Kelsey Ellis’ Research

Exploring variations in vitamin B12 requirements among bloom-forming marine diatoms

Because of their importance in oceanic carbon sequestration, diatoms are the focus of research efforts to determine what factors limit their growth in high-nutrient, low-chlorophyll (HNLC) regions of the world’s oceans. These are areas where limiting concentrations of the micronutrient iron limit phytoplankton growth despite high concentrations of macronutrients such as nitrate. Recent experiments have shown that iron fertilization of HNLC waters has the potential to create diatom-dominated phytoplankton blooms (Coale et al. 1996, Boyd et al. 2007).

Figure 1: A profile of the upper ocean off of the California coast showing the variability of vitamin B12 distribution in the water column. Warm colors denote higher B12 concentrations and cool colors are low or negligible concentrations (Sañudo-Wilhelmy et al. 2012).

Figure 1: A profile of the upper ocean off of the California coast showing the variability of vitamin B12 distribution in the water column. Warm colors denote higher B12 concentrations and cool colors are low or negligible concentrations (Sañudo-Wilhelmy et al. 2012).

More recent studies in certain iron-limited regions of the ocean have shown that by adding both vitamins and iron, diatom growth can be enhanced beyond the results from iron enrichment alone. Vitamin B12 (cobalamin) in particular is capable of increasing growth with or without iron under the right environmental conditions (Bertrand et al. 2007, Koch et al. 2011). Previous studies have found vitamin B12 to be a limiting nutrient for phytoplankton growth in areas such as the Ross Sea (Bertrand et al. 2007), the Southern Ocean (Panzeca et al. 2006), Long Island embayments (Sañudo-Wilhelmy et al. 2006), the Sargasso Sea (Menzel and Spaeth 1962), and the Gulf of Maine (Swift 1981) (Figure 1).

Vitamin B12, or cobalamin, is a biologically synthesized, cobalt-containing molecule that is produced by certain bacteria and archaea. Its main use is in the creation of one form of the methionine synthase enzyme (MetH) which catalyzes synthesis of the essential amino acid methionine (Rodionov et al. 2003). Some marine phytoplankton also possess an alternate form of the enzyme synthesis pathway that does not require B12 (MetE) but has lower efficiency. However, a survey of algal species determined that over half have an obligate requirement for B12 and only possess the MetH gene (González et al. 1992, Croft et al. 2005).

Figure 2: Pseudo-nitzschia sp. (top) and Fragilariopsis sp. (bottom) diatom cells. Both genera have been shown to bloom after iron fertilization experiments.

Figure 2: Pseudo-nitzschia granii (top) and Fragilariopsis sp. (bottom) diatom cells. Both genera have been shown to bloom after iron fertilization experiments.

The two main morphological diatom groups, centrics and pennates, diverged evolutionarily only 90 million years ago. However, a recent comparison between the genomes of a pennate (Phaeodactylum tricornutum) and centric (Thalassiosira pseudonana) diatom found that the two species share only 57% of their genes. Genetic diversity among diatom genera results in varying responses to changes in concentrations of nutrients such as vitamin B12 in oceanic environments. I am currently investigating how vitamin B12 requirements vary among diatom species, particularly the closely related pennate species Pseudo-nitzschia granii and Fragilariopsis cylindrus (Figure 2).

My research goals are to investigate possible phylogenetic patterns in diatom B12 dependence and to determine how physiological responses to B12 additions vary between closely related diatom genera. I am also looking at how gene expression of MetE and MetH relates to diatom growth and physiology. My research broadly seeks to connect differences in gene expression of methionine synthase isoforms under varying B12 concentrations to differences in physiological responses and ultimately, resultant variations in community composition. The initial composition of diatom assemblages in areas where Fe concentrations are limiting and B12 concentrations are low determines how those assemblages respond to inputs of Fe and alter diatom bloom dynamics. Changes in concentrations of vitamin B12 have the potential to affect carbon sequestration rates when some diatoms are able to grow without B12 and others cannot. In this light, variations in B12 concentrations could have far reaching influences on the biological carbon pump not only in present oceans but also those of the past and future.

References:

Bertrand, E.M., et al. 2007. Vitamin B12 and iron co-limitation of phytoplankton growth in the Ross Sea. Limnol. Oceanogr. 52: 1079–1093.
Boyd, P.W., et al. 2007. Mesoscale iron enrichment experiments 1993-2005: Synthesis and future directions. Science 315: 612-617. doi: 10.1126/science.1131669
Coale, K.H., et al. 1996. A massive phytoplankton bloom induced by an ecosystem-scale iron fertilization experiment in the equatorial Pacific Ocean. Nature 383: 495-501. doi: 10.1038/383495a0
Croft, M.T., et al. 2005. Algae acquire vitamin B12 through a symbiotic relationship with bacteria. Nature 438:90–93. doi: 10.1038/nature04056
González, J.C., et al. 1992. Comparison of cobalamin-independent and cobalamin-dependent methionine synthases from Escherichia coli: Two solutions to the same chemical problem. Biochemistry 31:6045–6056. doi: 10.1021/bi00141a013
Koch, F., et al. 2011. The effect of vitamin B12 on phytoplankton growth and community structure in the Gulf of Alaska. Limnol. and Oceanogr. 56: 1023-1034. doi: 10.4319/lo.2011.56.3.1023
Menzel, D.W. and J.P. Spaeth. 1962. Occurrence of vitamin B12 in the Sargasso Sea. Limnol. and Oceanogr. 7: 151–154.
Panzeca, C., et al. 2006. B vitamins as regulators of phytoplankton dynamics. EOS 87: 593–596. doi: 10.1029/2006EO520001
Rodionov, D.A., et al. 2003. Comparative genomics of the vitamin B12 metabolism and regulation in prokaryotes. J. Biol. Chem. 278: 41148-41159. doi: 10.1074/jbc.M305837200
Sañudo-Wilhelmy, S.A., et al. 2006. Regulation of phytoplankton dynamics by vitamin B12. Geophys. Res. Lett. 33: L04604. doi: 10.1029/2005GL025046
Sañudo-Wilhelmy, S.A., et al. 2012. Multiple B-vitamin depletion in large areas of the coastal ocean. PNAS 109: 14041-14045. doi: 10.1073/pnas.1208755109
Swift, D. 1981. Vitamin levels in the Gulf of Maine and ecological significance of vitamin B12 there. J. Mar. Res. 39: 375–403.