The potential environmental response to increasing ocean alkalinity for negative emissions

The negative emissions technology, artificial ocean alkalinization (AOA), aims to store atmospheric carbon dioxide (CO₂) in the ocean by increasing total alkalinity (TA). Calcium carbonate saturation state (ΩCaCO₃) and pH would also increase meaning that AOA could alleviate sensitive regions and ecosystems from ocean acidification. However, AOA could raise pH and ΩCaCO₃ well above modern-day levels, and very little is known about the environmental and biological impact of this. After treating a red calcifying algae (Corallina spp.) to elevated TA seawater, carbonate production increased by 60% over a control. This has implication for carbon cycling in the past, but also constrains the environmental impact and efficiency of AOA. Carbonate production could reduce the efficiency of CO₂ removal. Increasing TA, however, did not significantly influence Corallina spp. primary productivity, respiration, or photophysiology. These results show that AOA may not be intrinsically detrimental for Corallina spp. and that AOA has the potential to lessen the impacts of ocean acidification. However, the experiment tested a single species within a controlled environment to constrain a specific unknown, the rate change of calcification, and additional work is required to understand the impact of AOA on other organisms, whole ecosystems, and the global carbon cycle.