Biogeographical controls on the marine nitrogen fixers

We interpret the environmental controls on the global ocean diazotroph biogeography in the context of a three-dimensional global model with a self-organizing phytoplankton community. As is observed, the model's total diazotroph population is distributed over most of the oligotrophic warm subtropical and tropical waters, with the exception of the southeastern Pacific Ocean. This biogeography broadly follows temperature and light constraints which are often used in both field-based and model studies to explain the distribution of diazotrophs. However, the model suggests that diazotroph habitat is not directly controlled by temperature and light, but is restricted to the ocean regions with low fixed nitrogen and sufficient dissolved iron and phosphate concentrations. We interpret this regulation by iron and phosphate using resource competition theory which provides an excellent qualitative and quantitative framework.Gordon and Betty Moore Foundation (Marine Microbiology Initiative)United States. National Oceanic and Atmospheric AdministrationUnited States. National Aeronautics and Space AdministrationNational Science Foundation (U.S.

Antarctic ice sheet fertilises the Southern Ocean

Southern Ocean (SO) marine primary productivity (PP) is strongly influenced by the availability of iron in surface waters, which is thought to exert a significant control upon atmospheric CO2 concentrations on glacial/interglacial timescales. The zone bordering the Antarctic Ice Sheet exhibits high PP and seasonal plankton blooms in response to light and variations in iron availability. The sources of iron stimulating elevated SO PP are in debate. Established contributors include dust, coastal sediments/upwelling, icebergs and sea ice. Subglacial meltwater exported at the ice margin is a more recent suggestion, arising from intense iron cycling beneath the ice sheet. Icebergs and subglacial meltwater may supply a large amount of bioavailable iron to the SO, estimated in this study at 0.07–0.2 Tg yr−1. Here we apply the MIT global ocean model (Follows et al., 2007) to determine the potential impact of this level of iron export from the ice sheet upon SO PP. The export of iron from the ice sheet raises modelled SO PP by up to 40%, and provides one plausible explanation for seasonally very high in situ measurements of PP in the near-coastal zone. The impact on SO PP is greatest in coastal regions, which are also areas of high measured marine PP. These results suggest that the export of Antarctic runoff and icebergs may have an important impact on SO PP and should be included in future biogeochemical modelling.Leverhulme Trust (Philip Leverhulme Prize)Leverhulme Trust (Leverhulme Research Fellowship)Leverhulme Trust (PDRA grant F/00182/BY)Royal Society (Great Britain) (Fellowship)European Commission (Marie-Curie Intra-European Fellowship)Natural Environment Research Council (Great Britain) (NERC Fellowship NE/J019062/1

Iron conservation by reduction of metalloenzyme inventories in the marine diazotroph Crocosphaera watsonii

The marine nitrogen fixing microorganisms (diazotrophs) are a major source of nitrogen to open ocean ecosystems and are predicted to be limited by iron in most marine environments. Here we use global and targeted proteomic analyses on a key unicellular marine diazotroph Crocosphaera watsonii to reveal large scale diel changes in its proteome, including substantial variations in concentrations of iron metalloproteins involved in nitrogen fixation and photosynthesis, as well as nocturnal flavodoxin production. The daily synthesis and degradation of enzymes in coordination with their utilization results in a lowered cellular metalloenzyme inventory that requires ~40% less iron than if these enzymes were maintained throughout the diel cycle. This strategy is energetically expensive, but appears to serve as an important adaptation for confronting the iron scarcity of the open oceans. A global numerical model of ocean circulation, biogeochemistry and ecosystems suggests that Crocosphaera’s ability to reduce its iron-metalloenzyme inventory provides two advantages: It allows Crocosphaera to inhabit regions lower in iron and allows the same iron supply to support higher Crocosphaera biomass and nitrogen fixation than if they did not have this reduced iron requirement.National Science Foundation (U.S.). Chemical and Biological Oceanography Program (OCE-0452883)National Science Foundation (U.S.). Chemical and Biological Oceanography Program (OCE-0752291)National Science Foundation (U.S.). Chemical and Biological Oceanography Program (OCE-0723667)National Science Foundation (U.S.). Chemical and Biological Oceanography Program (OCE-0928414)National Science Foundation (U.S.). Polar Program (ANT-0732665)United States. Environmental Protection Agency (Star Fellowship)Woods Hole Oceanographic Institution. Ocean Life InstituteCenter for Microbial Oceanography: Research and EducationCenter for Environmental Bioinorganic Chemistr

Distribution of diverse nitrogen fixers in the global ocean

We employ a global three-dimensional model to simulate diverse phytoplanktonic diazotrophs (nitrogen fixers) in the oceans. In the model, the structure of the marine phytoplankton community self-assembles from a large number of potentially viable physiologies. Amongst them, analogs of Trichodesmium, unicellular diazotrophs and diatom-diazotroph associations (DDA) are successful and abundant. The simulated biogeography and nitrogen fixation rates of the modeled diazotrophs compare favorably with a compilation of published observations, which includes both traditional and molecular measurements of abundance and activity of marine diazotrophs. In the model, the diazotroph analogs occupy warm subtropical and tropical waters, with higher concentrations and nitrogen fixation rates in the tropical Atlantic Ocean and the Arabian Sea/Northern Indian Ocean, and lower values in the tropical and subtropical South Pacific Ocean. The three main diazotroph types typically co-exist in the model, although Trichodesmium analogs dominate the diazotroph population in much of the North and tropical Atlantic Ocean and the Arabian Sea, while unicellular-diazotroph analogs dominate in the South Atlantic, Pacific and Indian oceans. This pattern reflects the relative degree of nutrient limitation by iron or phosphorus. The model suggests in addition that unicellular diazotrophs could add as much new nitrogen to the global ocean as Trichodesmium.Gordon and Betty Moore Foundation (Marine Microbiology Initiative)United States. National Aeronautics and Space Administratio