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 (NO_x and SO_x) 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.

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].