Organisms within all domains of life require the cofactor cobalamin (vitamin B 12 ), which is produced only by a subset of bacteria and archaea. On the basis of genomic analyses, cobalamin biosynthesis in marine systems has been inferred in three main groups: select heterotrophic Proteobacteria, chemoautotrophic Thaumarchaeota, and photoautotrophic Cyanobacteria. Culture work demonstrates that many Cyanobacteria do not synthesize cobalamin but rather produce pseudocobalamin, challenging the connection between the occurrence of cobalamin biosynthesis genes and production of the compound in marine ecosystems. Here we show that cobalamin and pseudocobalamin coexist in the surface ocean, have distinct microbial sources, and support different enzymatic demands. Even in the presence of cobalamin, Cyanobacteria synthesize pseudocobalaminlikely reflecting their retention of an oxygen-independent pathway to produce pseudocobalamin, which is used as a cofactor in their specialized methionine synthase (MetH). This contrasts a model diatom, Thalassiosira pseudonana, which transported pseudocobalamin into the cell but was unable to use pseudocobalamin in its homolog of MetH. Our genomic and culture analyses showed that marine Thaumarchaeota and select heterotrophic bacteria produce cobalamin. This indicates that cobalamin in the surface ocean is a result of de novo synthesis by heterotrophic bacteria or via modification of closely related compounds like cyanobacterially produced pseudocobalamin. Deeper in the water column, our study implicates Thaumarchaeota as major producers of cobalamin based on genomic potential, cobalamin cell quotas, and abundance. Together, these findings establish the distinctive roles played by abundant prokaryotes in cobalamin-based microbial interdependencies that sustain community structure and function in the ocean. vitamin B 12 ) is synthesized by a select subset of bacteria and archaea, yet organisms across all domains of life require it (1-3). In the surface ocean, cobalamin auxotrophs (including most eukaryotic algae) (3) obtain the vitamin through direct interactions with cobalamin producers (3) or breakdown of cobalamin-containing cells (4,5). Interdependencies between marine cobalamin producers and consumers are critical in surface waters where primary productivity can be limited by the availability of cobalamin and the compound is short-lived (1, 6, 7). The exchange of cobalamin in return for organic compounds is hypothesized to underpin mutualistic interactions between heterotrophic bacteria and autotrophic algae (3,6,8,9). The apparent pervasiveness of cobalamin biosynthesis genes in chemoautotrophic Thaumarchaeota and photoautotrophic Cyanobacteria genomes (1, 10, 11) raises the question of whether cobalamin production by these autotrophs may underlie additional, unexplored microbial interactions.Cobalamin is a complex molecule with a central cobalt-containing corrin ring, an α ligand of 5,6-dimethylbenzimidizole (DMB), and a β ligand of either OH-, CN-, Me-, or Ado-(12) (Fig. 1). Pr...

A major percentage of fixed nitrogen (N) loss in the oceans occurs within nitrite-rich oxygen minimum zones (OMZs) via denitrification and anammox. It remains unclear to what extent ammonium and nitrite oxidation co-occur, either supplying or competing for substrates involved in nitrogen loss in the OMZ core. Assessment of the oxygen (O 2 ) sensitivity of these processes down to the O 2 concentrations present in the OMZ core (<10 nmol·L −1 ) is therefore essential for understanding and modeling nitrogen loss in OMZs. We determined rates of ammonium and nitrite oxidation in the seasonal OMZ off Concepcion, Chile at manipulated O 2 levels between 5 nmol·L −1 and 20 μmol·L −1 . Rates of both processes were detectable in the low nanomolar range (5-33 nmol·L −1 O 2 ), but demonstrated a strong dependence on O 2 concentrations with apparent half-saturation constants (K m s) of 333 ± 130 nmol·L −1 O 2 for ammonium oxidation and 778 ± 168 nmol·L −1 O 2 for nitrite oxidation assuming one-component Michaelis-Menten kinetics. Nitrite oxidation rates, however, were better described with a two-component Michaelis-Menten model, indicating a high-affinity component with a K m of just a few nanomolar. As the communities of ammonium and nitrite oxidizers were similar to other OMZs, these kinetics should apply across OMZ systems. The high O 2 affinities imply that ammonium and nitrite oxidation can occur within the OMZ core whenever O 2 is supplied, for example, by episodic intrusions. These processes therefore compete with anammox and denitrification for ammonium and nitrite, thereby exerting an important control over nitrogen loss.is a key factor regulating biogeochemical cycling in the marine environment (1). Although the vast majority of the ocean remains well oxygenated, subsurface regions of extreme oxygen depletion can persist along eastern boundaries of the world's ocean basins. These regions are known as oxygen minimum zones (OMZs) and are located within the eastern tropical North and South Pacific, the Arabian Sea, and off the coast of Namibia, where oxygen depletion results from poor ventilation and a high export of organic matter from productive surface waters, generating high rates of subsurface oxygen consumption (2, 3).Oceanic waters characterized by oxygen-deficient conditions (<4.5 μmol·L −1 O 2 ) account for <0.1% of total ocean volume but for >30% of fixed nitrogen (N) loss (3-6) due to the onset of anaerobic processes, including denitrification and anammox (7-10). Both field and modeling observations point to the expansion of low oxygen regions as a result of global warming (11). Thus, to evaluate the biogeochemical impact of these regions, it is imperative to understand fully how oxygen controls the cycling of substrates involved in nitrogen loss pathways.Recent studies have quantified the oxygen sensitivity of anaerobic OMZ nitrogen transformations, finding that denitrification has relatively low oxygen tolerance with a half-inhibition concentration (IC 50 ) of 0.3 μmol·L −1 , compared with higher values for...

Nitrosopumilus maritimus gen. nov., sp. nov., Nitrosopumilus cobalaminigenes sp. nov., Nitrosopumilus oxyclinae sp. nov., and Nitrosopumilus ureiphilus sp. nov., four marine ammonia-oxidizing archaea of the phylum Thaumarchaeota