Tokyo – October 12, 2026 -- NTT, Inc. has cut the maximum relative error of sensor-based CO2 absorption monitoring in microalgae cultures from approximately 3,300% to approximately 8%, measured against the conventional titration method.
The technology calculates dissolved inorganic carbon (DIC) concentrations from CO2 and other sensor readings. It does not require sampling the culture fluid for each measurement. The results were published in the journal Algal Research on August 12, 2026.
Titration is accurate but cannot deliver continuous data
Evaluating microalgal CO2 absorption means tracking DIC, which exists in culture fluid as dissolved CO2, bicarbonate ions and carbonate ions. Titration measures DIC accurately, but every reading requires a fluid sample and takes time and effort, so it does not suit continuous data acquisition.
Existing sensor-based calculations built on carbonate equilibrium allow continuous monitoring. Applied directly to microalgal culture fluid, however, they produced DIC values with relative errors of up to approximately 3,300% against titration.
A pH-based correction equation closes the gap
NTT's error analysis found that culture fluid pH rose from approximately 8.4 to approximately 10.0 as cultivation progressed. The company attributes this to a shift in carbonate equilibrium as microalgae consume DIC through photosynthesis. The large, rapid pH change made direct use of existing calculations likely to produce substantial errors.
NTT analyzed the relationship between relative error and pH and developed a correction equation that adjusts calculated DIC concentrations according to pH. The equation has been confirmed in a laboratory setting only, using a 5 L culture bottle at 25°C and a pH range of 8 to 10.
Light-dark tests show carbon uptake tracked over time
NTT added the carbon in corrected DIC to the carbon in CO2 in the air inside the culture vessel. It then compared a microalgal culture with an algae-free control under alternating 12-hour light and dark periods.
During light periods, the carbon in the culture system fell in a pattern consistent with CO2 absorption through photosynthesis. During dark periods, photosynthesis stopped and a different pattern appeared, reflecting effects including respiration.
Next tests target other species and commercial conditions
NTT plans to evaluate the technology across a wider range of conditions, including different algal species and culture conditions that reflect commercial production. Time-series DIC data can identify the conditions under which microalgae actively absorb CO2 and when absorption declines. NTT expects optimization based on this data to raise CO2 absorption and improve biomass production efficiency.
The company intends to combine the technology with environmental control systems built into cultivation equipment and with the expertise of algae producers. Microalgal biomass can serve as feedstock for animal feed, fuels and chemicals. NTT has also proposed using microalgae as feed for fish and shellfish.