AUTHOR=Miller Cale A. , Yang Sylvia , Love Brooke A. 

TITLE=Moderate Increase in TCO2 Enhances Photosynthesis of Seagrass Zostera japonica, but Not Zostera marina: Implications for Acidification Mitigation

JOURNAL=Frontiers in Marine Science

VOLUME=4

YEAR=2017

URL=https://www.frontiersin.org/journals/marine-science/articles/10.3389/fmars.2017.00228

DOI=10.3389/fmars.2017.00228

ISSN=2296-7745

ABSTRACT=<p>Photosynthesis and respiration are vital biological processes that shape the diurnal variability of carbonate chemistry in nearshore waters, presumably ameliorating (daytime) or exacerbating (nighttime) short-term acidification events, which are expected to increase in severity with ocean acidification (OA). Biogenic habitats such as seagrass beds have the capacity to reduce CO<sub>2</sub> concentration and potentially provide refugia from OA. Further, some seagrasses have been shown to increase their photosynthetic rate in response to enriched total CO<sub>2</sub> (TCO<sub>2</sub>). Therefore, the ability of seagrass to mitigate OA may increase as concentrations of TCO<sub>2</sub> increase. In this study, we exposed native <italic>Zostera marina</italic> and non-native <italic>Zostera japonica</italic> seagrasses from Padilla Bay, WA (USA) to various levels of irradiance and TCO<sub>2</sub>. Our results indicate that the average maximum net photosynthetic rate (<italic>P</italic><sub>max</sub>) for <italic>Z. japonica</italic> as a function of irradiance and TCO<sub>2</sub> was 3x greater than <italic>Z. marina</italic> when standardized to chlorophyll (360 ± 33 μmol TCO<sub>2</sub> mg chl<sup>−1</sup> h<sup>−1</sup> and 113 ± 10 μmol TCO<sub>2</sub> mg chl<sup>−1</sup> h<sup>−1</sup>, respectively). Additionally, <italic>Z. japonica</italic> increased its <italic>P</italic><sub>max</sub> ~50% when TCO<sub>2</sub> increased from ~1,770 to 2,051 μmol TCO<sub>2</sub> kg<sup>−1</sup>. In contrast, <italic>Z. marina</italic> did not display an increase in <italic>P</italic><sub>max</sub> with higher TCO<sub>2</sub>, possibly due to the variance of photosynthetic rates at saturating irradiance within TCO<sub>2</sub> treatments (coefficient of variation: 30–60%) relative to the range of TCO<sub>2</sub> tested. Our results suggest that <italic>Z. japonica</italic> can affect the OA mitigation potential of seagrass beds, and its contribution may increase relative to <italic>Z. marina</italic> as oceanic TCO<sub>2</sub> rises. Further, we extended our empirical results to incorporate various biomass to water volume ratios in order to conceptualize how these additional attributes affect changes in carbonate chemistry. Estimates show that the change in TCO<sub>2</sub> via photosynthetic carbon uptake as modeled in this study can produce positive diurnal changes in pH and aragonite saturation state that are on the same order of magnitude as those estimated for whole seagrass systems. Based on our results, we predict that seagrasses <italic>Z. marina</italic> and <italic>Z. japonica</italic> both have the potential to produce short-term changes in carbonate chemistry, thus offsetting anthropogenic acidification when irradiance is saturating.</p>