Compared to vitamin B12, vitamin B1 seems to play a less important role in the bloom dynamics of this HAB, despite C. polykrikoides’ absolute requirement for this vitamin . Although primary production rates during blooms were 10- fold higher than parallel non-bloom samples, B1 uptake was similar. Carbon-specific uptake rates of vitamin B1 in the >20 mm size class inside the bloom patches were lower than non-bloom plankton, indicating C. polykrikoides had a lower B1 requirement than comparably sized plankton. In addition similarly to B12 there was no significant difference (p > 0.05) between the carbon-specific B1 uptake of the 0.2–2 mm size fraction between the bloom and non-bloom despite the ten-fold higher biomass of the former (1553 _ 246 and 147 _ 20.5 mgC L_1, respectively). The similarity of carbon specific B1 uptake by picoplankton between bloom and non-bloom samples may be due to an absence of competition for assimilation of this vitamin with C. polykrikoides. Like C. polykrikoides, A. anophagefferens also displayed a larger requirement for vitamin B1 compared to B12 in culture but a lower requirement for vitamin B1 compared to B12 during blooms . Unlike B12, vitamin B1 is used in many different metabolic pathways and alternative compounds found in seawater that are not present in synthetic growth medium may be substituted by field populations. The biosynthesis of vitamins is complex and includes many precursor macromolecules . While no algae possess the pathways to biosynthesize B12 Helliwell et al. (2011), Bertrand and Allen (2012), and Croft et al. (2006) suggested that organisms that possess genes encoding for the synthesis of some of the precursors of B1 but not the entire molecule may also be able to utilize other partially assemble B1 from their environment and complete synthesis intracellulary. It has also been demonstrated that vitamin auxotrophy can vary even between strains of the same phytoplankton species isolated from the same field population on the same date . As such, it is possible that unlike C. polykrikoides clone CP1, some strains in the field do not require vitamin B1