B vitamins are some of the most commonly required biochemical cofactors in living systems. Therefore, cellular metabolism of marine vitamin-requiring (auxotrophic) phytoplankton and bacteria would likely be significantly compromised if B vitamins (thiamin B 1 , riboflavin B 2 , pyridoxine B 6 , biotin B 7 , and cobalamin B 12 ) were unavailable. However, the factors controlling the synthesis, ambient concentrations, and uptake of these key organic compounds in the marine environment are still not well understood. Here, we report vertical distributions of five B vitamins (and the amino acid methionine) measured simultaneously along a latitudinal gradient through the contrasting oceanographic regimes of the southern California-Baja California coast in the Northeast Pacific margin. Although vitamin concentrations ranged from below the detection limits of our technique to 30 pM for B 2 and B 12 and to ∼500 pM for B 1 , B 6 , and B 7 , each vitamin showed a different geographical and depth distribution. Vitamin concentrations were independent of each other and of inorganic nutrient levels, enriched primarily in the upper mesopelagic zone (depth of 100-300 m), and associated with water mass origin. Moreover, vitamin levels were below our detection limits (ranging from ≤0.18 pM for B 12 to ≤0.81 pM for B 1 ) in extensive areas (100s of kilometers) of the coastal ocean, and thus may exert important constraints on the taxonomic composition of phytoplankton communities, and potentially also on rates of primary production and carbon sequestration.T he evolution of organic catalysts (enzymes and coenzymes) that accelerated biochemical reactions by orders of magnitude compared with inorganic catalysis alone was a critical step for the emergence of major cellular processes (1). Well-known coenzyme molecules include the B vitamins, which catalyze many important biochemical reactions in central metabolism (2). Those reactions include rearrangement-reduction of C-C bonds and methyl transfer reactions, synthesis of deoxyribose/fatty acids/ carbohydrates/branched-chain amino acids, electron transfer in oxidation-reduction reactions, and CO 2 fixation (2). This means that without an exogenous source of B vitamins, central metabolism in vitamin-requiring (auxotrophic) organisms would be compromised and the cells would not be able to grow, a situation that could have great bearing on many biological processes in the ocean, including the biological carbon pump.In the marine environment, vitamins have been implicated as important factors regulating the growth and succession of phytoplankton species (3). This inference was based on the fact that many species of eukaryotic phytoplankton require vitamins when they are cultured in the laboratory (3-7). Although it was thought that marine prokaryotes were the major producers of vitamins in the sea (4), genomic data indicate that it is common for both eukaryotic phytoplankton and bacterioplankton to be auxotrophic for at least one B vitamin, mainly because they lack the complete biosynthetic...

We studied the nitrogen biogeochemistry of the ice‐covered eastern Bering Sea shelf using the isotope ratios (15N/14N and 18O/16O) of NO3− and other N species. The 15N/14N of late winter NO3− on the shelf decreases inshore and is inversely correlated with bottom water [NH4+], consistent with an input of low‐15N/14N NO3− from partial nitrification of NH4+ remineralized from the sediments. An inshore 15N/14N increase in total dissolved N (TDN) suggests that (1) the sediment‐derived NH4+ is elevated in 15N due to the same partial nitrification that yields the low‐15N/14N NO3−, and (2) 15N‐deplete NO3− from partial nitrification within the sediments is denitrified to N2. The proportion of newly nitrified NO3− on the shelf, evidenced by an inshore decrease in NO3− 18O/16O, is correlated with the N deficit, further implicating nitrification coupled to denitrification; however, a simple N isotope budget indicates a comparable rate of denitrification supported by diffusion of NO3− into the sediments. The isotopic impact of benthic N loss is further demonstrated by a correlation between the 15N/14N of shelf surface sediment and the N deficit of the overlying water column, both of which increase inshore and northward, as well as by Arctic NO3− isotope data indicating that the fixed N transported through Bering Strait has a 15N/14N higher than is found in the open Bering Sea. The significant net isotope effect of benthic N loss on the Bering shelf, 6–8 ‰, is at odds with previous assumptions regarding the global ocean's N isotope budget.

An extensive region of the Eastern Tropical South Pacific (ETSP) Ocean has surface waters that are nitrate-poor yet phosphate-rich. It has been proposed that this distribution of surface nutrients provides a geochemical niche favorable for N 2 fixation, the primary source of nitrogen to the ocean. Here, we present results from two cruises to the ETSP where rates of N 2 fixation and its contribution to export production were determined with a suite of geochemical and biological measurements. N 2 fixation was only detectable using nitrogen isotopic mass balances at two of six stations, and rates ranged from 0 to 23 μmol N m −2 d −1 based on sediment trap fluxes. Whereas the fractional importance of N 2 fixation did not change, the N 2 -fixation rates at these two stations were several-fold higher when scaled to other productivity metrics. Regardless of the choice of productivity metric these N 2 -fixation rates are low compared with other oligotrophic locations, and the nitrogen isotope budgets indicate that N 2 fixation supports no more than 20% of export production regionally. Although euphotic zone-integrated short-term N 2 -fixation rates were higher, up to 100 μmol N m, and detected N 2 fixation at all six stations, studies of nitrogenase gene abundance and expression from the same cruises align with the geochemical data and together indicate that N 2 fixation is a minor source of new nitrogen to surface waters of the ETSP. This finding is consistent with the hypothesis that, despite a relative abundance of phosphate, iron may limit N 2 fixation in the ETSP.nitrogen fixation | eastern tropical south Pacific | nitrogen budgets | nitrate | nitrogen isotopes

The oxygen triple‐isotope composition of dissolved O2 provides an integrative method to estimate the rates of Gross Photosynthetic Production (GPP) in the upper ocean, and combined with estimates of Net Community Production (NCP) yields an estimate of the net‐to‐gross (NCP/GPP) production ratios. However, derivations of GPP from oxygen triple‐isotope measurements have involved some mathematical approximations. We derive an exact expression for calculating GPP, and show that small errors associated with approximations result in a relative error of up to ∼38% in GPP, and up to ∼50% in N/G. In open ocean regimes with low primary production, the observed magnitude of the error is comparable to the combined methodological uncertainties. In highly productive ecosystems, the error arising from approximations becomes significant. Using data collected on the Bering Sea shelf, we illustrate the differences in GPP estimates in both high and low productivity regimes that arise from exact and approximated formulations.

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