Microalgae in Global CO2 Sequestration: Comparison
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Please note this is a comparison between Version 2 by Rita Xu and Version 1 by Shabnam Nisha.

The rising concentration of global atmospheric carbon dioxide (CO2) has severely affected our planet’s homeostasis. Efforts are being made worldwide to curb carbon dioxide emissions, but there is still no strategy or technology available to date that is widely accepted. 

  • microalgae
  • pyrenoid
  • carbon sequestration
  • carbon emissions

1. Introduction

Climate change is a major threat that severely hampers the survival of various plant and animal species as well as humans. The continuous increase in the emissions of several greenhouse gasses (GHGs), including carbon dioxide (CO2), water vapor, methane (CH4), nitrous oxide (N2O), and fluorinated gases, has aggravated climate change [1,2][1][2]. The rise in GHGs emissions is mostly associated with anthropogenic actions, with the use of fossil fuels being the largest contributor [3]. The world’s atmospheric CO2 has increased from ~313 ppm (in 1960) to ~411 ppm at present [4]. A high level of CO2 in the atmosphere raises the acidity of ocean water and affects the marine ecosystem to a significant extent [5,6][5][6]. Hence, it is highly imperative at this moment to develop an appropriate strategy to reduce or stabilize the CO2 content in the atmosphere. Various countries have signed many international protocols to curb GHGs emissions, e.g., COP26, Kyoto Protocol (1997), and the Paris agreement (2015).
Two basic approaches for reducing CO2 emissions include (i) the decreased use of fossil fuels complemented with the increased use of renewable energy sources; (ii) and carbon capture and storage via various biological, chemical, or physical methods [7,8][7][8]. The physical methods for carbon emission reduction have been extensively explored. Still, there are several technological and economic limitations with the existing technologies. Therefore, it is crucial to upgrade the existing technologies as well as develop suitable alternatives. Among various others, biological CO2 fixation seems to be a relatively cost-effective and eco-friendly approach in comparison to the physical and chemical methods. Photosynthetic organisms assimilate CO2 via the dark phase of photosynthesis and play a key role in maintaining the balance of CO2 levels in the atmosphere. Compared to other photosynthetic organisms, phytoplankton had higher CO2 fixation efficiency and biomass productivity [6]. Marine phytoplankton accounts for half of the total global primary productivity by fixing ~ 50 gigatons of CO2 annually [6]. In this context, research on CO2 sequestration by microalgae has attracted attention across the globe [9,10,11,12,13,14][9][10][11][12][13][14]. Microalgae can assimilate CO2 10–50 times more effectively, compared to vascular plants without competing or providing food to humans/animals [15,16,17][15][16][17]. Microalgae have a special mechanism to assimilate carbon dioxide known as the carbon concentration mechanism (CCM). In this mechanism, a specialized organelle i.e., pyrenoid increases the concentration of CO2 around the thylakoid membranes [18]. The increased concentration of carbon dioxide around the thylakoid membrane enhances the efficiency of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), an important photosynthetic enzyme for carbon assimilation or sequestration. Rubisco has a low affinity for carbon dioxide as it has been evolved in high CO2 and low O2 environments, so the pyrenoid constantly provides an environment for enhanced CO2 fixation [18,19][18][19]. It is evident from Table 1 that microalgae grown in various cultivation conditions for carbon sequestration showed higher tolerance for increased CO2 concentration. Some of the investigations also reported that microalgae can also be used for flue gas (NOx and SOx) sequestration [20]. Microalgae have high biomass yield and tolerance for adverse environmental conditions. Therefore, microalgae are considered a potential feedstock for CO2 sequestration and bioenergy production [21,22][21][22].
Microalgae are also used for wastewater treatment and biomass production, which can further be exploited for various applications, including biogas, bioplastics, and fertilizer production [23,24][23][24]. The low nutrition conditions and high photosynthetic efficiency have made it easy to cultivate algae for their exploitation for various applications. The previously published reviews extensively majorly explored the carbon sequestration of microalgae and their physiological mechanism, however detailed information on the role of pyrenoids in the sequestration of CO2 is missing, which are key aspects with specific reference to the global CO2 mitigation using algal technologies [25,26][25][26]. This review comprehensively discusses the physiological mechanism of carbon sequestration, the role of pyrenoids, and the impact of environmental factors in the carbon concentration mechanism. Besides this, the review also provides the current global CO2 emission status and scenarios.

1.1. Global CO2 Emission Status

Worldwide progress in the economy and the population boom have led to a continuously rapid rise in the emissions of carbon dioxide in the last few decades [27]. The rising level of CO2 in the atmosphere leads to an increased global average surface temperature, which directly and indirectly influences the global weather and climatic phenomenon (e.g., excessive rainstorms, drought) [27,28][27][28]. In order to combat the increasing earth’s surface temperature, the Paris agreement came into force, which was ratified by 196 countries to limit global warming below 1.5 °C compared to the pre-industrial era. This can be achieved through reducing greenhouse emissions by Nationally Determined Contributions (NDCs). The global carbon dioxide emissions have increased by 0.9% in 2019 compared to 2018. The largest emitters were China, USA, India, EU27 + UK, Russia, and Japan as per the Emission Database for Global Atmospheric Research (EDGAR) [29,30][29][30]. Demographically, these countries comprise 51% of the global population but contribute to ~67% of CO2 emissions. A detailed CO2 emission scenarios of major contributing countries from 1990 to 2020 is published elsewhere [29,30][29][30]. Surprisingly, compared to that in 2018, the level of CO2 emission in 2019 increased in China and India but decreased in EU28, the USA, and Russia (Figure 1). Global carbon emissions showed a 5% drop in the first quarter of 2020 compared to the first quarter of 2019, due to the decline in the demand for coal (8%), oil (4.5%), and natural gases (2.3%). In another report, the daily, weekly, and seasonal dynamics of CO2 emissions were presented and estimated a ~8.8% decrease in the CO2 emissions in the first half of 2020 [29,30][29][30]. The decline in global CO2 emissions in 2020 was due to the COVID-19 pandemic, which recorded the most significant decline since the end of World War II [28]. EDGAR estimated that 2020 showed a decline, with global anthropogenic fossil CO2 emissions 5.1% lower than in 2019, at 36.0 Gt CO2, just below the 36.2 Gt CO2 emission level registered in 2013 [29,30][29][30].
In 2019, global carbon emissions (fossil fuels) per unit of Gross Domestic Products (GDP) showed a declining trend reaching an average value of 0.298 tCO2/k USD/yrs., while per capita carbon emissions remained stable at 4.93TCO2/capita/yrs., confirming a 15.9% surge from 1990 [29,30][29][30] as published by [29. The COVID-19 pandemic has had a significant impact on social activities worldwide, and thus, on energy use and carbon emissions as well. According to the International Energy Agency (IEA), the energy demand in 2020 declined by 6%, which is seven times higher than the financial crisis energy demand of 2008 [31].

1.2. Carbon Sequestration Technologies

There are various physical, chemical, and biological methods in operation for reducing atmospheric CO2 emissions [7,8,32][7][8][32]. The carbon sequestration or fixation strategies are popularly known as carbon capture and storage/utilization (CCS/U). Carbon emission reductions in CCS is carried out in various stages such as CO2 capture, separation, transportation, utilization, and storage. A detailed discussion of all these steps is demonstrated elsewhere [7,8,32][7][8][32]. A major system frequently used for carbon capture comprises (i) pre-combustion, wherein CO2 is removed before combustion and the fuel is broken down to yield synthesis gas, a mixture of CO2 and H2; subsequently, CO2 is separated into various processes, and H2 is used as a clean fuel; (ii) post-combustion, where CO2 is captured after the combustion of fuels using chemical absorption; (iii) oxy-fuel where the fuel is combusted in the presence of pure oxygen to produce high levels of CO2; and (iv) chemical looping combustion, where oxygen carrier (solid metal oxides) particles are continuously circulated to supply oxygen to react with fuel, wherein the combustion of metal oxide and fuel produce metal, CO2, and H2O [33]. The separation of CO2 from flue gas also plays a vital role in carbon capture and storage technologies. Many separation techniques in operation include absorption/adsorption, membrane separation, and cryogenic distillation [34]. After capture at the source, CO2 needs to be transported to the sink, which requires further various methodologies described elsewhere [35].

2. Physiological Mechanism of Carbon Sequestration in Algae

Aquatic photosynthetic organisms, mainly phytoplankton, are responsible for 50% of the global carbon assimilation [36,37,38][36][37][38]. It has been stated in the literature that 1.0 kg of cultivated microalgae may assimilate 1.83 kg of CO2 [39,40][39][40]. There are three different processes i.e., photoautotrophic, heterotrophic, and mixotrophic metabolisms, involved in algae that help CO2 assimilation [41,42,43][41][42][43]. Microalgae take up inorganic carbon in three different ways: (i) The transformation of bicarbonates into CO2 by extracellular carbonic anhydrase that readily diffuses inside the cells without any hindrance; (ii) straight absorption of CO2 via the plasma membrane; and (iii) direct intake of bicarbonates by resolute carriers in the membrane, also known as dissolved inorganic carbon (DIC) pumps (Figure 1A) [44].
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Figure 1. Typical figures of carbon concentration mechanism in microalgae Chlamydomonas reinhardtii (A) showing (i) Bicarbonate/Ci pumps for Ci transportation; (ii) passive diffusion of CO2 through membrane pores; (iii) pyrenoid packed with Rubisco. (B) Magnified view of chloroplast where photosynthetic CO2 reduction takes place through Calvin–Benson cycle.

2.1. Photoautotrophic Metabolism

The majority of the microalgae are photoautotrophic, requiring inorganic carbon and light to transform (inorganic) CO2 into carbohydrates by photosynthesis. The algae fix CO2 through the Calvin–Benson cycle (Figure 1A), where the enzyme Rubisco plays a key role in converting CO2 into organic compounds [41,45][41][45]. In microalgae, the photosynthetic reaction can be classified as a light-dependent reaction and a light-independent or dark reaction (Figure 1B). The first phase of photosynthesis is light-driven, and here light transforms NADP+ and ADP into energy-storing NADPH and ATP molecules [46]. The second phase, i.e., the dark phase, consists of CO2 fixation and assimilation via the Calvin–Benson cycle in order to create organic compounds (glucose) with the aid of NADPH and ATP, produced in the first phase [47]. Here, Ribulose bisphosphate carboxylase/oxygenase (Rubisco) plays a significant role in the sequestration of CO2 [48,49][48][49]. Rubisco catalyzes the conversion of CO2 to 3-phosphoglycerate. However, due to the oxygenase character, Rubisco binds very weakly binds with CO2, which makes it a poor CO2 fixer [48,49][48][49]. These phosphoglycerates are then involved in yielding carbohydrates. Furthermore, these phosphoglycerates are mostly used to regenerate RuBP, which is then employed to continue the carbon-fixing cycle. The oxygen ion of Rubisco produces phosphoglycolate, which in turn hinders the carboxylase function of Rubisco. The phosphoglycolate is further transformed into phosphoglycerate (3-PGA) by exploiting ATP and releasing CO2. This reaction is known as photorespiration, in which O2 is utilized and CO2 is released [50]. Therefore, photorespiration leads to the wastage of carbon and energy, eventually decreasing the yield of photosynthesis [51]. Nonetheless, atmospheric O2 concentration usually remains higher compared to atmospheric CO2, thus further favoring the oxygenase functionality of Rubisco and thereby promoting photorespiration. To counter this situation, microalgae have developed CO2 concentrating mechanisms (CCMs) to enhance the concentrations of CO2 within close range of Rubisco [52,53][52][53].

2.2. Heterotrophic Metabolism

Heterotrophic metabolism occurs with or without solar energy, and it requires organic carbon. Although the majority of microalgae are photoautotrophic, there are cases where several microalgae can grow via heterotrophic metabolism under dark conditions or under low-light conditions, which is insufficient for autotrophic metabolism. These particular algae heterotrophically metabolize a wide range of organic carbon sources in these light-deprived environments [54,55,56,57][54][55][56][57]. This metabolism follows the pentose phosphate pathway (PPP), which involves the usage of organic carbons derived from acetate, glucose, lactate, and glycerol, and different enzymes involved in transportation, phosphorylation, anabolic and catabolic metabolism, and yielding energy via the substrate or respiration [43]. However, in a few algal strains, heterotrophy can also occur in the presence of light, and such processes are termed photoheterotrophy [58]. The characteristics of the heterotrophic microalgae cultivation are (i) comparably higher capacity to assimilate and grow under light-impoverished conditions; (ii) a fast growth rate; and (iii) the capability to metabolize various types of resources of organic carbon sources [56]. Numerous microalgal strains have been examined in heterotrophic conditions for the production of biomass, and various important metabolites using glucose as a carbon source [59,60][59][60].

2.3. Mixotrophic Metabolism

Mixotrophic metabolism obeys both autotrophic photosynthesis and heterotrophic assimilation. This metabolism can be considered a derivative of the heterotrophic metabolism as both CO2 and organic carbon are used together. Mixotrophic metabolism is accompanied by respiration and photosynthesis, resulting in maximum glucose usage [61]. Thus, mixotrophic metabolism can employ both organic and inorganic carbon, thereby leading to the high production of biomass [62,63][62][63]. The organic carbon is captured via aerobic respiration, whereas inorganic carbon is absorbed through photosynthesis [64]. Mixotrophic microalgae cultivation delivers higher cell yields per unit of energy input compared to autotrophic or heterotrophic cultivations [65]. Furthermore, mixotrophic metabolism manifests a lower energy-conversion efficiency compared to heterotrophic metabolism [65]. However, both these mechanisms preserve the important pigments and photosynthetic carotenoids under solar irradiation [66,67][66][67]. There are certain aspects where mixotrophic cultivation offers extra benefits over photoautotrophic cultivation, such as an increased growth rate, decreased growth cycles, insignificant decrement of biomass in the dark, and overall higher biomass yields [68,69][68][69]. However, mixotrophic metabolisms have their own disadvantages, i.e., comparably costly due to the high requirement of organic carbon resources and are vulnerable to intrusive heterotrophic bacteria in bare pond arrangements Moreover, balancing two kinds of metabolisms is another challenge for the mixotrophic mechanism. However, mixotrophic metabolisms have their own disadvantages, as they are costly due to their necessity of organic carbon resources and are vulnerable to intrusive heterotrophic bacteria in bare pond arrangements [70]. Moreover, balancing two kinds of metabolisms is another challenge for the mixotrophic mechanism.

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