1,720,982 research outputs found
A dissolved cobalt plume in the oxygen minimum zone of the eastern tropical South Pacific
Full text linkCobalt is a nutrient to phytoplankton, but knowledge about its biogeochemical cycling is limited, especially in the Pacific Ocean. Here, we report sections of dissolved cobalt and labile dissolved cobalt from the US GEOTRACES GP16 transect in the South Pacific. The cobalt distribution is closely tied to the extent and intensity of the oxygen minimum zone in the eastern South Pacific with highest concentrations measured at the oxycline near the Peru margin. Below 200 m, remineralization and circulation produce an inverse relationship between cobalt and dissolved oxygen that extends throughout the basin. Within the oxygen minimum zone, elevated concentrations of labile cobalt are generated by input from coastal sources and reduced scavenging at low O2. As these high cobalt waters are upwelled and advected offshore, phytoplankton export returns cobalt to low-oxygen water masses underneath. West of the Peru upwelling region, dissolved cobalt is less than 10 pM in the euphotic zone and strongly bound by organic ligands. Because the cobalt nutricline within the South Pacific gyre is deeper than in oligotrophic regions in the North and South Atlantic, cobalt involved in sustaining phytoplankton productivity in the gyre is heavily recycled and ultimately arrives from lateral transport of upwelled waters from the eastern margin. In contrast to large coastal inputs, atmospheric deposition and hydrothermal vents along the East Pacific Rise appear to be minor sources of cobalt. Overall, these results demonstrate that oxygen biogeochemistry exerts a strong influence on cobalt cycling.National Science Foundation (U.S.) (Award OCE-1237011
Iron and Cobalt Limitation in Atlantic Prochlorococcus MIT9301
Full text linkABSTRACT
Prochlorococcus is an abundant marine microorganism playing a vital role in the Earth’s carbon cycle and therefore climate. This organism is a cyanobacteria meaning it obtains energy through photosynthesis. A large portion of the ocean’s phytoplankton populations are limited by low concentrations of trace metals such as iron and cobalt. Future climate change will likely alter ocean circulation and supply of trace metals to the surface ocean, and may impact Prochlorococcus populations, which can affect marine food webs. We only have information on iron and cobalt requirements for a few strains of Prochlorococcus. Therefore, we performed culturing experiments to understand the growth and decay of Prochlorococcus in a lab environment with changes in levels of iron and cobalt. Iron limitation on Atlantic Prochlorococcus did not have a significant impact on growth rates, while cobalt limitation experiments did significantly impact the growth of these phytoplankton when lowered. The growth rates resulting from these experiments can be used to model future changes in Prochlorococcus populations as the impacts of climate change become more prevalent.
Keywords: Prochlorococcus, trace metal, cobalt, iron, growth rate, limitationProchlorococcus is an abundant marine microorganism playing a vital role in the Earth’s carbon cycle and therefore climate. This organism is a cyanobacteria meaning it obtains energy through photosynthesis. A large portion of the ocean’s phytoplankton populations are limited by low concentrations of trace metals such as iron and cobalt. Future climate change will likely alter ocean circulation and supply of trace metals to the surface ocean, and may impact Prochlorococcus populations, which can affect marine food webs. We only have information on iron and cobalt requirements for a few strains of Prochlorococcus. Therefore, we performed culturing experiments to understand the growth and decay of Prochlorococcus in a lab environment with changes in levels of iron and cobalt. Iron limitation on Atlantic Prochlorococcus did not have a significant impact on growth rates, while cobalt limitation experiments did significantly impact the growth of these phytoplankton when lowered. The growth rates resulting from these experiments can be used to model future changes in Prochlorococcus populations as the impacts of climate change become more prevalent.
Keywords: Prochlorococcus, trace metal, cobalt, iron, growth rate, limitatio
The Ocean’s Cobalt Cycle and its Correlations with other Metals
No full textThe metal cobalt is an essential nutrient for many important marine organisms, but its distribution across the oceans is not well known. When cobalt concentrations correlate with phosphate it is acting as a nutrient to marine organisms, but when it correlates with manganese it is normally an indication that there is a strong geological influence. Other metals such as manganese and phosphate are prominent elements in seawater compared to cobalt which is why they are used as proxies to cobalt (Zeng, 2019). Cobalt concentrations for over 100 samples from the TARA Oceans expedition were measured across the global ocean. The focus is on four regions: the North Pacific, South Pacific, West Pacific and the North Atlantic, which demonstrate how other elements can trace the cobalt cycle in the ocean. Cobalt exhibits similar behavior to phosphate in some regions and manganese in others. In the North-east Pacific region, Mexican and Central American coastal waters had a small correlation with manganese: cobalt concentration is higher when manganese is high. In the Southeast Pacific and South American coast, an increase in cobalt is detected when manganese is elevated. Cobalt concentrations from the Western Pacific region have not been reported previously. We observed increased cobalt concentrations off the coast of Australia, New Caledonia, and the coast of Papua New Guinea due to input from river systems from the land masses, as well as at the equator, to the upwelling of deep nutrient rich waters. Interestingly, Co and phosphate are correlated in the North Atlantic and Caribbean Sea. In this paper cobalt is traced by two metals from different sources that can paint a picture of the metal’s cycle and influence. By correlating cobalt with these two metals that are more easily measured elements, we can broaden our understanding of the cobalt cycle and its sources
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Siderophore-based microbial adaptations to iron scarcity across the eastern Pacific Ocean
Full text linkNearly all iron dissolved in the ocean is complexed by strong organic ligands of unknown composition. The effect of ligand composition on microbial iron acquisition is poorly understood, but amendment experiments using model ligands show they can facilitate or impede iron uptake depending on their identity. Here we show that siderophores, organic compounds synthesized by microbes to facilitate iron uptake, are a dynamic component of the marine ligand pool in the eastern tropical Pacific Ocean. Siderophore concentrations in iron-deficient waters averaged 9 pM, up to fivefold higher than in iron-rich coastal and nutrient-depleted oligotrophic waters, and were dominated by amphibactins, amphiphilic siderophores with cell membrane affinity. Phylogenetic analysis of amphibactin biosynthetic genes suggests that the ability to produce amphibactins has transferred horizontally across multiple Gammaproteobacteria, potentially driven by pressures to compete for iron. In coastal and oligotrophic regions of the eastern Pacific Ocean, amphibactins were replaced with lower concentrations (1–2 pM) of hydrophilic ferrioxamine siderophores. Our results suggest that organic ligand composition changes across the surface ocean in response to environmental pressures. Hydrophilic siderophores are predominantly found across regions of the ocean where iron is not expected to be the limiting nutrient for the microbial community at large. However, in regions with intense competition for iron, some microbes optimize iron acquisition by producing siderophores that minimize diffusive losses to the environment. These siderophores affect iron bioavailability and thus may be an important component of the marine iron cycle.National Science Foundation (U.S.) (OCE-1356747)National Science Foundation (U.S.) (OCE-1233261)National Science Foundation (U.S.) (OCE-1237034)National Science Foundation (U.S.) (DBI-0424599)Gordon and Betty Moore Foundation (Grant GBMF3298)Gordon and Betty Moore Foundation (Grant GBMF3934)Simons Foundation (329108
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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