Renewable Energy Global Innovations features: Co-cultivation of microalgae and nitrifiers for higher biomass production and better carbon capture

Significance Statement

Dragoljub Bilanovic and colleagues investigated the co-cultivation of nitrifiers with microalgae as a non-intrusive technique for selective removal of oxygen generated by microalgae. The study is now published in peer-reviewed journal, Bioresource Technology.

According to the researchers, to tackle the challenge of climate change, processes and practices, elimination of all anthropogenic carbon emissions from CO₂ must be deployed.  Current biological, chemical and physical processes being developed for carbon capture and storage are expensive to operate even when capturing and storing carbon from concentrated CO₂ sources. Photosynthesis was found helpful in reducing CO₂ emissions and will not decrease the concentration of the atmospheric CO₂.

Microalgae will produce about 280 ton of biomass per hectare per year, which indicate that a hectare microalgal reactor will release > 500 ton CO₂ to the atmosphere per year. Microalgal photosynthesis, has been found to be an excellent-natural carbon capture and microalgae biomass.

The authors calculated the oxygen excreted by microalgae into growth medium intentionally to balance the amount of oxygen needed for full oxidation of both ammonia and nitrite to provide N-NO₃ for microalgal growth. The research team assumed composition of ammonia oxidizers and nitrite oxidizing to be the same. Nitrifying bacteria use large quantities of oxygen to convert ammonia to nitrate, to avoid unnecessary accumulation of extracellular products, generated by Chlorella and nitrifying bacteria the experiments were terminated at the end of exponential phase. The team immobilized nitrifying bacteria to know how immobilization affects growth and oxygen concentration in mixed culture of suspended C. vulgaris and immobilized nitrifiers.

They observed that a higher chlorophyll concentration, and higher microalgae biomass production in the mixed-culture, which was due to nitrifying bacteria decreasing the local concentration of photosynthetic oxygen at the surface of microalgae cells thereby improving C. vulgaris growth. The lowest chlorophyll concentration was measured in mixed-culture in which the initial [N]/[A B] ratio was 4.20 indicating that the concentration of nitrifiers was sufficient to affect the concentration of dissolved oxygen but not sufficiently high to eliminate oxygen inhibition.

A somewhat smaller chlorophyll concentration was found with immobilized nitrifiers, up to 30% smaller, than with nitrifiers grown in suspended mode. They found biomass and chlorophyll concentration to be significantly higher in cultures where the dissolved oxygen concentration was kept below 9.0μL L-1 this shows microalgae to be more sensitive to oxygen inhibition than currently thought.

The study concluded higher biomass production is the main way to reduce atmospheric CO₂. Nitrifiers was able eliminate oxygen inhibition, enabling high levels of CO₂ conversion to organic compounds via photosynthesis. The microalgae biomass production method developed in this study can be scaled up into building reactors for removal of CO₂ straight from the atmosphere.

 

Journal Reference

Dragoljub Bilanovic¹ ,Mark Holland², Jeanna Starosvetsky³, Robert Armon³, Co-Cultivation of Microalgae and Nitrifiers for Higher Biomass Production and Better Carbon Capture, Bioresource Technology 220 (2016) 282–288.

Show Affiliations

1. Center for Environmental, Earth, and Space Studies, Bemidji State University, Bemidji, MN, USA.
2. Department of Biological Sciences, Salisbury University, Salisbury, MD, USA.
3. Division of Environmental, Water, and Agriculture Engineering, Faculty of Civil and Environmental Engineering, Technion, Israel Institute of Technology, Haifa 32000, Israel.

 

 

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