Carbon dioxide (CO₂) biofixation by microalgae and its potential for biorefinery and biofuel production

Highlights

• CO₂ biofixation by Chlorella sp. and T. suecica

• CO₂ concentration effects on pH and microalgae growth

• Both microalgae capable to growth in elevated CO₂ concentration

• Organic acid and solvent can be produced from fermentation of microalgae biomass.

Abstract

Carbon dioxide (CO₂) using biological process is one of the promising approaches for CO₂ capture and storage. Recently, biological sequestration using microalgae has gained many interest due to its capability to utilize CO₂ as carbon source and biomass produced can be used as a feedstock for other value added product for instance biofuel and chemicals. In this study, the CO₂ biofixation by two microalgae species, Chlorella sp. and Tetraselmis suecica was investigated using different elevated CO₂ concentration. The effect of CO₂ concentration on microalgae growth kinetic, biofixation and its chemical composition were determined using 0.04, 5, 15 and 30% CO₂. The variation of initial pH value and its relationship on CO₂ concentration toward cultivation medium was also investigated. The present study indicated that both microalgae displayed different tolerance toward CO₂ concentration. The maximum biomass production and biofixation for Chlorella sp. of 0.64 g L^− 1 and 96.89 mg L^− 1 d^− 1 was obtained when the cultivation was carried out using 5 and 15% CO_2, respectively. In contrast, the maximum biomass production and CO₂ biofixation for T. suecica of 0.72 g L^− 1 and 111.26 mg L^− 1 d^− 1 were obtained from cultivation using 15 and 5% CO₂. The pH value for the cultivation medium using CO₂ was between 7.5 and 9, which is favorable for microalgal growth. The potential of biomass obtained from the cultivation as a biorefinery feedstock was also evaluated. An anaerobic fermentation of the microalgae biomass by bacteria Clostridium saccharoperbutylacenaticum N1-4 produced various type of value added product such as organic acid and solvent. Approximately 0.27 and 0.90 g L^− 1 of organic acid, which corresponding to acetic and butyric acid were produced from the fermentation of Chlorella sp. and T. suecica biomass. Overall, this study suggests that Chlorella sp. and T. suecica are efficient microorganism that can be used for CO₂ biofixation and as a feedstock for chemical production.

Introduction

The rapid development of the human population and uncontrolled energy consumption has led to the depletion of global energy resources reserves (BP, 2016). Furthermore, over-consumption of energy through heat, electricity and transportation fuel has been identified as the primary cause of global warming and environmental pollution (EIA, 2016). Besides, burning of fossil fuels has released substantial amounts of carbon dioxide (CO₂) into the atmosphere, which could affect global climate change in a very short period of time (Olivier et al., 2015). CO₂ emissions from industrial activities have been identified as one of the main greenhouse gases (GHG), which could lead to global warming and environmental unsustainability.

Several CO₂ sequestration approaches have been introduced in order to capture CO₂ for instance, physical processes (biochar burial, ocean storage, geological sequestration), chemical processes (mineral carbonization and chemical scrubber) and biological processes (reforestation, agriculture and photosynthetic microorganisms) (Grover et al., 2015, Leung et al., 2014, Sanna et al., 2014). Biosequestration of CO₂ using photosynthetic micro-organisms particularly microalgae is believed to be one of the promising approaches to fix CO₂ (Cheah et al., 2015). Generally, microalga are microscopic organism having high growth rate and capability to utilize CO₂ as a carbon source for its biomass production (Zeng et al., 2012).

Biofixation using microalgae involves a complex metabolism during the cultivation process. Microalgae have been reported to have high CO₂ emission fixation compared to terrestrial plants (Chen et al., 2013). This can be explained by the presence of carbon concentrating mechanisms (CCMs) present in many types of microalgae species, enabling them to utilize and accumulate carbon under different concentrations of CO₂ (Barber and Price, 2003). Theoretically, providing CO₂ in microalgae cultivations could also promote photosynthesis and enhance microalgae growth rate (Benavente-Valdés et al., 2016). The biomass produced from the cultivation that contain valuable chemical components such as lipid, protein and carbohydrate can be converted into other value-added products such as biofuel, chemicals and biopolymer-based materials (Halim et al., 2011, Harun et al., 2010, Kassim et al., 2014b).

The efficiency of biofixation using microalgae could be contributed by several factors, for instance, cultivation condition, technical operation of different types of bioreactor and engineering aspect (Kumar et al., 2010, Praveenkumar et al., 2014, Raeesossadati et al., 2014). However, based on the current technology maturity and limitations, there is no efficient way to mitigate CO₂ in a sustainable manner. A suitable bioreactor that could provide sufficient light and temperature to support microalgae growth are also important to obtain maximum biomass production and CO₂ biofixation (Pires et al., 2014, Renaud et al., 1995). Furthermore, the available technologies such as CO₂ fixation using bioreactor consume substantial amount of energy and very costly when up-scaled to commercial stage (Lam et al., 2012). Another factors that could play crucial role and need to be considered in the CO₂ biofixation is selection of the suitable microalgae strain used for the process. Generally, the capability of microalgae to tolerate CO₂ concentration is species specific and can be grouped into two different groups namely CO₂-sensitive (less 2–5%) and CO₂ tolerant (5–20%) ((Miyachi et al., 2003). Since, the typical flue gas generated from the power plant is contain between 3 and 15% of CO₂, hydrogen sulphate (H₂S), nitrogen and other hydrocarbon that could inhibit microalgae growth. Thus the microalgae which has capability to growth and utilize high CO₂ (beyond atmospheric CO₂) concentration is the best candidate that can be used for biofixation (Ji et al., 2016, Last and Schmick, 2011). Therefore, the objective of this study is to evaluate the growth performance and biofixation capability of two different microalgae species namely, Chlorella sp. and Tetraselmis suecica cultivated in a medium supplied with elevated CO₂. The potential of microalgal biomass produced from this cultivation for use as chemical production feedstock is also determined in this present study.