Analysis of direct and indirect quantification methods of CO₂ fixation via microalgae cultivation in photobioreactors: A critical review

Anthropogenic CO₂ emissions are the main contributors to climate change. Among the various efforts to reduce atmospheric CO₂ levels, cultivation of microalgae is the most promising approach owing to its high photosynthetic rates and CO₂ fixation efficiencies than terrestrial counterparts. However, the accurate quantification method of CO₂ fixation during the cultivation of microalgae in photobioreactors (PBRs) is lacking. Present methods for the determination of CO₂ fixation during microalgae cultivation include direct and indirect methods, where 79% of direct method studies of the bibliometric analysis compared to 21% of indirect method studies. Direct methods evaluate the carbon content in microalgae biomass using assumptive values, though it results in significant errors as high as 50% in quantifying the CO₂ fixation. This can be improved by measuring the carbon contents using elemental and total organic carbon analysis. On the other hand, indirect methods quantify CO₂ concentration at inlet and outlet of PBRs by using gas chromatography or infrared sensors. It is rather difficult to validate the accuracy of direct and indirect methods due to the lack of comparative works and analysis among the methods. Additionally, there are no current studies that provide in-depth discussion and perspectives on the CO₂ fixation methods. Therefore, the main aim of this critical review is to analyse, contrast and discuss the differences as well as inaccuracies of direct and indirect microalgae CO₂ fixation quantifications in PBRs. This is followed by the recommendations for further improvements, and standard guidance for future studies in applying appropriate CO₂ fixation quantification methods.