Effect of B-vitamins (B₁, B₁₂) and inorganic nutrients on algal bloom dynamics in a coastal ecosystem

Vitamins play an integral role in the cellular biochemistry of algae, but the effect of these organic metabolites on the growth and diversity of phytoplankton communities has been poorly studied. We integrated newly developed techniques to directly measure vitamins B₁ and B₁₂ with field-based amendment experiments to elucidate the role of B-vitamins in phytoplankton population dynamics in coastal marine environments. Two sites on Long Island, New York, USA, were monitored from spring through fall: the tidal Old Fort Pond (OFP) and the brackish Peconic River (PR) estuary. Vitamin B ₁₂ levels were similar between sites (OFP: 1.6 to 21 pM; PR: 1.6 to 17 pM) and were significantly correlated with bacterial densities, dissolved organic nitrogen and dissolved organic phosphorus at OFP, suggesting that B ₁₂ behaves like regenerated organic nutrients. Concentrations of vitamin B₁ were substantially higher in the freshwater dominated site (PR: 12 to 190 pM; OFP: 9 to 43 pM) and were inversely correlated with salinity, suggesting that rivers and groundwater may be an important source of vitamin B₁. During dinoflagellate blooms (>10⁴ cells ml^-1), occurring in late summer and early fall, vitamin B ₁₂ and B₁ levels in PR decreased 90 % relative to pre-bloom levels, while levels temporarily increased to seasonal maxima in OFP, likely reflecting vitamin synthesis and/or regeneration by microbial communities. Nutrient amendment experiments conducted at both sites during summer demonstrated that algal communities were primarily N-limited, while those conducted during early fall showed that vitamins B₁ and B ₁₂ were each capable of significantly enhancing the biomass of larger phytoplankton (>5 μm). The autumnal shift in phytoplankton communities from dinoflagellates to diatoms, as vitamin levels became depleted and algal communities were limited by vitamin B₁₂, suggests that B-vitamins may influence the succession of coastal phytoplankton.