The effect of vitamin B12 on phytoplankton growth and community structure in the Gulf of Alaska

A majority of eukaryotic phytoplankton species require an exogenous source of vitamin B-12 for growth and recent field studies in some coastal and polar regions indicate that the addition of vitamin B-12 alone, or with another limiting nutrient can influence the accumulation of phytoplankton biomass. We quantified the concentrations and uptake rates of vitamin B-12, characterized phytoplankton community composition, and examined the ability of vitamin B-12 to alter the growth and composition of phytoplankton communities in the Gulf of Alaska. Picoplankton (0.2-2 mu m) were responsible for the majority of vitamin B-12 uptake in both coastal and high-nutrient low-chlorophyll (HNLC) regions and B-12 concentrations and uptake rates were higher in HNLC regions compared to coastal regions with higher iron (Fe) concentrations. During vitamin amendment experiments, B-12 alone or in conjunction with other limiting nutrients (N or Fe) significantly enhanced algal biomass and increased the growth rates of multiple groups of larger (> 2 mu m) phytoplankton. This included ecologically significant, B-12 auxotrophs such as Gymnodinium sp. and Alexandrium sp. (in the costal experiment) as well as Chaetoceros sp. and Gymnodinium sp. (in the HNLC experiment). The ability of vitamin B-12 to shape algal community composition in coastal and HNLC areas of the Gulf of Alaska, even in cases where it does not limit total phytoplankton production, suggests that it may influence carbon export in this and other polar ecosystems.