Alkalization of seawater enhances calcification of large benthic foraminifers.

There is concern that the increasing pCO₂ following the Industrial Revolution Period might lead to ocean acidification, which could affect calcifying organisms in the oceans. Recently, negative emission technology has been attracting attention as an effective countermeasure for greenhouse gas emissions. In the ocean, ocean alkalinization technology is proposed to neutralize acidified oceans and enhance the absorption capacity of CO₂ in the oceans. The potential effectiveness of ocean alkalinization technology is also suggested by the history of the Earth. During the Cretaceous period, when pCO₂ is interpreted >1,000 ppm, calcifying organisms thrived in the Cretaceous oceans. It is hypothesized that it was due to the total alkalinity (TA) of the seawater being maintained higher, thereby kept the calcium carbonate saturation state at necessary. In this study, we examined this hypothesis as well as attempted to predict the effects of the application of current alkalinization techniques in the ocean on calcifying organisms.

Clonal populations of large benthic foraminifers were cultured in highly alkalinized seawater under high pCO₂ conditions, and amounts of calcification (weight and volume) were measured (Group 1: high TA and high pCO₂). Specimens taken from same clonal population were kept in modern surface seawater (Group 2: low TA and low pCO₂) as a control treatment. The same experiments were also conducted as Group 3 (low TA and high pCO₂) to simulate future ocean acidification conditions, and as Group 4 (high TA and low pCO₂) to simulate alkalinized ocean under a low pCO₂ environment. It was showed significant differences in the amount of calcification in each of the Groups after three months cultivation. The amount of calcification in Group 1 was almost the same as that in the control treatment, confirming the possibility of maintaining the growth of calcifying organisms by alkalinization. Calcification amount in Group 3 was the smallest among all groups, indicating that future ocean acidification may inhibit calcification of large benthic foraminifers. In addition, the calcification rate was the greatest in Group 4, it is indicated that ocean alkalinization may enhance the calcification of the organisms. Finally, these results suggest that the calcium carbonate saturation state of seawater is an important parameter for calcification.