Ocean alkalinity enhancement using sodium carbonate salts does not lead to measurable changes in Fe dynamics in a mesocosm experiment

<p>The addition of carbonate minerals to seawater through an artificial ocean alkalinity enhancement (OAE) process increases the concentrations of hydroxide, bicarbonate, and carbonate ions. This leads to changes in the pH and the buffering capacity of the seawater. Consequently, OAE could have relevant effects on marine organisms and in the speciation and concentration of trace metals that are essential for their physiology. During September and October 2021, a mesocosm experiment was carried out in the coastal waters of Gran Canaria (Spain), consisting on the controlled variation of total alkalinity (TA). Different concentrations of carbonate salts (NaHCO<span class="inline-formula">₃</span> and Na<span class="inline-formula">₂</span>CO<span class="inline-formula">₃</span>) previously homogenized were added to each mesocosm to achieve an alkalinity gradient between <span class="inline-formula">Δ</span>0 to <span class="inline-formula">Δ</span>2400 <span class="inline-formula">µ</span>mol L<span class="inline-formula">⁻¹</span>. The lowest point of the gradient was 2400 <span class="inline-formula">µ</span>mol kg<span class="inline-formula">⁻¹</span>, being the natural alkalinity of the medium, and the highest point was 4800 <span class="inline-formula">µ</span>mol kg<span class="inline-formula">⁻¹</span>. Iron (Fe) speciation was monitored during this experiment to analyse total dissolved iron (TdFe, unfiltered samples), dissolved iron (dFe, filtered through a 0.2 <span class="inline-formula">µ</span>m pore size filter), soluble iron (sFe, filtered through a 0.02 <span class="inline-formula">µ</span>m pore size filter), dissolved labile iron (dFe<span class="inline-formula">^′</span>), iron-binding ligands (LFe), and their conditional stability constants (<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M15" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi>K</mi><mi mathvariant="normal">FeL</mi><mo>′</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="23pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="71dd64848558b1d613703afb4f6eca40"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-21-2705-2024-ie00001.svg" width="23pt" height="15pt" src="bg-21-2705-2024-ie00001.png"/></svg:svg></span></span>) because of change due to OAE and the experimental conditions in each mesocosm. Observed iron concentrations were within the expected range for coastal waters, with no significant increases due to OAE. However, there were variations in Fe size fractionation during the experiment. This could potentially be due to chemical changes caused by OAE, but such an effect is masked by the stronger biological interactions. In terms of size fractionation, sFe was below 1.0 nmol L<span class="inline-formula">⁻¹</span>, dFe concentrations were within 0.5–4.0 nmol L<span class="inline-formula">⁻¹</span>, and TdFe was within 1.5–7.5 nmol L<span class="inline-formula">⁻¹</span>. Our results show that over 99 % of Fe was complexed, mainly by <span class="inline-formula"><i>L</i>₁</span> and <span class="inline-formula"><i>L</i>₂</span> ligands with <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M21" display="inline" overflow="scroll" dspmath="mathml"><mrow><msubsup><mi>k</mi><mrow><msup><mi mathvariant="normal">Fe</mi><mo>′</mo></msup><mi mathvariant="normal">L</mi></mrow><mo>′</mo></msubsup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="24pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="85c04a9f29d686cdd800dea220bbda6e"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-21-2705-2024-ie00002.svg" width="24pt" height="15pt" src="bg-21-2705-2024-ie00002.png"/></svg:svg></span></span> ranging between 10.92 <span class="inline-formula">±</span> 0.11 and 12.68 <span class="inline-formula">±</span> 0.32, with LFe ranging from 1.51 <span class="inline-formula">±</span> 0.18 to 12.3 <span class="inline-formula">±</span> 1.8 nmol L<span class="inline-formula">⁻¹</span>. Our data on iron size fractionation, concentration, and iron-binding ligands substantiate that the introduction of sodium salts in this mesocosm experiment did not modify iron dynamics. As a consequence, phytoplankton remained unaffected by alterations in this crucial element.</p>