. Previous studies have shown that vitamin B12 utilization is dominated by the picoplankton , even during algal blooms , with multiple lines of evidence suggesting heterotrophic bacteria are the primary vitamin users . In contrast, during C. polykrikoides blooms B12 utilization occurred primarily in larger plankton (>2 mm; 64.4 _ 7.10%; Suppl. Table 1) suggesting that C. polykrikoides is better able to gain access to the pM concentrations of vitamins than other phytoplankton, including A. anophagefferens, and bacteria .