Temporal variation in vitamin B12 concentration and their impact on phytoplankton composition of surface waters of a coastal ocean off Japan (Ariake Sea)

Most phytoplankton, including harmful algal bloom (HAB)-forming species, have an absolute requirement for vitamin B12 (cobalamin). However, the cycling of vitamin B12 in the ocean is still poorly understood. For example, in the Ariake Sea, where local fisheries have been affected by HABs, the impact of vitamin B12 on phytoplankton dynamics has never been evaluated. This study reports the surface distribution of cyanocobalamin (CN-B12, a type of vitamin B12) in the Ariake Sea and its adjacent areas. The field measurements were complemented with an additional onboard CN-B12 incubation experiment to elucidate the importance of the coenzyme on the growth of natural phytoplankton communities. Dissolved CN-B12 behaved dynamically and was often depleted in surface waters. Dominant phytoplankton in the Ariake Sea changed seasonally; all these taxa include vitamin B12 auxotrophs. The distribution of CN-B12 was controlled by dynamic production and consumption by natural phytoplankton and bacterial communities. We attempted to measure the concentration of particulate CN-B12; it was undetectable suggesting that CN-B12 form of vitamin B12 is not dominant inside the cell. No clear relationship existed between CN-B12 and any oceanographic parameters, suggesting that river discharge is unlikely a major source of CN-B12 in this area. Bottle incubation experiments showed that CN-B12 and nitrate additions promoted the growth of diatom Chaetoceros spp., implying that some phytoplankton groups are co-limited by them. These preliminary results suggest that vitamin B12 may affect the composition of the phytoplankton community in the Ariake Sea.