Hydrothermal venting along mid-ocean ridges exerts an important control on the chemical composition of sea water by serving as a major source or sink for a number of trace elements in the ocean. Of these, iron has received considerable attention because of its role as an essential and often limiting nutrient for primary production in regions of the ocean that are of critical importance for the global carbon cycle. It has been thought that most of the dissolved iron discharged by hydrothermal vents is lost from solution close to ridge-axis sources and is thus of limited importance for ocean biogeochemistry. This long-standing view is challenged by recent studies which suggest that stabilization of hydrothermal dissolved iron may facilitate its long-range oceanic transport. Such transport has been subsequently inferred from spatially limited oceanographic observations. Here we report data from the US GEOTRACES Eastern Pacific Zonal Transect (EPZT) that demonstrate lateral transport of hydrothermal dissolved iron, manganese, and aluminium from the southern East Pacific Rise (SEPR) several thousand kilometres westward across the South Pacific Ocean. Dissolved iron exhibits nearly conservative (that is, no loss from solution during transport and mixing) behaviour in this hydrothermal plume, implying a greater longevity in the deep ocean than previously assumed. Based on our observations, we estimate a global hydrothermal dissolved iron input of three to four gigamoles per year to the ocean interior, which is more than fourfold higher than previous estimates. Complementary simulations with a global-scale ocean biogeochemical model suggest that the observed transport of hydrothermal dissolved iron requires some means of physicochemical stabilization and indicate that hydrothermally derived iron sustains a large fraction of Southern Ocean export production.

The factors that control the growth and nitrogen fixation rates of marine diazotrophs such as Trichodesmium have been intensively studied because of the role that these processes have in the global cycling of carbon and nitrogen, and in the sequestration of carbon to the deep sea. Because the phosphate concentrations of many ocean gyres are low, the bioavailability of the larger, chemically heterogeneous pool of dissolved organic phosphorus could markedly influence Trichodesmium physiology. Here we describe the induction, by phosphorus stress, of genes from the Trichodesmium erythraeum IMS101 genome that are predicted to encode proteins associated with the high-affinity transport and hydrolysis of phosphonate compounds by a carbon-phosphorus lyase pathway. We show the importance of these genes through expression analyses with T. erythraeum from the Sargasso Sea. Phosphonates are known to be present in oligotrophic marine systems, but have not previously been considered to be bioavailable to marine diazotrophs. The apparent absence of genes encoding a carbon-phosphorus lyase pathway in the other marine cyanobacterial genomes suggests that, relative to other phytoplankton, Trichodesmium is uniquely adapted for scavenging phosphorus from organic sources. This adaptation may help to explain the prevalence of Trichodesmium in low phosphate, oligotrophic systems.

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Ammonia-oxidizing archaea (AOA) are now implicated in exerting significant control over the form and availability of reactive nitrogen species in marine environments. Detailed studies of specific metabolic traits and physicochemical factors controlling their activities and distribution have not been well constrained in part due to the scarcity of isolated AOA strains. Here, we report the isolation of two new coastal marine AOA, strains PS0 and HCA1. Comparison of the new strains to Nitrosopumilus maritimus strain SCM1, the only marine AOA in pure culture thus far, demonstrated distinct adaptations to pH, salinity, organic carbon, temperature, and light. Strain PS0 sustained nearly 80% of ammonia oxidation activity at a pH as low as 5.9, indicating that coastal strains may be less sensitive to the ongoing reduction in ocean pH. Notably, the two novel isolates are obligate mixotrophs that rely on uptake and assimilation of organic carbon compounds, suggesting a direct coupling between chemolithotrophy and organic matter assimilation in marine food webs. All three isolates showed only minor photoinhibition at 15 μE·m −2 ·s −1 and rapid recovery of ammonia oxidation in the dark, consistent with an AOA contribution to the primary nitrite maximum and the plausibility of a diurnal cycle of archaeal ammonia oxidation activity in the euphotic zone. Together, these findings highlight an unexpected adaptive capacity within closely related marine group I Archaea and provide new understanding of the physiological basis of the remarkable ecological success reflected by their generally high abundance in marine environments.marine ammonia-oxidizing archaea | ecophysiology | urea utilization

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Marine picocyanobacteria, comprised of the genera Synechococcus and Prochlorococcus, are the most abundant and widespread primary producers in the ocean. More than 20 genetically distinct clades of marine Synechococcus have been identified, but their physiology and biogeography are not as thoroughly characterized as those of Prochlorococcus. Using clade-specific qPCR primers, we measured the abundance of 10 Synechococcus clades at 92 locations in surface waters of the Atlantic and Pacific Oceans. We found that Synechococcus partition the ocean into four distinct regimes distinguished by temperature, macronutrients and iron availability. Clades I and IV were prevalent in colder, mesotrophic waters; clades II, III and X dominated in the warm, oligotrophic open ocean; clades CRD1 and CRD2 were restricted to sites with low iron availability; and clades XV and XVI were only found in transitional waters at the edges of the other biomes. Overall, clade II was the most ubiquitous clade investigated and was the dominant clade in the largest biome, the oligotrophic open ocean. Co-occurring clades that occupy the same regime belong to distinct evolutionary lineages within Synechococcus, indicating that multiple ecotypes have evolved independently to occupy similar niches and represent examples of parallel evolution. We speculate that parallel evolution of ecotypes may be a common feature of diverse marine microbial communities that contributes to functional redundancy and the potential for resiliency.

The Costa Rica upwelling dome (CRD; ϳ8.67ЊN and 90.6ЊW) was characterized chemically for cobalt and nickel abundances and speciation and biologically using cyanobacterial abundances and phylogeny. Total dissolved cobalt was 93 pmol L Ϫ1 at 90 m depth and decreased in surface waters to 45 pmol L Ϫ1 at 10 m. Cobalt was 40% labile at 90 m but was completely complexed by strong ligands at 10 m. A surface transect out of the dome showed decreasing total dissolved cobalt from 57 pmol L Ϫ1 to 12 pmol L Ϫ1 . Detection window studies showed that natural cobalt-ligand complexes have conditional stability constants greater than 10 16.8 and that competition with nickel did not release cobalt bound to organic complexes, consistent with natural cobalt ligands being Co(III) complexes. Synechococcus cell densities at the CRD are among the highest reported in nature, varying between 1.2 ϫ 10 6 and 3.7 ϫ 10 6 cells ml Ϫ1 . Phylogenetic analysis using the 16S-23S rDNA internally transcribed spacer showed the majority of clones were related to Synechococcus strain MIT S9220, while the remaining subset form a novel group within the marine Synechococcus lineage. In a bottle incubation experiment chlorophyll increased with cobalt and iron additions relative to each element alone and the unamended control treatment. Cobalt speciation analysis of incubation experiments revealed large quantities of strong cobalt-ligand complexes in the cobalt addition treatments (401 pmol L Ϫ1 ), whereas cobalt added to a 0.2-m filtered control remained predominantly labile (387 pmol L Ϫ1 ), demonstrating that the Synechococcus-dominated community is a source of strong cobalt ligands.Processes that influence the production of metal binding ligands in the surface ocean are not well understood. Many of the important biologically used metals (Fe, Cu, Zn, Co, and Cd) have a chemical speciation that is dominated by their complexation with strong organic ligands (e.g., Rue and Bruland 1995;Saito and Moffett 2001; Morel et al. and references therein;Morel et al. 2003). While the chemical structures of these metal-complexing ligands in the natural environment are currently unknown, their destruction by ultraviolet-irradiation provides evidence for their organic composition. Furthermore, conditional stability constants indicate these ligands are very strong, much stronger than most inorganic complexes (i.e., chloro or carbonate complexes).1 Corresponding author (mak@whoi.edu). AcknowledgmentsWe are grateful to co-chief scientists Ken Bruland and Dave Hutchins for providing space aboard the cruise, helpful discussions, and ancillary data. Thanks to G. Smith, J. Conn, and L. M. Lessin for water samples and nutrient and chlorophyll analyses. Thanks to the captain and crew of the R/V Melville. We are indebted to J. Waterbury, E. Webb, and two anonymous reviewers' comments on the manuscript and to Val Franck for helpful discussions.

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