 only possess a cobalamin-dependent methionine synthase (MetH) whereas those with a facultative requirement possess MetH and a cobalamin-independent version (MetE). Pseudo-nitzshia and Fragilariopsis are two ecologically important diatom genera that often dominate phytoplankton assemblages following iron enrichment in iron-limited oceans. P.granii appears to possess MetH whereas F. cylindrus possesses both MetH and MetE, suggesting a fundamental difference in vitamin B12 needs between members of the two closely related genera. Here we show that P. granii has an obligate requirement for cobalamin whereas F. cylindrus growth is unaffected by varying B12 availability. P. granii cells ceased growth without vitamin B12, while MetH was constitutively expressed under vitamin B12-deficient and replete conditions. MetH in F.cylindrus was also constitutively expressed under both treatments. MetE was highly expressed when F. cylindrus cells were grown without vitamin B12, whereas upon resupply of the vitamin the expression of MetE decreased 102-fold. Our findings provide a mechanism for observed differences in diatom assemblages following iron-enrichment within iron-limited regions and highlight the important role vitamins could play in areas where their supply may be variable and limiting.