Abstract
The variability in the extracellular release of organic ligands by Emiliania huxleyi under four different pCO2 scenarios (225, 350, 600 and 900 μatm), was determined. Growth in the batch cultures was promoted by enriching them only with major nutrients and low iron concentrations. No chelating agents were added to control metal speciation. During the initial (IP), exponential (EP) and steady (SP) phases, extracellular release rates, normalized per cell and day, of dissolved organic carbon (DOCER), phenolic compounds (PhCER), dissolved combined carbohydrates (DCCHOER) and dissolved uronic acids (DUAER) in the exudates were determined. The growth rate decreased in the highest CO2 treatment during the IP (＜48 h), but later increased when the exposure was longer (more than 6 days). DOCER did not increase significantly with high pCO2. Although no relationship was observed between DCCHOER and the CO2 conditions, DCCHO was a substantial fraction of the freshly released organic material, accounting for 18% to 37%, in EP, and 14% to 23%, in SP, of the DOC produced. Growth of E. huxleyi induced a strong response in the PhCER and DUAER. While in EP, PhCER were no detected, the DUAER remained almost constant for all CO2 treatments. Increases in the extracellular release of these organic ligands during SP were most pronounced under high pCO2 conditions. Our results imply that, during the final growth stage of E. huxleyi, elevated CO2 conditions will increase its excretion of acid polysaccharides and phenolic compounds, which may affect the biogeochemical behavior of metals in seawater.
Highlights
Increasing atmospheric carbon dioxide levels causes rapid alterations of the physical and chemical seawater conditions, affecting ecosystem dynamics [1]
Our results indicated that the E. huxleyi populations adapted to experimental acidification, displaying a slight increase in growth rates during steady phase (SP)
In the open ocean dissolved iron is found at subnanomolar levels [77], the concentrations fixed in the present study (2.5 nM) could be considered as low Fe conditions, since the abundances of E. huxleyi, AIMS Environmental Science reached during experimental cultures were between one and three orders of magnitude higher than the biomass of photosynthetic eukaryotes found in different marine systems [78]
Summary
Increasing atmospheric carbon dioxide levels causes rapid alterations of the physical and chemical seawater conditions, affecting ecosystem dynamics [1]. Emission projections of CO2 for this century expect an important increase in total dissolved inorganic carbon and a concomitant decrease in pH, from its current value of 8.1 to 7.8 [3]. This last process, termed ocean acidification, may increase carbon fixation rates in some photosynthetic organisms [4]. The variations projected for the marine carbonate chemistry [3] might decrease the need for employing CO2-concentrating mechanisms in autotrophic microorganisms [12], modifying the carbon fixation efficiency and, the release of organic matter by marine phytoplankton [13]
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