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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