Abstract
The potential effect of ocean acidification on the exudation of organic matter by phytoplankton and, consequently, on the iron redox chemistry is largely unknown. In this study, the cocccolithophorid Emiliania huxleyi was exposed to different pCO2 conditions (225-900 μatm), in order to determine the role of natural organic ligands on the Fe(II) oxidation rate. Oxidation kinetics of Fe(II) were studied as a function of pH (7.75-8.25) and dissolved organic carbon levels produced (0-141.11 μmol C L-1) during the different growth stages. The Fe(II) oxidation rate always decreased in the presence of exudates as compared to that in the exudates-free seawater. The organic ligands present in the coccolithophorid exudates were responsible for this decrease. The oxidation of Fe(II) in artificial seawater was also investigated at nanomolar levels over a range of pH (7.75–8.25) at 25 oC in the presence of different glucuronic acid concentrations. Dissolved uronic acids (DUA) slightly increased the experimental rate compared to control artificial seawater (ASW) which can be ascribed to the stabilization of the oxidized form by chelation. This behavior was a function of the Fe(II):DUA ratio and was a pH dependent process. A kinetic model in ASW, with a single organic ligand, was applied for computing the equilibrium constant (log KFeCHO+ = 3.68 ± 0.81 M-1) and the oxidation rate (log kFeCHO+ = 3.28 ± 0.41 M-1 min-1) for the Fe(II)-DUA complex (FeCHO+), providing an excellent description of data obtained over a wide range of DUA concentrations and pH conditions. Considering the Marcus theory the Fe(III) complexing constant with DUA was limited to between 1013-1016. For the seawater enriched with exudates of E. huxleyi a second kinetic modelling approach was carried out for fitting the Fe(II) speciation, and the contribution of each Fe(II) species to the overall oxidation rate as a function of the pH/pCO2 conditions. The influence of organic ligands in the Fe(II) speciation diminished as pH decreased in solution. During the stationary growth phase, the FeCHO+ complex became the most important contributor to the overall oxidation rate when pH was lower than 7.95. Because CO2 levels modify the composition of excreted organic ligands, the
Highlights
Iron is an essential micronutrient required for important phytoplankton physiological processes, such as nutrient assimilation and electron transport during respiration and photosynthesis (Behrenfeld and Milligan, 2013)
The kinetic studies were performed in seawater enriched with nutrients (SWEN) and seawater enriched with nutrients and exudates of E. huxleyi (SWEX), in order to determine the effect of organic ligands released under different pH conditions
To demonstrate that the total organic ligands exuded during the microalgae growth can be indirectly related to organic matter released (Strmecki et al, 2010; González et al, 2014), the dissolved organic carbon exuded (DOCE) by E. huxleyi has to be monitored under different pH/pCO2 conditions
Summary
Iron is an essential micronutrient required for important phytoplankton physiological processes, such as nutrient assimilation and electron transport during respiration and photosynthesis (Behrenfeld and Milligan, 2013). Exudates from bacteria and phytoplankton are not well characterized, sources of these biologically-mediated weak ligands include mainly exo-polymeric substances (EPS), since they form a major biological component of the total dissolved organic carbon (DOC) pool (Pakulski and Benner, 1994). Using Fourier transform infrared spectrum analysis of EPS, Ozturk et al (2014) determined that the carboxylic groups of uronic acids confer the negative net charge of the polymer that serves both to bind metallic cations and to form resilient linkages between the polymer chains. Surface active saccharides, such as uronic acids, play a key role in oceanic carbon cycling, through coagulation and aggregation processes, by acting as precursors for transparent exopolymer particles (Passow, 2002) and representing an abiotic linkage between dissolved and particulate phases of organic carbon (Engel et al, 2004; Verdugo et al, 2004)
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