Bibliography - Francois Morel

1. Xu, Yan, Dalin Shi, Ludmilla Aristilde, and Francois Morel, 2012: The effect of pH on the uptake of zinc and cadmium in marine phytoplankton: Possible role of weak complexes. Limnology and Oceanography, 57(1), 293-304
   [ Abstract ]

   In natural samples from the New Jersey coast and the Gulf of Alaska, zinc (Zn) and cadmium (Cd) uptake rates by phytoplankton decreased on average about 30% as pH was decreased from 8.5 to 7.9 or 7.7, and the partial pressure of carbon dioxide (PCO₂) increased accordingly. The underlying mechanism was explored with the model species, Thalassiosira weissflogii and Emiliania huxleyi, using ethylenediaminetetraacetic acid (EDTA), desferrioxamine B, phytochelatin, and cysteine as complexing agents. Experiments with single complexing agents did not reproduce the effect of pH seen in field samples, ruling out two possible mechanisms: a direct effect on the uptake machinery or down-regulation of uptake at high PCO₂. Zn and Cd bioavailability must thus somehow decrease at low pH in natural seawater, which is counterintuitive since the protonation of complexing agents at low pH should increase the total free concentration of metals. However, in the presence of both a strong and a weak complexing agent, metal uptake rate may decrease at low pH if formation of the weak complex decreases and the metal in the weak complex is more "available" than in the strong complex. We obtained proof of concept for such a two-ligand mechanism for Zn uptake in the presence of EDTA ± phytochelatin and EDTA + cysteine. Weak ligands that bind a small fraction of essential metals in surface seawater may thus be important in metal uptake by phytoplankton, and the dual effects of strong and weak complexing agents may control not just the magnitude but also the sign of the effect of pH-PCO₂ on metal uptake rates.

2. Hopkinson, Brian M., Christopher L. Dupont, Andrew E. Allen, and Francois Morel, March 2011: Efficiency of the CO₂-concentrating mechanism of diatoms. Proceedings of the National Academy of Sciences of the United States of America, 108(10), doi:10.1073/pnas.1018062108 3830-3837
   [ Abstract ]

   Diatoms are responsible for a large fraction of CO₂ export to deep seawater, a process responsible for low modern-day CO₂ concentrations in surface seawater and the atmosphere. Like other photosynthetic organisms, diatoms have adapted to these low ambient concentrations by operating a CO₂ concentrating mechanism (CCM) to elevate the concentration of CO₂ at the site of fixation. We used mass spectrometric measurements of passive and active cellular carbon fluxes and model simulations of these fluxes to better understand the stoichiometric and energetic efficiency and the physiological architecture of the diatom CCM. The membranes of diatoms are highly permeable to CO₂, resulting in a large diffusive exchange of CO₂ between the cell and external milieu. An active transport of carbon from the cytoplasm into the chloroplast is the main driver of the diatom CCM. Only one-third of this carbon flux is fixed photosynthetically, and the rest is lost by CO₂ diffusion back to the cytoplasm. Both the passive influx of CO₂ from the external medium and the recycling of the CO₂ leaking out of the chloroplast are achieved by the activity of a carbonic anhydrase enzyme combined with the maintenance of a low concentration of HCO₃⁻ in the cytoplasm. To achieve the CO₂ concentration necessary to saturate carbon fixation, the CO₂ is most likely concentrated within the pyrenoid, an organelle within the chloroplast where the CO₂-fixating enzyme is located.

3. Egleston, Eric S., Christopher L. Sabine, and Francois Morel, January 2010: Revelle revisited: Buffer factors that quantify the response of ocean chemistry to changes in DIC and alkalinity. Global Biogeochemical Cycles, AGU, (VOL. 24, GB1002), doi:10.1029/2008GB003407 1-9
   [ Abstract ]

   We derive explicit expressions of the Revelle factor and several other buffer factors of interest to climate change scientists and those studying ocean acidification. These buffer factors quantify the sensitivity of CO₂ and H⁺ concentrations ([CO₂] and [H+]) and CaCO₃saturation (¥Ø ) to changes in dissolved inorganic carbon concentration (DIC) and alkalinity (Alk). The explicit expressions of these buffer factors provide a convenient means to compare the degree of buffering of [CO₂], [H⁺], and Ω in different regions of the oceans and at different times in the future and to gain insight into the buffering mechanisms. All six buffer factors have roughly similar values, and all reach an absolute minimum when DIC = Alk (pH ~ 7.5). Surface maps of the buffer factors generally show stronger buffering capacity in the subtropical gyres relative to the polar regions. As the dissolution of anthropogenic CO₂ increases the DIC of surface seawater over the next century, all the buffer factors will decrease, resulting in a much greater sensitivity to local variations in DIC and Alk. For example, diurnal and seasonal variations in pH and Ω caused by photosynthesis and respiration will be greatly amplified. Buffer factors provide convenient means to quantify the effect that changes in DIC and Alk have on seawater chemistry. They should also help illuminate the role that various physical and biological processes have in determining the oceanic response to an increase in atmospheric CO₂.

4. Hopkinson, Brian M., Yan Xu, Dalin Shi, Patrick J. McGinn, and Francois Morel, 2010: The effect of CO₂ on the photosynthetic physiology of phytoplankton in the Gulf of Alaska. Limnology and Oceanography, 55(5), doi:10.4319/lo.2010.55.5.2011 2011-2024
   [ Abstract ]

   In the high-nutrient, low-chlorophyll waters of the Gulf of Alaska, microcosm manipulation experiments were used to assess the effect of CO₂ on growth and primary production under iron-limited and iron-replete conditions. As expected, iron had a strong effect on growth and photosynthesis. A modest and variable stimulation of growth and biomass production by CO₂ (high CO₂: 77-122 Pa; low CO₂: 11-17 Pa) was observed under both iron-replete and iron-limited conditions, though near the limit of precision of our measurements in slow-growing low-iron experiments. Physiological acclimations responsible for the changes in growth were assessed. Under iron-limited conditions, growth stimulation at high CO₂ appeared to result from an increase in photosynthetic efficiency, which we attribute to energy savings from down-regulation of the carbon concentrating mechanisms. In some cases, iron-rich photosynthetic proteins (PsbA, PsaC, and cytochrome b⁶) were down-regulated at elevated CO₂ in iron-limited controls. Under iron-replete conditions, there was an increase in growth rate and biomass at high CO₂ in some experiments. This increase was unexpectedly supported by reductions in cellular carbon loss, most likely decreased respiration. We speculate that this effect may be due to acclimation to decreased pH rather than high CO₂. The variability in responses to CO₂ among experiments did not appear to be caused by differences in phytoplankton community structure and may reflect the sensitivity of the net response of phytoplankton to antagonistic effects of the several parameters that co-vary with CO₂.

5. Shi, Dalin, Yan Xu, Brian M. Hopkinson, and Francois Morel, January 2010: Effect of Ocean Acidification on Iron Availability to Marine Phytoplankton. Science, New York, AAAS, (January 14, 2010), doi:10.1126/science.1183517 1-8
   [ Abstract ]

   The acidification caused by dissolution of anthropogenic CO₂ in the ocean changes the chemistry and, hence, the bioavailability of iron (Fe), a limiting nutrient in large oceanic regions. Here, we show that the bioavailability of dissolved Fe may decline due to ocean acidification. Acidification of media containing various Fe compounds decreases the Fe uptake rate of diatoms and coccolithophores to an extent predicted by the changes in Fe chemistry. A slower Fe uptake by a model diatom with decreasing pH is also seen in experiments with Atlantic surface water. The Fe requirement of model phytoplankton remains unchanged with increasing CO₂ . The ongoing acidification of seawater is likely to increase the Fe-stress of phytoplankton populations in some areas of the ocean.

6. Xu, Y., C.T. Supuran, and Francois Morel, 2010: Cadmium-carbonic anhyrase (In Press). Handbook of Metalloproteins,
7. Shi, Dalin, Yan Xu, and Francois Morel, 2009: Effects of the pH/pCO₂ control method on medium chemistry and phytoplankton growth. Biogeosciences, http://www.biogeosciences.net/6/1199/2009/bg-6-1199-2009.pdf, 6(1199-1207),
   [ Abstract ]

   The control of key chemical parameters in phytoplankton cultures, such as pCO₂, pH and Ù (the saturation state of calcium carbonate), is made difficult by the interdependence of these parameters and by the changes resulting from the growth of the organisms, such as andCO₂ fixation, nutrient uptake and, for coccolithophores, calcite precipitation. Even in cultures where pandCO₂ or pH is maintained constant, other chemical parameters change substantially at high cell densities. Experimentally we observed that various methods of adjustment of pandCO₂/pH – acid or base addition, use of buffers or pH-stats, or bubbling of andCO₂-enriched air – can be used, the choice of one or the other depending on the goals of the experiments. At seawater pH, we measured the same growth rates in cultures of the diatom Thalassiosira weissflogii where the pandCO₂/pH was controlled by these different methods. The pH/pandCO₂control method also did not affect the rates of growth or calcification of the coccolithophore Emiliania huxleyi at seawater pH. At lower pH/higher pandCO₂, in the E. huxleyi strain PLY M219, we observed increases in rates of carbon fixation and calcification per cell, along with a slight increase in growth rate, except in bubbled cultures. In our hands, the bubbling of cultures seemed to induce more variable results than other methods of pandCO₂/pH control. While highly convenient, the addition of pH buffers to the medium apparently induces changes in trace metal availability and cannot be used under trace metal-limiting conditions.

8. Xu, Y., J. M. Boucher, and Francois Morel, 2009: Expression and Diversity of Alkaline Phosphatase EHAP1 in Emiliania Huxleyi (Prymnesiophyceae). Journal of Phycology, 46(1), doi:10.1111/j.1529-8817.2009.00788.x 85-92
   [ Abstract ]

   Emiliania huxleyi (Lohmann) W. W. Hay et H. Mohler is a cosmopolitan coccolithophore species that forms massive blooms in low phosphorus seawater, partly due to its ability to utilize organic phosphate via extracellular alkaline phosphatase (AP). A novel AP gene, ehap1, was identified from the strain CCMP374. In this study, we examined the expression of ehap1 in various E. huxleyi strains and its genetic diversity in those strains and field populations. Two EHAP1 proteins (EHAP1a, 75 kDa and EHAP1b, 110 kDa) with virtually identical sequence were expressed under P limitation in all strains except one; a third protein (EHAP1c, 115 kDa) was expressed in a few strains. The correlation between AP activity and protein abundance suggests that EHAP1b is inactive and probably the precursor of EHAP1a. The transcript of ehap1 was induced by P depletion in all strains. The ehap1 gene sequence is highly conserved in these strains and field populations with <3% nucleic acid substitution. Most of the ehap1 sequences from one site in the English Channel and three sites in the Gulf of Alaska were essentially identical to one another. No EHAP1-like protein can be detected in other phytoplankton species tested via Western blot analysis. The rapid induction and high activity of EHAP1 in E. huxleyi suggest that it plays a significant role in P regeneration in the oligotrophic ocean where E. huxleyi is abundant. The EHAP1 antibody and gene-specific primers are well suited to study the dynamics of P limitation in field populations of E. huxleyi.

9. Morel, Francois, A. B. Kustka, and Y. Shaked, 2008: The role of unchelated Fe in the iron nutrition of phytoplankton. Limnology and Oceanography, http://geoweb.princeton.edu/research/tracemetals/pdf/morel2008.pdf, 53(1), 400-404
   [ Abstract ]

   The important question of iron bioavailability in the sea has become complicated by the discovery that marine phytoplankton can take up Fe bound in very stable chelates via reductive processes, and some particular Fe species through specialized transport mechanisms. As a result there is some question of whether the small fraction of Fe that is ‘‘free’’ or unchelated in seawater is important in the nutrition of natural phytoplankton assemblages. A careful examination of published laboratory studies on Fe uptake by model organisms all support the idea that unchelated Fe (III) is highly available for uptake and that it is an important source of the Fe taken up by phytoplankton under a variety of experimental conditions. Comparing these results with field data on Fe speciation shows that unchelated Fe can be an important source of Fe to the phytoplankton in the sea: it is likely sufficient to contribute the bulk of the Fe supporting primary production in regions that are not limited by Fe and a significant fraction everywhere, including high-nutrient low-chlorophyll areas.

10. Riebesell, U., R.G.J. Bellerby, A. Engel, V. J. Fabry, D. A. Hutchins, T. B. H. Reusch, K. G. Schulz, and Francois Morel, 2008: Comment on Phytoplankton Calcification in a High-CO₂ World. Science, 322, doi:10.1126/science.1161096 1466
    [ Abstract ]

    Iglesias-Rodriguez et al. (Research Articles, 18 April 2008, p. 336) reported that the coccolithophore Emiliania huxleyi doubles its organic matter production and calcification in response to high carbon dioxide partial pressures, contrary to previous laboratory and field studies. We argue that shortcomings in their experimental protocol compromise the interpretation of their data and the resulting conclusions.

11. Kraepiel, A. M., Klaus Keller, H. B. Chin, E. G. Malcolm, and Francois Morel, 2003: Sources and Variations of Mercury in Tuna. Environmental Science and Technology, 37, doi:10.1021/es0340679 5551-5558
    [ Abstract ]

    While the bulk of human exposure to mercury is through the consumption of marine fish, most of what we know about mercury methylation and bioaccumulation is from studies of freshwaters. We know little of where and how mercury is methylated in the open oceans, and there is currently a debate whether methylmercury concentrations in marine fish have increased along with global anthropogenic mercury emissions. Measurements of mercury concentrations in Yellowfin tuna caught off Hawaii in 1998 show no increase compared to measurements of the same species caught in the same area in 1971. On the basis of the known increase in the global emissions of mercury over the past century and of a simple model of mercury biogeochemistry in the Equatorial and Subtropical Pacific ocean, we calculate that the methylmercury concentration in these surface waters should have increased between 9 and 26% over this 27 years span if methylation occurred in the mixed layer or in the thermocline. Such an increase is statistically inconsistent with the constant mercury concentrations measured in tuna. We conclude tentatively that mercury methylation in the oceans occurs in deep waters or in sediments.

12. Tortell, P. D., G. R. Di Tullio, Daniel Sigman, and Francois Morel, 2002: CO₂ effects on taxonomic composition and nutrient utilization in an Equatorial Pacific phytoplankton assemblage. Marine Ecology Progress Series, 236, doi:10.3354/meps236037 37-43
    [ Abstract ]

    We report the results of a field incubation experiment demonstrating a substantial shift in the taxonomic composition of Equatorial Pacific phytoplankton assemblages exposed to CO₂ levels of 150 and 750 ppm (dissolved CO₂ ~3 to 25 μM). By the end of the experiment, the phytoplankton community in all samples was dominated by diatoms and Phaeocystis sp. However, the relative abundance of these phytoplankton taxa differed significantly between CO₂ treatments. Taxonomic pigment analysis and direct microscopic examination of samples revealed that the abundance of diatoms decreased by ~50% at low CO₂ relative to high CO₂, while the abundance of Phaeocystis sp. increased by ~60% at low CO₂. This CO₂-dependent shift was associated with a significant change in nutrient utilization, with higher ratios of nitrate:silicate (N:Si) and nitrate:phosphate (N:P) consumption by phytoplankton in the low CO₂ treatment. Despite the significant changes in taxonomic composition and nutrient consumption ratios, total biomass and primary productivity did not differ significantly between the CO₂ treatments. Our results suggest that CO₂ concentrations could potentially influence competition among marine phytoplankton taxa and affect oceanic nutrient cycling.

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