[A-CG51] Coastal Ecosystems-2. Coral reefs, seagrass and macroalgal beds, and mangroves

[ACG51-03] Experiment and modeling of the response of Sargassum Horneri to ocean acidification

*Marina Orita¹, Aya Suzuki¹, Takashi Kobayashi¹, Masaki Hisada¹ (1.NTT Space Environment and Energy laboratories)

Keywords:Macroalgae, Ocean Acidification, Ecosystem Model, Carbon Cycle

Ocean acidification has various effects on marine lives. To conserve marine ecosystems, it is important to build marine ecosystem models that consider the effects of ocean acidification and to forecast changes in marine biomass. However, the impacts of ocean acidification on marine lives are still largely unknown, and existing marine ecosystem models do not take them. In addition, although various experiments have been conducted on the response of macroalgae, which fixes carbon and suppresses acidification, to ocean acidification, different results have been obtained for each species. For example, the growth rate of the larvae of wakame seaweed increases as the CO₂ concentration increases (Leal, P. et al., 2017). On the other hand, the germination and juvenile stages of Sargassum Horneri are inhibited by low pH environments (Fukami, T. et al., 2021). Also, the growth and photosynthetic function of the sporophyte of Laminaria japonica decrease in high CO₂ concentrations (Chu, Y. et al., 2019). Some macroalgae have their photosynthetic functions activated by high CO₂ concentrations, while others have their growth inhibited by too high CO₂ concentrations. Therefore, we hypothesized that the optimal pH range for each macroalgae would differ and proposed a model that considers ocean acidification by incorporating a pH-dependent function (Figure).
In this study, we conducted tank experiments using Sargassum Horneri to obtain the data on the effects of acidification on macroalgae. We chose its macroalgae because it is small enough for tank experiments over several months. Also, we verified the effectiveness of the pH-dependent function by building the model that reproduced the tank experiment.
We prepared three tanks with pH controlled to 8.2, 7.5 and 6.0. pH 8.2 is the level of current seawater, and pH 7.5 is the level of ocean acidification that occurs locally. pH 6.0 was set so that the results would be clearly visible. The experimental period was 22 days, referring to Fukami, T. et al., 2021, and the measurements were the biomass of Sargassum Horneri, water temperature, light intensity, pH, nutrient concentration, and photosynthetic efficiency. The results showed that at pH 6.0, the growth and photosynthesis of Sargassum Horneri were suppressed and withered leaves were observed. This showed that when the pH is around 7.5, the growth of Sargassum Horneri is not affected, but if the acidification progresses further, it may lead to inhibit growth.
Next, to verify the effectiveness of the pH-dependent function, we built the tank experiment model. In this model, we set the input values as water temperature, light intensity, pH, amount of nutrient added, and biomass of each life and the concentration of each substance at the beginning of the experiment and set up differential equations with respect to the biomass of Sargassum Horneri and phytoplankton, the concentration of organic phosphorus and nitrogen, inorganic phosphorus and nitrogen. Also, we were able to identify the coefficients of the pH-dependent function, which is a combination of Gaussian functions, from the experimental results and built the model that includes the effects of acidification on macroalgae.
In conclusion, we obtained data on the effects of acidification on Sargassum Horneri and built the model of it. In the experiment, we considered only the direct effects of acidification on Sargassum Horneri. However, in nature, there are various species living together, and other species may be indirectly affected by changes in the ecosystem balance and habitat environment. Therefore, our future task is to conduct additional experiments involving competition with phytoplankton or other macroalgae and build the model that includes the results. In addition, the model of Sargassum Horneri tank is a simplified model of actual marine environment. We would like to use the results of this study to simulate the marine ecosystem in acidified ocean areas.