Abstract. Nitrous oxide (N 2 O) is a trace gas that contributes to the greenhouse effect and stratospheric ozone depletion. The N 2 O yield from nitrification (moles N 2 O-N produced per mole ammonium-N consumed) has been used to estimate marine N 2 O production rates from measured nitrification rates and global estimates of oceanic export production. However, the N 2 O yield from nitrification is not constant. Previous culture-based measurements indicate that N 2 O yield increases as oxygen (O 2 ) concentration decreases and as nitrite (NO − 2 ) concentration increases. Here, we have measured yields of N 2 O from cultures of the marine β-proteobacterium Nitrosomonas marina C-113a as they grew on low-ammonium (50 µM) media. These yields, which were typically between 4 × 10 −4 and 7 × 10 −4 for cultures with cell densities between 2×10 2 and 2.1×10 4 cells ml −1 , were lower than previous reports for ammonia-oxidizing bacteria. The observed impact of O 2 concentration on yield was also smaller than previously reported under all conditions except at high starting cell densities (1.5 × 10 6 cells ml −1 ), where 160-fold higher yields were observed at 0.5% O 2 (5.1 µM dissolved O 2 ) compared with 20% O 2 (203 µM dissolved O 2 ). At lower cell densities (2 × 10 2 and 2.1 × 10 4 cells ml −1 ), cultures grown under 0.5% O 2 had yields that were only 1.25-to 1.73-fold higher than cultures grown under 20% O 2 . Thus, previously reported many-fold increases in N 2 O yield with dropping O 2 could be reproduced only at cell densities that far exceeded those of ammonia oxidizers in the ocean. The presence of excess NO − 2 (up to 1 mM) in the growth medium also increased N 2 O yields by an average of 70% to 87% depending on O 2 concentration. We made stable

We present full-depth zonal sections of total dissolved cobalt, iron, manganese, and labile cobalt from the South Atlantic Ocean. A basin-scale plume from the African coast appeared to be a major source of dissolved metals to this region, with high cobalt concentrations in the oxygen minimum zone of the Angola Dome and extending 2500 km into the subtropical gyre. Metal concentrations were elevated along the coastal shelf, likely due to reductive dissolution and resuspension of particulate matter. Linear relationships between cobalt, N 2 O, and O 2 , as well as low surface aluminum supported a coastal rather than atmospheric cobalt source. Lateral advection coupled with upwelling, biological uptake, and remineralization delivered these metals to the basin, as evident in two zonal transects with distinct physical processes that exhibited different metal distributions. Scavenging rates within the coastal plume differed for the three metals; iron was removed fastest, manganese removal was 2.5 times slower, and cobalt scavenging could not be discerned from water mass mixing. Because scavenging, biological utilization, and export constantly deplete the oceanic inventories of these three hybrid-type metals, point sources of the scale observed here likely serve as vital drivers of their oceanic cycles. Manganese concentrations were elevated in surface waters across the basin, likely due to coupled redox processes acting to concentrate the dissolved species there. These observations of basin-scale hybrid metal plumes combined with the recent projections of expanding oxygen minimum zones suggest a potential mechanism for effects on ocean primary production and nitrogen fixation via increases in trace metal source inputs.

Nitrous oxide (N₂O) is a trace gas that contributes to greenhouse warming of the atmosphere and stratospheric ozone depletion. The N₂O yield from nitrification (moles N₂O-N produced/mole ammonium-N consumed) has been used to estimate marine N₂O production rates from measured nitrification rates and global estimates of oceanic export production. However, the N₂O yield from nitrification is not constant. Previous culture-based measurements indicate that N₂O yield increases as oxygen (O₂) concentration decreases and as nitrite (NO₂^&minus;) concentration increases. These results were obtained in substrate-rich conditions and may not reflect N₂O production in the ocean. Here, we have measured yields of N₂O from cultures of the marine β-proteobacterium <i>Nitrosomonas marina</i> C-113a as they grew on low-ammonium (50 &mu;M) media. These yields were lower than previous reports, between 4&times;10^&minus;4 and 7&times;10^&minus;4 (moles N/mole N). The observed impact of O₂ concentration on yield was also smaller than previously reported under all conditions except at high starting cell densities (1.5&times;10<sup6</sup> cells ml^&minus;1), where 160-fold higher yields were observed at 0.5% O₂ compared with 20% O₂. At environmentally relevant cell densities (2&times;10² to 2.1&times;10⁴ cells ml^&minus;1), cultures grown under 0.5% O₂ had yields that were only 1.25- to 1.73-fold higher than cultures grown under 20% O₂. Thus, previously reported many-fold increases in N₂O yield with dropping O₂ could be reproduced only at cell densities that far exceeded those of ammonia oxidizers in the ocean. The presence of excess NO₂^&minus; (up to 1 mM) in the growth medium also increased N₂O yields by an average of 70% to 87% depending on O₂ concentration. We made stable isotopic measurements on N₂O from these cultures to identify the biochemical mechanisms behind variations in N₂O yield. Based on measurements of &delta;¹⁵N, site preference (SP=&delta;¹⁵N^&alpha; - &delta;¹⁵N^&beta;), and &delta;¹⁸O, we estimate that nitrifier-denitrification produced between 11% and 26% of N₂O from cultures grown under 20% O<s...

The stable isotopic composition of dissolved nitrous oxide (N 2 O) is a tracer for the production, transport, and consumption of this greenhouse gas in the ocean. Here we present dissolved N 2 O concentration and isotope data from the South Atlantic Ocean, spanning from the western side of the mid-Atlantic Ridge to the upwelling zone off the southern African coast. In the eastern South Atlantic, shallow

In an effort to better understand the factors influencing carbon fixation by Prochlorococcus, and to elucidate the effects of these cyanobacteria on the ocean carbon cycle, the kinetic parameters and isotopic discrimination of form IA ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO) from Prochlorococcus marinus MIT 9313 were determined. The RubisCO genes (cbbL and cbbS) were cloned and expressed in Escherichia coli. Enzyme was purified via ammonium sulfate precipitation, and the optimum pH and temperature, as well as Michaelis-Menton constants, were determined radiometrically. The degree to which this RubisCO discriminates against 13 CO 2 during fixation was determined by the high-precision substrate depletion method. Purified enzyme had a pH optimum of 7.5, was most active between 20uC and 30uC, had a moderate V max (0.41 mmol min 21 mg protein 21 ), and had the highest K CO2 value (0.75 mmol L 21 ) for a form I RubisCO characterized to date. The e value, e 5 1,000[(k 12 /k 13 ) 2 1], for the enzyme was determined to be 24.0% (95% C.I. 5 22.2-25.6%), within the range observed for other form I RubisCOs. This e value is a critical baseline for interpreting the d 13 C values of marine environmental samples, particularly those collected from the open ocean, where P. marinus is responsible for a substantial fraction of carbon fixation.