Bibliography - Yan Xu

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., 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.

3. 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.

4. 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.

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