Seagrasses (Zostera marina) and (Zostera japonica) display a differential photosynthetic response to TCO2: implications for acidification mitigation

Excess atmospheric CO2 is being absorbed at an unprecedented rate by the global and coastal oceans, shifting the baseline pCO2 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 Zostera marina and non-native Zostera japonica seagrasses from Padilla Bay, WA, to various irradiance and total CO2 (TCO2) concentrations ranging from ~1770 – 2100 μmol TCO2 kg-1.

Our results indicate that the 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 ± 74 μmol TCO2 mgchl-1 hr-1 and 113 ± 21 μmol TCO2 mgchl-1 hr-1, respectively). In addition, Z. japonica increased its Pmax 77% (± 56%) when TCO2 increased from ~1770 to 2050 μmol TCO2 kg-1, whereas Z. marina did not display an increase in Pmax with higher TCO2. The lack of response by Z. marina to TCO2 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 TCO2 on Z. marina given the range of concentrations tested. Because previous findings have shown that Z. marina is saturated with respect to HCO3- at low pH (≥ 7.5) we, therefore, suggest that the unequivocal positive response of Z. japonica to TCO2 is a result of increased HCO3- utilization in addition to increased CO2 uptake.

Considering that Z. japonica displays a greater photosynthetic rate than Z. marina when normalized to chlorophyll, particularly under enhanced TCO2 conditions, the ability of Z. japonica to mitigate OA may also increase relative to Z. marina in the future ocean. Higher photosynthetic rates by Z. japonica 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 Z. marina and Z. japonica’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.

Miller C. A., 2016. Seagrasses (Zostera marina) and (Zostera japonica) display a differential photosynthetic response to TCO2: implications for acidification mitigation. MSc thesis, Western Washington University, 70 p. Thesis.


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