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 B 1 and B 12 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 B 1 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 B 1 . During dinoflagellate blooms (>10 4 cells ml -1), occurring in late summer and early fall, vitamin B 12 and B 1 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 B 1 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 B 12 , suggests that B-vitamins may influence the succession of coastal phytoplankton. KEY WORDS: B-vitamins · Phytoplankton · Harmful algal blooms · Inorganic nutrients Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 49: [181][182][183][184][185][186][187][188][189][190][191][192][193][194] 2007 1980, Croft et al. 2005). A recent compilation by Croft et al. (2005) indicated that half of the algal species studied in culture (n = 326) cannot grow without an exogenous supply of vitamin B 12 . Sources of vitamins in marine systems include bacterial biosynthesis and allochthonous sources, such as influxes from rivers and streams (Haines & Guillard 1974, Bruno & Staker 1978, Swift 1980. Few studies have examined the effect of ambient vitamin levels on phytoplankton dynamics in marine ecosystems. These studies have generally suggested that vitamins can be present at picomolar concentrations in seawater and may sometimes be depleted to growth-limiting levels (Swift 1980, Croft et al. 2005, Panzeca et al. 2006, Bertrand et al. 2007. Recent independent studies have demonstrated that phytoplankton communities in the S...

A majority of eukaryotic phytoplankton species require an exogenous source of vitamin B 12 for growth and recent field studies in some coastal and polar regions indicate that the addition of vitamin B 12 alone, or with another limiting nutrient can influence the accumulation of phytoplankton biomass. We quantified the concentrations and uptake rates of vitamin B 12 , characterized phytoplankton community composition, and examined the ability of vitamin B 12 to alter the growth and composition of phytoplankton communities in the Gulf of Alaska. Picoplankton (0.2-2 mm) were responsible for the majority of vitamin B 12 uptake in both coastal and high-nutrient low-chlorophyll (HNLC) regions and B 12 concentrations and uptake rates were higher in HNLC regions compared to coastal regions with higher iron (Fe) concentrations. During vitamin amendment experiments, B 12 alone or in conjunction with other limiting nutrients (N or Fe) significantly enhanced algal biomass and increased the growth rates of multiple groups of larger (. 2 mm) phytoplankton. This included ecologically significant, B 12 auxotrophs such as Gymnodinium sp. and Alexandrium sp. (in the costal experiment) as well as Chaetoceros sp. and Gymnodinium sp. (in the HNLC experiment). The ability of vitamin B 12 to shape algal community composition in coastal and HNLC areas of the Gulf of Alaska, even in cases where it does not limit total phytoplankton production, suggests that it may influence carbon export in this and other polar ecosystems.

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Without an adequate supply of dissolved vitamins, many species of phytoplankton do not grow. Additions of inorganic nutrients like phosphorus and nitrogen, and trace metals like iron, are not alone adequate to sustain life—a practical lesson learned quickly by experimental biologists when they try to keep eukaryotic phytoplankton cultures alive in their labs. The reason is that coenzymes such as B vitamins are also required for many metabolic pathways. For example, vitamin B, serves as a cofactor for a large number of enzymatic systems, including the pyruvate dehydrogenase complex required for the metabolism of carbohydrates (glycolysis) and amino acid synthesis [Vandamme, 1989]. Vitamin B 12 is used primarily to assist two enzymes: methionine synthase, which is involved in DNA synthesis, and methylmalonyl CoA mutase, which is required for inorganic carbon assimilation [Lindemans and Abels, 1985].

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