Cobalt (Co) is an important bioactive trace metal that is the metal
cofactor in cobalamin (vitamin B12) which can limit or co-limit
phytoplankton growth in many regions of the ocean. Total dissolved ...and
labile Co measurements in the Canadian sector of the Arctic Ocean during the
U.S. GEOTRACES Arctic expedition (GN01) and the Canadian International Polar
Year GEOTRACES expedition (GIPY14) revealed a dynamic biogeochemical cycle
for Co in this basin. The major sources of Co in the Arctic were from shelf
regions and rivers, with only minimal contributions from other freshwater
sources (sea ice, snow) and eolian deposition. The most striking feature
was the extremely high concentrations of dissolved Co in the upper 100 m,
with concentrations routinely exceeding 800 pmol L−1 over the shelf
regions. This plume of high Co persisted throughout the Arctic basin and
extended to the North Pole, where sources of Co shifted from primarily
shelf-derived to riverine, as freshwater from Arctic rivers was entrained in
the Transpolar Drift. Dissolved Co was also strongly organically complexed
in the Arctic, ranging from 70 % to 100 % complexed in the surface and deep
ocean, respectively. Deep-water concentrations of dissolved Co were
remarkably consistent throughout the basin (∼55 pmol L−1), with concentrations reflecting those of deep Atlantic water and
deep-ocean scavenging of dissolved Co. A biogeochemical model of Co cycling
was used to support the hypothesis that the majority of the high surface Co
in the Arctic was emanating from the shelf. The model showed that the high
concentrations of Co observed were due to the large shelf area of the
Arctic, as well as to dampened scavenging of Co by manganese-oxidizing (Mn-oxidizing)
bacteria due to the lower temperatures. The majority of this scavenging
appears to have occurred in the upper 200 m, with minimal additional
scavenging below this depth. Evidence suggests that both dissolved Co (dCo) and labile Co (LCo) are increasing over time on the Arctic shelf, and these limited temporal results are consistent
with other tracers in the Arctic. These
elevated surface concentrations of Co likely lead to a net flux of Co out of
the Arctic, with implications for downstream biological uptake of Co in the
North Atlantic and elevated Co in North Atlantic Deep Water. Understanding
the current distributions of Co in the Arctic will be important for
constraining changes to Co inputs resulting from regional intensification of
freshwater fluxes from ice and permafrost melt in response to ongoing
climate change.
Throughout the open ocean, a minimum in dissolved iron concentration (dFe) overlaps with the deep chlorophyll maximum (DCM), which marks the lower limit of the euphotic zone. Maximizing light capture ...in these dim waters is expected to require upregulation of Fe-bearing photosystems, further depleting dFe and possibly leading to co-limitation by both iron and light. However, this effect has not been quantified for important phytoplankton groups like Prochlorococcus, which contributes most of the productivity in the oligotrophic DCM. Here, we present culture experiments with Prochlorococcus strain MIT1214, a member of the Low Light 1 ecotype isolated from the DCM in the North Pacific subtropical gyre. Under a matrix of iron and irradiance matching those found at the DCM, the ratio of Fe to carbon in Prochlorococcus MIT1214 cells ranged from 10-40 × 10
mol Fe:mol C and increased with light intensity and growth rate. These results challenge theoretical models predicting highest Fe:C at lowest light intensity, and are best explained by a large photosynthetic Fe demand that is not downregulated at higher light. To sustain primary production in the DCM with the rigid Fe requirements of low-light-adapted Prochlorococcus, dFe must be recycled rapidly and at high efficiency.
Cobalt is an important micronutrient for ocean microbes as it is present in vitamin B12 and is a co‐factor in various metalloenzymes that catalyze cellular processes. Moreover, when seawater ...availability of cobalt is compared to biological demands, cobalt emerges as being depleted in seawater, pointing to a potentially important limiting role. To properly account for the potential biological role for cobalt, there is therefore a need to understand the processes driving the biogeochemical cycling of cobalt and, in particular, the balance between external inputs and internal cycling. To do so, we developed the first cobalt model within a state‐of‐the‐art three‐dimensional global ocean biogeochemical model. Overall, our model does a good job in reproducing measurements with a correlation coefficient of >0.7 in the surface and >0.5 at depth. We find that continental margins are the dominant source of cobalt, with a crucial role played by supply under low bottom‐water oxygen conditions. The basin‐scale distribution of cobalt supplied from margins is facilitated by the activity of manganese‐oxidizing bacteria being suppressed under low oxygen and low temperatures, which extends the residence time of cobalt. Overall, we find a residence time of 7 and 250 years in the upper 250 m and global ocean, respectively. Importantly, we find that the dominant internal resupply process switches from regeneration and recycling of particulate cobalt to dissolution of scavenged cobalt between the upper ocean and the ocean interior. Our model highlights key regions of the ocean where biological activity may be most sensitive to cobalt availability.
Plain Language Summary
Biological activity in the sea requires cobalt, primarily due to its role in vitamin B12 but also because it is required in other cellular enzymes. While our observations of cobalt distributions in the ocean is growing, we do not have a quantitative understanding of the role of different external sources of cobalt or how it is internally processed by different biological and chemical processes in the ocean. To answer these questions, we built the first ever global ocean cobalt model that coupled the cycling of cobalt to the major biogeochemical processes occurring in the ocean. Using this model, we identified that sediments are the major cobalt source and that the combination of oxygen levels and scavenging removal by bacteria allow externally supplied cobalt to pervade the ocean as a whole. We find that in certain regions of the upper ocean, cobalt levels may be low enough to affect biological activity but that to quantify this requires further work on how we represent cellular biochemistry.
Key Points
The first global model of ocean cobalt cycling is presented
Sediments are the major global source of cobalt
Suppression of Co scavenging by low oxygen and reduced bacterial activity is key in extending the residence time of externally supplied Co
Organic ligands form strong complexes with many trace elements in seawater. Various metals can compete for the same ligand chelation sites, and the final speciation of bound metals is determined by ...relative binding affinities, concentrations of binding sites, uncomplexed metal concentrations, and association/dissociation kinetics. Different ligands have a wide range of metal affinities and specificities. However, the chemical composition of these ligands in the marine environment remains poorly constrained, which has hindered progress in modeling marine metal speciation. In this study, we detected and characterized natural ligands that bind copper (Cu) and nickel (Ni) in the eastern South Pacific Ocean with liquid chromatography tandem inductively coupled plasma mass spectrometry (LC-ICPMS), and high resolution electrospray ionization mass spectrometry (ESIMS). Dissolved Cu, Ni, and ligand concentrations were highest near the coast. Chromatographically unresolved polar compounds dominated ligands isolated near the coast by solid phase extraction. Offshore, metal and ligand concentrations decreased, but several new ligands appeared. One major ligand was detected that bound both Cu2+ and Ni2+. Based on accurate mass and fragmentation measurements, this compound has a molecular formula of C20H21N4O8S2 + M+ (M = metal isotope) and contains several azole-like metal binding groups. Additional lipophilic Ni complexes were also present only in oligotrophic waters, with masses of 649, 698, and 712 m/z (corresponding to the 58Ni metal complex). Molecular formulae of C32H54N3O6S2Ni+ and C33H56N3O6S2Ni+ were determined for two of these compounds. Addition of Cu and Ni to the samples also revealed the presence of additional compounds that can bind both Ni and Cu. Although these specific compounds represent a small fraction of the total dissolved Cu and Ni pool, they highlight the compositional diversity and spatial heterogeneity of marine Ni and Cu ligands, as well as variability in the extent to which different metals in the same environment compete for ligand binding.
Trace metals such as iron, nickel, copper, zinc, and cadmium (Fe, Ni, Cu, Zn, and Cd) are essential micronutrients (and sometimes toxins) for phytoplankton, and the analysis of trace-metal stable ...isotopes in seawater is a valuable tool for exploring the biogeochemical cycling of these elements in the ocean. However, the complex and often time-consuming chromatography process required to purify these elements from seawater has limited the number of trace-metal isotope samples which can be easily processed in biogeochemical studies. To facilitate the trace-metal stable isotope analysis, here, we describe a new rapid procedure that utilizes automated chromatography for extracting and purifying Ni and Cu from seawater for isotope analysis using a prepFAST-MC™ system (Elemental Scientific Inc.).
We have tested the matrix removal effectiveness, recoveries, and procedural blanks of the new purification procedure with satisfactory results. A nearly complete recovery of Ni and a quantitative recovery of Cu are achieved. The total procedural blanks are 0.33 ± 0.24 ng for Ni and 0.42 ± 0.18 ng for Cu, which is negligible for natural seawater samples. The new procedure cleanly separates Ni and Cu from key seawater matrix elements that may cause interferences during mass spectrometry analysis. When the new procedure was used to purify seawater samples for Ni and Cu stable isotope analysis by multi-collector ICP-MS, we achieved an overall uncertainty of 0.07 ‰ for δ60Ni and 0.09 ‰ for δ65Cu (2 SD). The new purification procedure was also tested using natural seawater samples from the South Pacific, for comparison of δ60Ni and δ65Cu achieved in the same samples purified by traditional hand columns. Both methods produced similar results, and the results from both methods are consistent with analyses of δ60Ni and δ65Cu from other ocean locations as reported by other laboratories.
This study presents a new rapid procedure for seawater stable-metal isotope analysis by automating the chromatography step. We anticipate that the automated chromatography described here will facilitate the rapid and accurate analysis of seawater δ60Ni and δ65Cu in future studies, and may be adapted in the future to automate chromatographic purification of Fe, Zn, and Cd isotopes from seawater.
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•We develop a procedure for Ni–Cu isotopic analysis using automated chromatography.•Nickel and Cu are fully recovered and cleanly separated from matrix elements.•The method was tested on seawater samples and yielded accurate and precise data.•The method may be adapted for the purification of Fe, Zn, and Cd in the future.
We examined the biogeochemical impact of paired mesoscale cyclones and anticyclones in spatial proximity (<200 km apart) in the North Pacific Subtropical Gyre. While previous studies have ...demonstrated that upwelling associated with the intensification of cyclonic eddies can supply nutrients supporting plankton productivity, we observed that steeper vertical gradients in inorganic nutrients increased nutrient fluxes due to diapycnal mixing during the mature stage of cyclonic eddies. The increased diapycnal nutrient supply was linked with expansion of eukaryotic phytoplankton biomass and intensification of the deep chlorophyll maximum (DCM) layer. This perturbation in the plankton community was associated with increased fluxes of biominerals (specifically particulate inorganic carbon and particulate silica) and isotopically enriched organic nitrogen in particles exported in the cyclone. The time‐integrated effects of thermocline vertical displacements on the lower euphotic zone were predictable deficits and surpluses of inorganic nutrients and dissolved oxygen, respectively. However, the stoichiometry of oxygen and inorganic nutrients differed from that predicted for production and consumption of phytoplankton biomass, requiring additional biological processes that decouple changes in oxygen and nutrient concentrations. The dynamics revealed by this study may be a common feature of oligotrophic ecosystems, where mesoscale biogeochemical perturbations are buffered by the DCM layer, which limits the ecological impact of eddies in the well‐lit, near‐surface ocean.
Key Points
The steepness of the nutricline in mature mesoscale eddies is an important driver of variability in pelagic ecosystems
Differences in eukaryotic phytoplankton communities alter the export of particulate inorganic carbon and particulate silica across eddies
Relative changes in oxygen and nutrients linked to displacements of the thermocline reflect anomalous cycling of organic matter
Nickel and copper are cofactors in key phytoplankton enzymes and the stable isotope composition of Ni and Cu (δ60Ni and δ65Cu) in seawater have the potential to identify major processes that ...influence their biogeochemistry. However, accurate analysis of δ60Ni and δ65Cu is challenging because of the difficulties in separating these metals from interfering elements in the seawater matrix. Here we report a fast and simple method for purification of Ni and Cu from seawater samples that is able to completely remove interfering elements Mn, Ti, Cr, and Fe. This method was verified by analyzing four reference materials that contain significant levels of interfering elements (powdered plankton, natural soils, and two marine sediments). Using this technique, we generated a dataset of 49 seawater δ60Ni and δ65Cu measurements from the upper water column of the North Pacific Ocean, which show preferential uptake of light Ni isotopes by phytoplankton (αbio-sw = 0.9997 ± 1) but no net fractionation of Cu isotopes. This new method simplifies treatment of seawater samples for Ni and Cu isotope analysis, enabling high-throughput investigations of δ60Ni and δ65Cu throughout the global ocean.
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•A new method for purification of Ni and Cu for isotopic analyses is presented.•Method effectively removes interfering and matrix elements in seawater, marine sediments, and other geological materials.•A dataset of 49 new samples from the North Pacific Ocean reveals biological fractionation of δ60Ni but not δ65Cu.
Despite very low concentrations of cobalt in marine waters, cyanobacteria in the genus Prochlorococcus retain the genetic machinery for the synthesis and use of cobalt-bearing cofactors (cobalamins) ...in their genomes. We explore cobalt metabolism in a Prochlorococcus isolate from the equatorial Pacific Ocean (strain MIT9215) through a series of growth experiments under iron- and cobalt-limiting conditions. Metal uptake rates, quantitative proteomic measurements of cobalamin-dependent enzymes, and theoretical calculations all indicate that Prochlorococcus MIT9215 can sustain growth with less than 50 cobalt atoms per cell, ∼100-fold lower than minimum iron requirements for these cells (∼5,100 atoms per cell). Quantitative descriptions of Prochlorococcus cobalt limitation are used to interpret the cobalt distribution in the equatorial Pacific Ocean, where surface concentrations are among the lowest measured globally but Prochlorococcus biomass is high. A low minimum cobalt quota ensures that other nutrients, notably iron, will be exhausted before cobalt can be fully depleted, helping to explain the persistence of cobalt-dependent metabolism in marine cyanobacteria.
The 2018, subaerial eruption of Kīlauea volcano, Hawaii, resulted in a 5‐km‐long stretch of coastline that actively drained lava into the ocean. Nutrients were added to the surrounding ocean through ...the dissolution of basaltic rock and thermal upwelling of deep water, thereby fueling a large phytoplankton bloom. Lava‐impacted, surface seawater had high suspended particle loads, and concentrations of chlorophyll, silicic acid, phosphate (Pi), nitrate, and iron that were elevated up to 12, 36, 5, 960, and 1,400 times, respectively, above the background oligotrophic levels. Widespread precipitation of iron oxyhydroxides (Feox) led to extensive scavenging of the dissolved Pi pool, similar to what occurs along mid‐ocean ridge hydrothermal systems. This scavenging transformed a “fertilization” event into a Pi sink near the coast of the ocean entry; however, nutrient data from outside the bloom suggest that Pi could also desorb from the Feox as it is dispersed into the open ocean. From lava quench experiments, we estimate that the hydration state of the Feox precipitate (H2O/Fe) was 5.2–5.7, and that the equilibrium partition coefficient of Pi into Feox (solid/liquid) was 106. In addition, 33Pi radiotracer incubations were used to differentiate between biotic and abiotic uptake of Pi at Kīlauea's ocean entry. These findings are important for understanding modern‐day volcanic fertilization events, modeling nutrient dynamics during major events in Earth history (such as oxygenation of the atmosphere and the formation of large igneous provinces), and predicting the marine response to greater continental weathering in a warming climate.
Key Points
Seawater collected offshore of a lava ocean entry zone in Hawaii had elevated particle, chlorophyll, nutrient, and iron concentrations
Precipitation of iron oxyhydroxide in this volcanic fertilization event led to extensive scavenging of dissolved phosphate
Laboratory experiments yielded estimates for phosphate partitioning and diffusive exchange into iron oxyhydroxide
In the ocean, dissolved cobalt is affected by both nutrient cycling and scavenging onto manganese oxides. The latter process concentrates Co in pelagic sediments, resulting in a small deep water ...inventory. While the flux of scavenged cobalt to sediments appears steady on timescales>100,000years, its residence time in the water column is short, approximately 130years. Using results from recent GEOTRACES expeditions, we show net removal of dissolved Co from the deep ocean on the order of 0.043pMyear−1, which corresponds to a turnover time of 980years. Scavenging in deep ocean water masses is too slow to match cobalt accumulation rates in marine sediments, requiring most of the scavenging flux to derive from the mesopelagic ocean (<1500m depth) where nutrient cycling is active. Based on differences between the Co:P stoichiometry in particles sinking from the euphotic zone and dissolved Co:P remineralization ratios, we calculate areal scavenging rates in the North Atlantic and South Pacific basins on the order of 1.5 and 0.7μmol m−2year−1, respectively, which agree with long-term accumulation rates in Atlantic and Pacific sediments. In both basins, over 50% of the scavenged flux of cobalt occurs in the upper 500m, resulting in decadal turnover times in the mesopelagic. An assessment of sources suggests that the marine cobalt cycle is approximately in balance, but that this inventory may be sensitive to long term trends in the intensity of oxygen minimum zones, which account for ~25% of the annual cobalt source to the modern oceans.
•The residence time of cobalt is short due to scavenging.•Dissolved cobalt removal from the deep ocean is slow.•Most of the cobalt scavenging flux originates in the upper 500m.•Water column scavenging and sedimentary accumulation rates agree.
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