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Science Highlights

Siderophores facilitate microbial adaptation to iron limitation in the eastern tropical Pacific Ocean

Siderophores are organic compounds secreted by microbes to facilitate iron uptake. Using new methods to characterize trace metal organic ligands in seawater, Boiteau and colleagues (2016, see reference below) measured the distribution of siderophores along the US East Pacific Zonal Transect (EPZT; GEOTRACES GP16). The cruise track crossed from the highly productive Peruvian coastal upwelling region into the oligotrophic central gyre. The study revealed important changes in siderophore composition and concentration across different nutrient regimes (see figure below). Siderophores were found to be nine times more abundant in the most iron-depleted areas of the transect compared to the iron-rich coastal zone. Companion phylogenetic analysis of siderophore synthesis genes in the TARA Oceans metagenomic catalogue led Boiteau and co-workers to suggest that lateral transfer of siderophore synthesis genes help microbes adapt to low iron conditions found in many regions of the ocean.

17 RepetaFigure: Distribution of siderophores across the GEOTRACES EPZT. Amphibactin siderophores (top panel) appear as peaks in the trace of organic iron isolated from seawater (middle panels). Each peak represents a different molecular form of amphibactin. Concentrations of amphibactins across the ETPZ were low in the high iron coastal region, high in the HNLC region, and low again in the low iron oligotraphic region (lower panel). Please click here to view the figure larger.

Reference :

Boiteau, R. M., Mende, D. R., Hawco, N. J., McIlvin, M. R., Fitzsimmons, J. N., Saito, M. A., Sedwick, P. N., DeLong, E. F., Repeta, D. J. (2016). Siderophore-based microbial adaptations to iron scarcity across the eastern Pacific Ocean. Proceedings of the National Academy of Sciences of the United States of America, 113(50), 14237–14242. DOI: 10.1073/pnas.1608594113

Dissolved iron isotopes reveal that distinct processes are controlling this micronutrient distribution in the ocean

Abadie and co-workers propose new dissolved iron concentration and isotopic composition distributions (DFe and ICFe respectively) along the Bonus-Goodhope IPY section (GIPY4), in the Atlantic sector of the Southern Ocean. DFe vertical profiles display a continuous increase with depth (see figure B), classically interpreted as due to biological uptake at the surface followed by remineralization at depth. However, heterogeneous profiles of ICFe (see figure B) suggest a more complicated story driven by distinct processes, discussed here for the first time. Indeed, the authors demonstrate that in the intermediate waters, DFe primarily originates from remineralization of organic matter and the redistribution of this regenerated DFe through mixing. Moreover it is also due to horizontal advection of DFe released by reducing sediments of the nearby South African coast. The scheme changes deeper, where abiotic processes are dominating the DFe distribution as for example non-reductive release of DFe from lithogenic particles. This last process would add an additional source to the global oceanic DFe budget, which should be considered in the biogeochemical models. In addition, it suggests that the oceanic DFe budget could be more sensitive than previously thought to continental erosion, particle transport, and dissolved/particle interactions.

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Figure: (A) Position of the 5 stations sampled for iron isotopes during the Bonus-Goodhope IPY GEOTRACES cruise; (B) Examples of profiles for dissolved iron concentration (DFe) and dissolved iron isotopic composition (expressed by δ56DFe) obtained in one of the 5 stations sampled (other stations show similar patterns); Iron isotopes show a very sharp minimum in intermediate depths (i.e. between about 200 and 1500 m below the surface). The contrast between these intermediate depths and the deep ocean (3000-5000 m) demonstrates that two different processes dominate dissolved iron sources in the ocean at these two levels. (C) Dissolved iron isotopic composition (δ56DFe) along the Bonus-Goodhope section. Negative values (in cold colours, blue, green) indicate iron that is naturally enriched in light isotopes, while high values (in warm colours, red, orange) indicate heavy iron isotopes enrichment. Click here to view the figure larger.

 Reference:

Abadie, C., Lacan, F., Radic, A., Pradoux, C., Poitrasson F. (2017) Iron isotopes reveal distinct dissolved iron sources and pathways in the intermediate versus deep Southern Ocean, PNAS, DOI: 10.1073/pnas.1603107114

Water mass circulation and weathering inputs in the Labrador Sea based on coupled hafnium-neodymium isotope compositions and rare earth element distributions

Filippova and co-authors (2017, see reference below) show distinct water mass signatures in the Labrador Sea revealed by combined dissolved hafnium (Hf) and neodymium (Nd) isotope compositions and REE distribution patterns along the AR7W transect in May 2013.

The new data show that in a semi-enclosed basin such as the Labrador Sea, the radiogenic Hf isotope signatures can serve as a highly sensitive tracer of water mass mixing processes given that they allow distinction of particular water masses that do not differ in their Nd isotope compositions. Based on the new data, the authors suggest that the residence time of Hf in the Labrador Sea can only be on the order of decades in order to sustain the observed variability. The high sensitivity of Hf isotopes to decadal ocean circulation changes in the Labrador Sea suggests a potential prospect for their application in other restricted basins with similar geological settings and pronounced short-term hydrographic variability.

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 17 Filipovafig2Figures: (top) Schematic map of the study area. Blue arrows represent cold deep currents and red arrows denote warm surface currents. Red dots indicate the positions of the stations occupied during CCGS Hudson Cruise 2013. A schematic representation of the geology of the surrounding landmasses is shown and includes average ɛHf and ɛNd values of the rocks. Please click here to view the figure larger. (bottom) Water masses distribution versus depth in the Labrador Sea based on their ɛNd (A) and ɛHf (B) signatures. Please click here to view the figure larger.

Reference:

Filippova, A., Frank, M., Kienast, M., Rickli, J., Hathorne, E., Yashayaev, I.M., and Böning, P. (2017): Water mass circulation and weathering inputs in the Labrador Sea based on coupled Hf-Nd isotope compositions and rare earth element distributions.- Geochimica et Cosmochimica Acta 199, 164-184. DOI: 10.1016/j.gca.2016.11.024

Up to four mercury species measured along the GEOTRACES East Pacific Zonal Transect

Total mercury (HgT), elemental mercury (Hg0), monomethyl-mercury (MMHg), and dimethyl-mercury (DMHg) were measured at an unprecedent resolution so far along the East Pacific Zonal Transect (EPZT) GEOTRACES cruise (GP16). Knowing that MMHg is a neurotoxin that accumulates in the trophic chain, establishing the full speciation of this element is required in order to better understand oceanic Hg cycle and more specifically where and how MMHg is formed and accumulated.

The other exciting feature of this article is that contrasted biogeographic oceanic provinces have been sampled during EPZT (Peru upwelling region, a suboxic OMZ, and an expansive submarine hydrothermal vent plume).

Bowman and co-authors results show that while HgT is not enriched in the hydrothermal plume, total Hg is accumulated with age and that DMHg is the dominant methylated species in deep waters. Importantly, filtered HgT is accumulated in upwelled waters near the coast of Peru where oxygen concentrations were the lowest, and MMHg represented 10–20% of the total Hg upwelling flux. This is the first identification of Hg behaviour along the Peru coast, a region that supports the largest fisheries of the world...

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Figures:
Images depict mercury concentrations in the water column from the coast of Peru (right), across the Eastern Pacific Rise (EPR), to Tahiti (left), measured during the 2013 U.S. GEOTRACES Eastern Pacific Zonal Section. Total mercury (HgT), gaseous elemental mercury (Hg0), methylmercury (MMHg), and dimethylmercury (DMHg) in filtered water is shown, in addition to methylmercury in suspended particles (MMHgpart.). Click here to view the figure larger.

Reference:

Bowman, K. L., Hammerschmidt, C. R., Lamborg, C. H., Swarr, G. J., & Agather, A. M. (2016). Distribution of mercury species across a zonal section of the eastern tropical South Pacific Ocean (U.S. GEOTRACES GP16). Marine Chemistry, 186, 156–166. DOI: 10.1016/j.marchem.2016.09.005

Oxygen biogeochemistry exerts a strong influence on cobalt cycling

This is an important result of the US GEOTRACES East Pacific Zonal Transect (EPZT) cruise (GP16) discussed by Hawco and his co-workers (2016, see reference below). The distribution of dissolved cobalt and labile cobalt along this section is closely tied to the oxygen minimum zone. This work also shows that (1) elevated concentrations of labile cobalt are generated by input from coastal sources and reduced scavenging at low oxygen; (2) atmospheric deposition and hydrothermal vents along the East Pacific Rise are contrastingly minor sources of cobalt; (3) high cobalt waters are further upwelled and advected offshore and; (4) phytoplankton export returns cobalt to low-oxygen water masses underneath. These processes result in covariation of dissolved cobalt with oxygen and phosphates, schematically represented in the Figure below.

16 Hawco lFigure: In the South Pacific Ocean, high levels of cobalt are harbored in waters that are devoid of dissolved oxygen (upper panel, warm colors). This plume of cobalt stems from the Peru coast and is enhanced by degradation of cobalt-bearing phytoplankton in these waters, and by the absence of removal processes (scavenging) when oxygen is low (lower panel). Please click here to view the figure larger.

Reference:

Hawco, N. J., Ohnemus, D. C., Resing, J. A., Twining, B. S., & Saito, M. A. (2016). A dissolved cobalt plume in the oxygen minimum zone of the eastern tropical South Pacific. Biogeosciences, 13(20), 5697–5717. DOI: 10.5194/bg-13-5697-2016

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