Seawater carbonate chemistry and photosynthesis of seagrass Zostera japonica and Zostera marina

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 CO2 concentration and potentially provide refugia from OA. Further, some seagrasses have been shown to increase their photosynthetic rate in response to enriched total CO2 (TCO2). Therefore, the ability of seagrass to mitigate OA may increase as concentrations of TCO2 increase. In this study, we exposed native Zostera marina and non-native Zostera japonica seagrasses from Padilla Bay, WA (USA) to various levels of irradiance and TCO2. Our results indicate that the average maximum net photosynthetic rate (Pmax) for Z. japonica as a function of irradiance and TCO2 was 3x greater than Z. marina when standardized to chlorophyll (360 ± 33 μmol TCO2 mg/chl/h and 113 ± 10 μmol TCO2 mg/chl/h, respectively). Additionally, Z. japonica increased its Pmax ~50% when TCO2 increased from 1,770 to 2,051 μmol TCO2/kg. In contrast, Z. marina did not display an increase in Pmax with higher TCO2, possibly due to the variance of photosynthetic rates at saturating irradiance within TCO2 treatments (coefficient of variation: 30–60%) relative to the range of TCO2 tested. Our results suggest that Z. japonica can affect the OA mitigation potential of seagrass beds, and its contribution may increase relative to Z. marina as oceanic TCO2 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 TCO2 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 Z. marina and Z. japonica both have the potential to produce short-term changes in carbonate chemistry, thus offsetting anthropogenic acidification when irradiance is saturating.

光合作用与呼吸作用是调控近岸水域碳酸盐化学日变化的核心生物学过程，可在日间缓解、夜间加剧短期酸化事件——而这类事件的严重程度预计将随海洋酸化（OA）加剧而提升。生源栖息地如海草床能够降低水体二氧化碳浓度，进而为生物提供躲避海洋酸化的潜在避难所。此外，已有研究证实部分海草可通过提升光合速率响应总二氧化碳（TCO2）浓度升高。因此，海草缓解海洋酸化的能力或随总二氧化碳浓度上升而增强。

本研究以美国华盛顿州帕迪拉湾（Padilla Bay）的原生鳗草（Zostera marina）与外来种日本鳗草（Zostera japonica）为实验材料，设置不同梯度的辐照度与总二氧化碳（TCO2）水平开展暴露实验。结果显示，以叶绿素标准化后，日本鳗草的平均最大净光合速率（Pmax）随辐照度与总二氧化碳的变化幅度是鳗草的3倍（分别为360 ± 33 μmol TCO2 mg/chl/h与113 ± 10 μmol TCO2 mg/chl/h）。此外，当总二氧化碳浓度从1770 μmol TCO2/kg升高至2051 μmol TCO2/kg时，日本鳗草的最大净光合速率提升约50%；与之形成鲜明对比的是，鳗草的最大净光合速率并未随总二氧化碳浓度升高出现显著上升，这可能是因为在各总二氧化碳处理组中，饱和辐照度下的光合速率变异系数（30%~60%）大于本次实验设置的总二氧化碳浓度梯度范围。

本研究结果表明，日本鳗草可影响海草床缓解海洋酸化的潜力，且随着海洋总二氧化碳浓度上升，其相较于鳗草的贡献度或将进一步提升。此外，我们结合实验结果，纳入不同生物量与水体体积比参数，以量化分析这些额外属性对碳酸盐化学变化的调控作用。模型估算显示，本研究中通过光合固碳引起的总二氧化碳浓度变化，可使pH与文石饱和状态出现日间正向变化，其变化幅度与已报道的完整海草生态系统观测结果处于同一数量级。基于上述结果，我们预测鳗草与日本鳗草均具备引发碳酸盐化学短期变化的潜力，在辐照度处于饱和状态时，可抵消人为活动引起的海洋酸化。