NTT Establishes Technology to Accurately Track When and How Much CO₂ Algae Absorb
Enabling culture optimization to increase CO₂ absorption and improve biomass production efficiency

News Highlights:

Figure 1. Comparison of conventional and new methods for measuring DIC dynamics in microalgal cultures

Background

Microalgae grow by using CO₂ and other forms of inorganic carbon through photosynthesis. Biomass⁴ derived from microalgae can be used as a feedstock for animal feed, fuels, and chemicals. Replacing a portion of conventional feedstocks with this biomass is expected to contribute to sustainable production and lower greenhouse gas emissions.

In culture fluid, CO₂ exists not only as dissolved CO₂ but also as bicarbonate and carbonate ions. Together, these forms are known as dissolved inorganic carbon, or DIC (Figure 2). Evaluating CO₂ absorption by microalgae therefore requires tracking the behavior of DIC as a whole as CO₂ changes form in water.

NTT previously proposed a concept that uses microalgae that grow by utilizing CO₂ as feed for fish and shellfish, aiming to address environmental issues while supporting food production.⁵ NTT has also conducted research and development to identify genes that improve the efficiency of CO₂ utilization by algae⁶ and to advance algal breeding technologies.⁷ Accurately tracking changes in DIC concentration during cultivation is essential for validating this concept and evaluating CO₂ absorption by microalgae.

Figure 2. Forms of CO₂ in aqueous solution

The conventional titration method can measure DIC concentrations accurately, but it requires culture fluid sampling for every measurement. Because each measurement also takes time and effort, the method is not suitable for continuous data acquisition. Existing calculation methods based on sensor data and carbonate equilibrium⁸ enable continuous calculation. However, when applied directly to microalgal culture fluid, the resulting DIC values showed relative errors of up to approximately 3,300% with respect to conventional titration measurements, making accurate calculation of DIC concentrations difficult. NTT therefore developed a measurement technology that uses sensors to calculate DIC concentrations accurately and continuously during cultivation and to track changes over time.

Key Features of the Technology

■ Correction technique designed for pH changes specific to microalgal cultivation

Analysis of the error in sensor-based DIC calculations showed that the pH⁹ of the culture fluid increased from approximately 8.4 to approximately 10.0 as cultivation progressed (Figure 3). This rise in pH is considered to reflect a shift in the carbonate equilibrium of the culture fluid as microalgae consume DIC through photosynthesis. In microalgal culture fluid undergoing such a large pH change over a short period, direct application of existing carbonate-equilibrium calculations to sensor measurements was considered likely to produce substantial errors in calculated DIC concentrations.

Figure 3. Change in pH of the algal culture fluid over time

The technology focuses on the difference between DIC concentrations measured by titration and those calculated from sensor measurements. NTT analyzed the relationship between relative error and pH, then developed a correction equation that adjusts calculated DIC concentrations according to pH (Figure 4, left).

■ Correction reduces maximum relative error from approximately 3,300% to approximately 8%

Applying the correction equation reduced the relative error of calculated DIC concentrations with respect to titration measurements from a maximum of approximately 3,300% before correction to a maximum of approximately 8% after correction (Figure 4, right). The correction equation has been confirmed to be effective in a laboratory environment using a 5 L culture bottle at 25℃ and within a pH range of 8 to 10. These results confirm that DIC concentrations can be calculated easily and accurately from sensor data.

Figure 4. pH-based correction of calculated DIC concentrations and its effect

Application Example

NTT calculated the amount of carbon contained in DIC in the culture fluid from the corrected DIC concentration and added it to the amount of carbon contained in CO₂ in the air inside the culture vessel. NTT then compared a microalgal culture with an algae-free control under alternating 12-hour light and dark periods¹⁰ to evaluate changes in the amount of carbon in the culture system associated with CO₂ utilization by microalgae (Figure 5). During the light period, the amount of carbon decreased in a manner consistent with CO₂ absorption through microalgal photosynthesis. During the dark period, photosynthesis stopped and a different pattern was observed, reflecting effects including respiration.

The results confirm that the technology can continuously and accurately capture changes over time in the amount of carbon associated with microalgal CO₂ absorption under different light conditions.

Figure 5. Change in the amount of carbon in the culture system over time under alternating light and dark periods
Negative values indicate that the amount of carbon in the culture system was lower in the algal culture than in the control.

Future Development

NTT will further evaluate the technology under a wider range of conditions, including different algal species and culture conditions that reflect commercial production, to expand its applicability.

Time-series DIC concentration data obtained using the technology can identify conditions under which microalgae actively absorb CO₂ and the timing of declines in CO₂ absorption. Optimizing culture conditions based on this information is expected to increase CO₂ absorption by microalgae and improve biomass production efficiency. By combining the technology with environmental control technologies built into cultivation systems and the cultivation expertise of algae producers, NTT aims to enable more advanced culture management, increase productivity, and contribute to the industrial use of algal biomass.

Related Press Releases

Notes

¹Dissolved inorganic carbon (DIC): A collective term for inorganic carbon dissolved in water. It exists mainly as dissolved CO₂, bicarbonate ions, and carbonate ions.

²Titration method: A chemical analysis method in which a reagent of known concentration is added to a sample and the amount of a component in the sample is determined from the amount of reagent required for the reaction.

³Microalgae: A collective term for microscopic algae whose individual cells are difficult to see with the naked eye. Like plants, many microalgae grow by using CO₂ and other forms of inorganic carbon through photosynthesis.

⁴Biomass: A collective term for resources derived from living organisms. In this release, the term refers primarily to microalgal cells and useful components obtained from them, including proteins, lipids, and carbohydrates.

⁵https://group.ntt/en/newsrelease/2021/11/12/211112a.html

⁶https://group.ntt/en/newsrelease/2023/02/09/230209c.html

⁷https://group.ntt/en/newsrelease/2026/07/07/260707a.html

⁸Carbonate equilibrium: A state in which dissolved CO₂, bicarbonate ions, carbonate ions, and other forms coexist in water while interconverting. Their relative proportions depend mainly on pH and are also affected by temperature, salinity, and other factors.

⁹pH: A measure of the acidity or alkalinity of an aqueous solution. A pH near 7 is neutral; higher values indicate greater alkalinity and lower values indicate greater acidity.

¹⁰Light and dark periods: The light period is the period during which the culture is illuminated, and the dark period is the period without illumination. In this study, 12-hour light and 12-hour dark periods were alternated.

About NTT

NTT is a leading global technology innovator, providing a broad range of services to both consumers and businesses. As a mobile operator and provider of infrastructure, networks, and services, NTT is dedicated to promoting a sustainable future through cutting-edge innovations. Our portfolio includes business consulting, AI-powered solutions, application services, global networks, cybersecurity, data center and edge computing, all supported by our deep global industry expertise. Generating over $90 billion in revenue and employing 340,000 professionals, we allocate 30% of our annual profits to fundamental research and development. With operations spanning more than 70 countries and regions, our clients include over 75% of Fortune Global 100 companies, alongside thousands of enterprises, government organizations, and millions of consumers.