title: Seagrasses (Zostera marina) and (Zostera japonica) Display a Differential Photosynthetic Response to TCO2: Implications for Acidification Mitigation
Creators: name: Miller, Cale A.
Publisher: name: Western Washington University
Year: 2016

description: "<p>Excess atmospheric CO<sub>2</sub> is being absorbed at an unprecedented rate by the global and coastal oceans, shifting the baseline <em>p</em>CO<sub>2</sub> and altering seawater carbonate chemistry in a process known as ocean acidification (OA). Recent attention has been given to near-shore vegetated habitats, such as seagrass beds, which may have the potential to mitigate the effects of acidification on vulnerable calcifying organisms via photosynthesis. Seagrasses are capable of raising seawater pH and calcium carbonate saturation state during times of high photosynthetic activity. To better understand the photosynthetic potential of seagrass OA mitigation, we exposed Pacific Northwest populations of native <em>Zostera marina</em> and non-native <em>Zostera japonica</em> seagrasses from Padilla Bay, WA, to various irradiance and total CO<sub>2</sub> (TCO<sub>2</sub>) concentrations ranging from ~1770 – 2100 μmol TCO<sub>2</sub> kg<sup>-1</sup>.</p>
<p>Our results indicate that the maximum net photosynthetic rate (<em>P</em><sub>max</sub>) for <em>Z. japonica</em> as a function of irradiance and TCO<sub>2</sub> was 3x greater than <em>Z. marina</em> when standardized to chlorophyll (360 ± 74 μmol TCO<sub>2</sub> mgchl<sup>-1</sup> hr<sup>-1</sup> and 113 ± 21 μmol TCO<sub>2</sub> mgchl<sup>-1</sup> hr<sup>-1</sup>, respectively). In addition, <em>Z. japonica </em>increased its <em>P</em><sub>max</sub> 77% (± 56%) when TCO<sub>2 </sub>increased from ~1770 to 2050 μmol TCO<sub>2</sub> kg<sup>-1</sup>, whereas <em>Z. marina</em> did not display an increase in <em>P</em><sub>max</sub> with higher TCO<sub>2</sub>. The lack of response by <em>Z. marina</em> to TCO<sub>2</sub> is a departure from previous findings; however, it is likely that the variance within our treatments (coefficient of variation: 30 – 60%) obscured any positive effect of TCO<sub>2</sub> on <em>Z. marina</em> given the range of concentrations tested. Because previous findings have shown that <em>Z. marina</em> is saturated with respect to HCO<sub>3</sub><sup>-</sup> at low pH (≥ 7.5) we, therefore, suggest that the unequivocal positive response of <em>Z. japonica </em>to TCO<sub>2</sub> is a result of increased HCO<sub>3</sub><sup>-</sup> utilization in addition to increased CO<sub>2</sub> uptake.</p>
<p>Considering that <em>Z. japonica</em> displays a greater photosynthetic rate than <em>Z. marina</em> when normalized to chlorophyll, particularly under enhanced TCO<sub>2</sub> conditions, the ability of <em>Z. japonica</em> to mitigate OA may also increase relative to <em>Z. marina</em> in the future ocean. Higher photosynthetic rates by <em>Z. japonica</em> result in a greater potential, on a per chlorophyll basis, to increase pH and calcium carbonate saturation state—both of which affect acid-base regulation and calcification of calcifying organisms vulnerable to acidification. While it is important to consider genotypic differences throughout <em>Z. marina </em>and <em>Z. japonica</em>’s biogeographical distribution, our findings help elucidate the potential contribution both seagrasses have on variations in carbonate chemistry. Further, our results could be applied to ecosystem service models aimed at determining how specific seagrass species can be grown in a controlled setting to help mitigate OA hotspots that affect commercial shellfish aquaculture.</p>"