102 research outputs found
Constraints on the vital effect in coccolithophore and dinoflagellate calcite by oxygen isotopic modification of seawater
Author Posting. © The Author(s), 2014. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 141 (2014): 612-627, doi:10.1016/j.gca.2014.05.002.In this study, we show that there are independent controls of 18O/16O and 13C/12C
fractionation in coccolithophore and dinoflagellate calcite due to the contrasting kinetics of
each isotope system. We demonstrate that the direction and magnitude of the oxygen isotope
fractionation with respect to equilibrium is related to the balance between calcification rate
and the replenishment of the internal pool of dissolved inorganic carbon (DIC). As such, in
fast growing cells, such as those of Emiliania huxleyi and Gephyrocapsa oceanica (forming
the so-called âheavy groupâ), calcification of the internal carbon pool occurs faster than
complete isotopic re-adjustment of the internal DIC pool with H2O molecules. Hence,
coccoliths reflect the heavy oxygen isotope signature of the CO2 overprinting the whole DIC
pool. Conversely, in large and slow growing cells, such as Coccolithus pelagicus ssp.
braarudii, complete re-equilibration is achieved due to limited influx of CO2 leading to
coccoliths that are precipitated in conditions close to isotopic equilibrium (âequilibrium
groupâ). Species exhibiting the most negative oxygen isotope composition, such as
Calcidiscus leptoporus (âlight groupâ), precipitate coccolith under increased pH in the
coccolith vesicle, as previously documented by the âcarbonate ion effectâ. We suggest that,
for the carbon isotope system, any observed deviation from isotopic equilibrium is only
âapparentâ, as the carbon isotopic composition in coccolith calcite is controlled by a Rayleigh
fractionation originating from preferential incorporation of 12C into organic matter. Therefore,
species with low PIC/POC ratios as E. huxleyi and G. oceanica are shifted towards positive
carbon isotope values as a result of predominant carbon fixation into the organic matter. By
contrast, cells with higher PIC/POC as C. braarudii and C. leptoporus maintain, to some
extent, the original negative isotopic composition of the CO2. The calcareous dinoflagellate
Thoracosphaera heimii exhibits different behaviour for both isotopic systems, in particular with respect to its very negative carbon isotope composition, owing to coeval intra and
extracellular biomineralisation in this group. In this study, we also investigate the sensitivity
of 18O/16O fractionation to varying ambient oxygen isotope composition of the medium for
inorganic, coccolithophore, and dinoflagellate calcite precipitated under controlled laboratory
conditions. The varying responses of different taxa to increased oxygen isotope composition
of the growth medium may point to a potential bias in sea surface temperature reconstructions
that are based on the oxygen isotopic compositions of sedimentary calcite, especially during
times of changing seawater oxygen isotopic composition. Overall, this study represent an
important step towards establishing a mechanistic understanding of the âvital effectâ in
coccolith and dinoflagellate calcite, and provides valuable information for interpreting the
geochemistry of the calcareous nannofossils in the sedimentary record, at both monospecific
and interspecies levels.MH is grateful to the Natural Environment Research Council (NERC) for funding
through Postdoctoral Fellowship (NE/H015523/1). TJH is supported by the Postdoctoral
Scholar Program at the Woods Hole Oceanographic Institution, with funding provided by the
Doherty Foundation. REMR was supported through European Research Council (ERC) grant
SP2-GA-2008-200915
Reply to Morel : cadmium as a micronutrient and macrotoxin in the oceans
Author Posting. © The Author(s), 2013. This is the author's version of the work. It is posted here by permission of National Academy of Sciences for personal use, not for redistribution. The definitive version was published in Proceedings of the National Academy of Sciences of the United States of America 110 (2013): E1878, doi:10.1073/pnas.1305068110.We thank François Morel for his interest in our study. Morel states that our conclusions are based on
the approximate match between the Cd-isotope composition of cultured bacteria and the fractionation
of Cd isotopes seen in seawater (1). This match is only a minor component of our argument, and we
welcome the opportunity to reiterate our case
Glacial-interglacial changes in bottom-water oxygen content on the Portuguese margin
During the last and penultimate glacial maxima, atmospheric CO2 concentrations were lower than present, possibly in part because of increased storage of respired carbon in the deep oceans. The amount of respired carbon present in a water mass can be calculated from its oxygen content through apparent oxygen utilization; the oxygen content can in turn be calculated from the carbon isotope gradient within the sediment column. Here we analyse the shells of benthic foraminifera occurring at the sediment surface and the oxic/anoxic interface on the Portuguese Margin to reconstruct the carbon isotope gradient and hence bottom-water oxygenation over the past 150,000 years. We find that bottom-water oxygen concentrations were 45 and 65âÎŒmol kgâ1 lower than present during the last and penultimate glacial maxima, respectively. We calculate that concentrations of remineralized organic carbon were at least twice as high as today during the glacial maxima. We attribute these changes to decreased ventilation linked to a reorganization of ocean circulation and a strengthened global biological pump. If the respired carbon pool was of a similar size throughout the entire glacial deep Atlantic basin, then this sink could account for 15 and 20 per cent of the glacial PCO2 drawdown during the last and penultimate glacial maxima
Declining metal availability in the Mesozoic seawater reflected in phytoplankton succession
Variable trace metal concentrations in the Precambrian ocean were closely linked to oxygen availability, although less is known about the drivers of seawater trace metal chemistry after the spread of complex life into the Phanerozoic eon. A major phytoplankton succession took place at the transition from the Palaeozoic to the Mesozoic era (~250 Myr ago), from an ocean dominated by the green Archaeplastida to secondary endosymbiotic algae with red-algal-derived plastids. Here, our comparative genomic analysis of 26 complete proteomes and metal domain analysis of additional 608 partially complete sequences of phytoplankton reveal that groups with different evolutionary history have distinct metal-binding proteins and contrasting metal acquisition strategies, adapted to differing availability of trace metals. The secondary-endosymbiont-bearing lineages are better adapted to well-oxygenated, nutrient-poor environments. This is supported by an enhanced thiol-based binding affinity of their transporters, coupled with minimized proteomic requirement for trace elements such as iron, copper and zinc at both protein and domain levels. Such different metal requirements across these lineages suggest a drastic decline in open-ocean trace metal concentrations at the inception of the Mesozoic, contributing to the shifts in phytoplankton communities that drove major changes in ocean chemical buffering.info:eu-repo/semantics/publishedVersio
The cadmium-phosphate relationship in brine: biological versus physical control over micronutrients in sea ice environments
Despite supporting productive ecosystems in the high latitudes, the relationship between macro- and micronutrients in sea ice environments and their impact on surface productivity is poorly documented. In seawater, the macronutrient phosphate and the micronutrient cadmium follow similar distributions, which are controlled by biological processes in surface waters. We investigated cadmium and phosphate in sea ice brine, and the biological and physical processes controlling their distribution. Cadmium concentrations in sea ice brine ranged from 0.092.4 nmol kg-1, and correlated well with salinity. Our results show that micronutrients in sea ice are most probably sourced from the seawater from which it froze rather than external sources such as atmospheric deposition. The weak correlation between sea ice cadmium and phosphate, and the positive relationship between cadmium and biomass, suggests against biological uptake being a principal control over micronutrient distribution even in a highly productive setting. Instead, brine expulsion and dilution play a dominant role in cadmium distribution in sea ice. Nutrient dilution within brine channels during melting, and contrasting sea ice and open water phytoplankton populations, suggests that late spring sea ice is not a significant source of nutrients or biomass to seawater. We suggest that future changes in sea ice seasonality may impact nutrient distribution and Antarctic marine ecosystems. © 2009 Antarctic Science Ltd
Pore fluid modeling approach to identify recent meltwater signals on the west Antarctic Peninsula
The sensitivity of sea level to melting from polar ice sheets and glaciers during recent natural and
anthropogenic climate fluctuations is poorly constrained beyond the period of direct observation by satellite.
We have investigated glacial meltwater events during the Anthropocene by adapting the pioneering
approach of modeling trends in d18O in the pore waters of deepâsea cores, previously used to constrain
the size of ice sheets during the Last Glacial Maximum. We show that during recent warm periods, meltwater
from glacier retreat drains into the coastal fjords, leaving a signature of depleted d18O values and low
Cl concentrations in the pore water profiles of rapidly accumulating sediments. Here we model such pore
water profiles in a piston core to constrain the timing and magnitude of an ice sheet retreat event at Caley
Glacier on the west Antarctic Peninsula, and the result is compared with local ice front movement. This
approach of pore water modeling was then applied in another kasten core and tested by a series of sensitivity
analyses. The results suggest that our approach may be applied in fjords of different sedimentary settings
to reconstruct the glacier history and allow insight into the sensitivity of polar glaciers to abrupt
warming events
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Large variation in the Rubisco kinetics of diatoms reveals diversity among their carbon-concentrating mechanisms
While marine phytoplankton rival plants in their contribution to global primary productivity, our understanding of their photosynthesis remains rudimentary. In particular, the kinetic diversity of the COâ-fixing enzyme, Rubisco, in phytoplankton remains unknown. Here we quantify the maximum rates of carboxylation (Kcatá¶), oxygenation (Kcatá”), Michaelis constants (K m) for COâ (K C) and Oâ (K O), and specificity for COâ over Oâ (SC/O) for Form I Rubisco from 11 diatom species. Diatom Rubisco shows greater variation in KC (23-68 ÎŒM), SC/O (57-116mol molâ»Âč), and KO (413-2032 ÎŒM) relative to plant and algal Rubisco. The broad range of KC values mostly exceed those of Câ plant Rubisco, suggesting that the strength of the carbon-concentrating mechanism (CCM) in diatoms is more diverse, and more effective than previously predicted. The measured k cat c for each diatom Rubisco showed less variation (2.1-3.7sâ»Âč), thus averting the canonical trade-off typically observed between KC and kcatᶠfor plant Form I Rubisco. Uniquely, a negative relationship between KC and cellular Rubisco content was found, suggesting variation among diatom species in how they allocate their limited cellular resources between Rubisco synthesis and their CCM. The activation status of Rubisco in each diatom was low, indicating a requirement for Rubisco activase. This work highlights the need to better understand the correlative natural diversity between the Rubisco kinetics and CCM of diatoms and the underpinning mechanistic differences in catalytic chemistry among the Form I Rubisco superfamily
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The role of Rubisco kinetics and pyrenoid morphology in shaping the CCM of haptophyte microalgae
The haptophyte algae are a cosmopolitan group of primary producers that contribute significantly to the marine carbon cycle and play a major role in paleo-climate studies. Despite their global importance, little is known about carbon assimilation in haptophytes, in particular the kinetics of their Form 1D CO2-fixing enzyme, Rubisco. Here we examine Rubisco properties of three haptophytes with a range of pyrenoid morphologies (Pleurochrysis carterae, Tisochrysis lutea, and Pavlova lutheri) and the diatom Phaeodactylum tricornutum that exhibit contrasting sensitivities to the trade-offs between substrate affinity (Km) and turnover rate (kcat) for both CO2 and O2. The pyrenoid-containing T. lutea and P. carterae showed lower Rubisco content and carboxylation properties (KC and kCcat) comparable with those of Form 1D-containing non-green algae. In contrast, the pyrenoid-lacking P. lutheri produced Rubisco in 3-fold higher amounts, and displayed a Form 1B Rubisco kCcatâKC relationship and increased CO2/O2 specificity that, when modeled in the context of a C3 leaf, supported equivalent rates of photosynthesis to higher plant Rubisco. Correlation between the differing Rubisco properties and the occurrence and localization of pyrenoids with differing intracellular CO2:O2 microenvironments has probably influenced the divergent evolution of Form 1B and 1D Rubisco kinetics
Calcification, dissolution and test properties of modern planktonic foraminifera from the central Atlantic Ocean
This research was supported in part by a Royal Society Newton International postdoctoral Fellowship to SZ from the Royal Society of London. JWBR acknowledges funding from the European Research Council under the European Unionâs Horizon 2020 research and innovation program (grant agreement 805246). We also acknowledge support from U.K. NERC Grant (PUCCA) NE/V011049/1.The mass of well-preserved calcite in planktonic foraminifera shells provides an indication of the calcification potential of the surface ocean. Here we report the shell weight of 8 different abundant planktonic foraminifera species from a set of core-to sediments along the Mid-Atlantic Ridge. The analyses showed that near the equator, foraminifera shells of equivalent size weigh on average 1/3 less than those from the middle latitudes. The carbonate preservation state of the samples was assessed by high resolution X-ray microcomputed tomographic analyses of Globigerinoides ruber and Globorotalia truncatulinoides specimens. The specimen preservation was deemed good and does not overall explain the observed shell mass variations. However, G. ruber shell weights might be to some extent compromised by residual fine debris internal contamination. Deep dwelling species possess heavier tests than their surface-dwelling counterparts, suggesting that the weight of the foraminifera shells changes as a function of the depth habitat. Ambient seawater carbonate chemistry of declining carbonate ion concentration with depth cannot account for this interspecies difference. The results suggest a depth regulating function for plankton calcification, which is not dictated by water column acidity.Publisher PDFPeer reviewe
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