Regulation of phytoplankton dynamics by vitamin B₁₂

Despite the biological necessity of vitamin B₁₂ (cobalamin), its importance in phytoplankton ecology has been ignored for nearly three decades. Here we report strong and selective responses of phytoplankton communities to varying low levels (5-87 pM) of dissolved B₁₂ in several coastal embayments. The ecological importance of this vitamin is inferred from observed declines in dissolved B₁₂ levels as field populations of large (>5 μm) phytoplankton increased. In contrast, biomass of small (<5 μm) phytoplankton varied independently of B₁₂ concentrations. These observations were corroborated by field-based nutrient amendment experiments, in which B₁₂ additions stimulated growth of large phytoplankton taxa 6-fold over unamended controls. In contrast, small taxa, (<5 μm) were largely unaffected. This study provides the first evidence of vitamin B₁₂'s influence on phytoplankton field population dynamics based on direct chemical measurements of cobalamin, and implicates B₁₂ as an important organic regulator of photoautotrophic fertility in marine systems.