The impending danger of climate change and pollution can now be seen on the world panorama. The concentration of CO2, the most important Green House Gas (GHG), has reached to formidable levels. Although carbon capture and storage (CCS) methods have been largely worked upon, they are cumbersome in terms of economy and their long term environmental safety raises a concern. Alternatively, bio-sequestration of CO2 using microalgal cell factories has emerged as a promising way of recycling CO2 into biomass via photosynthesis which in turn could be used for the production of bioenergy and other value-added products. Despite enormous potential, the production of microalgae for low-value bulk products and bulk products such as biofuels, is heretofore, not feasible. To achieve economic viability and sustainability, major hurdles in both, the upstream and downstream processes have to be overcome. Recent technoeconomic analyses and life-cycle assessments of microalgae-based production systems have suggested that the only possible way for scaling up the production is to completely use the biomass in an integrated biorefinery set-up wherein every valuable component is extracted, processed and valorized. This article provides a brief yet comprehensive review of the present carbon sequestration and utilization technologies, focusing primarily on biological CO2 capture by microalgae in the context of bio-refinery. The paper discusses various products of microalgal biorefinery and aims to assess the opportunities, challenges and current state-of-the-art of microalgae-based CO2 bioconversion, which are essential to the sustainability of this approach in terms of the environment as well as the economy. Introduction The increased concentration of Green House Gases (GHGs) are causing dramatic climatic changes (rise in temperature, changes in the distribution, intensity and pattern of rainfall, rising sea levels, floods, droughts and increased occurrence of extreme climatic phenomena) as a result of well-known phenomenon “Global Warming” (Alexander et al., 2006; Church and White, 2006; Rignot and Kanagaratnam, 2006; Meinshausen et al., 2009; Rockstrom et al., 2009; Solomon et al., 2009; Dawson et al., 2011). The temperature of the planet has risen by 0.85°C from 1880 to 2012 and it has been forecasted that by the end of this century, a rise of 1.4–5.8°C would be witnessed (De Silva et al., 2015). The concentration of CO2, the most important GHG and the major contributor to global warming, has reached to formidable levels. Corresponding to a 32% increase, from around 280 ppm to 400 ppm, since the industrial revolution (De Silva et al., 2015). The primary causes being irrational use of fossil fuels and change in land use pattern (Goldemberg, 2007; Atsumi et al., 2009). Not merely global warming, the increased CO2 concentration in the atmosphere has also led to a 30% increase in the ocean acidity, which in turn is affecting the biodiversity adversely (Doney et al., 2009; Hofmann and Schellnhuber, 2010; Farrelly et al., 2013). The Kyoto Protocol and the Paris Agreement (2015), have set a number of policy actions for participating countries to curb climate change impact. The major requirement being reduced CO2 emissions by reduced fossil fuel utilization and increased carbon capture and sequestration (Cheah et al., 2016; Pires, 2017). This minireview aims to discuss briefly yet comprehensively the various CCS methodologies, focusing mainly on the potential of microalgae mediated carbon capture within the framework of a biorefinery approach: bioconversion and valorization of captured CO2, current state of the technology, recent developments, challenges and future prospects. CO2 Capture and Storage Methods Currently there are many physico-chemical carbon capture and sequestration strategies that are combinedly categorized as carbon capture and storage (CCS) methodologies. CCS operate over 3 major steps: CO2 capture, CO2 transportation and CO2 storage. CO2 capture is done from large point sources such as power plants and cement manufacturing plants. The separation and capture of CO2 from other exhaust components is usually done via following methods: (i) chemical absorption; (ii) physical adsorption; (iii) membrane separation; and (iv) cryogenic distillation (Figueroa et al., 2008; Pires et al., 2011, 2012). This highly concentrated CO2 is then compressed and transported to storage points via pipelines or ship (Svensson et al., 2004; McCoy and Rubin, 2008). Next, the captured CO2 is stored into reservoirs, viz. geological storage, oceanic storage wherein the CO2 is directly injected deep into the ocean, saline formations, aquifers or depleted oil/gas wells (Lackner, 2003). Despite remarkable storage potential of the aforementioned CCS, considerable drawbacks remain, including expensive operation and transportation, environmental threat of long term CO2 leakage and other uncertainties (Lam et al., 2012; De Silva et al., 2015). Moreover, physico-chemical CCS methods are practically successful only for capturing CO2 from point sources producing high concentrations of CO2 i.e., diffused, non-point emissions and low concentrations of CO2 cannot be captured (Nouha et al., 2015). Table 1 briefly illustrates the various CCS methodologies, their mechanisms, merits and limitations with respective references. Aside to physical and chemical CCS, the biological route can be taken for capturing CO2 via natural sinks: (i) forestation; afforestation, reforestation, and the farming of crops and livestock, the biomass can be further valorized (Farrelly et al., 2013; Cheah et al., 2016). (ii) ocean fertilization; fertilizing oceans with iron and other nutrients prompting increased carbon dioxide uptake by the phytoplanktons (Williamson et al., 2012) (iii) microalgae cultivation (Lam et al., 2012; Cheah et al., 2016; Yadav and Sen, 2017; Zhou et al., 2017). CO2 Capture by Microalgae The term “microalgae” is generally used for both prokaryotic blue green algae (cyanobacteria) and eukaryotic microalgae including green algae, red algae, and diatoms. Microalgae are being sought as alluring biofactories for the sequestration of CO2 and simultaneous production of renewable biofuels, food, animal and aquaculture feed products and other value-added products such as cosmetics, nutraceuticals, pharmaceuticals, bio-fertilizers, bioactive substances (Ryan, 2009; Harun et al., 2010). Microalgae possess strategies, well known as CO2 concentrating mechanism (CCM) for efficiently photosynthesizing by acquiring inorganic carbon even from very low atmospheric CO2 concentrations (Whitton, 2012). These microorganisms surpass other feedstocks in terms of their abilities to flourish in extreme environments and simple yet versatile nutritional requirements. Microalgae do not require arable land and are capable of surviving well in places that other crop plants cannot inhabit, such as saline-alkaline water, land and wastewater (Searchinger et al., 2008; Wang et al., 2008). Furthermore, microalgae can be fed with notorious waste gasses such as CO2 and NOx, SOx from flue gas, inorganic and organic carbon, N, P and other pollutants from agricultural, industrial and sewage wastewater sources so as to provide us with opportunities to transform them into bioenergy, valuable products and forms that cause least harm to the environment (Chisti, 2007; Hu et al., 2008; Pires et al., 2012; Singh and Thakur, 2015). The uncomplicated cellular structures and rapid growth of microalgae endow them with CO2 fixation efficiency as higher as 10–50 folds than terrestrial plants (Li Y. et al., 2008; Khan et al., 2009). Recently, many research studies have come up showing the positive impact of growing microalgae under high concentrations of Ci in the form of pure gaseous CO2, real or simulated flue gas, or soluble carbonate (bicarbonate), reporting increased carbon bio-fixation and biomass productivity (Ho et al., 2010; Sydney et al., 2010; Yoo et al., 2010; Tang et al., 2011; Singh et al., 2014; Aslam et al., 2017; Kuo et al., 2017). Detailed information can be found in elaborated reviews by Lam et al. (2012); Cheah et al. (2015); Thomas et al. (2016); Vuppaladadiyam et al. (2018). The fate of the supplied carbon can end up in making skeleton for lipids, proteins, sugars and pigments (Sydney et al., 2010). Despite such remarkable potential, the production of microalgae for low-value bulk products, such as proteins for food/feed applications, fatty acids for nutraceuticals or bulk products such as biofuels, is heretofore, not economically feasible (Williams and Laurens, 2010; Zhou et al., 2017). Recent technoeconomic analyses and life-cycle assessments of microalgae- based production systems have suggested that the only possible way of realizing the potential production is to completely use the biomass in an integrated biorefinery set-up wherein every valuable component is extracted, processed and valorized (Chew et al., 2017). Biorefinery Concept of Microalgal Biomass The concept of valorization of a raw material into marketable products is well known in fossil fuel refinery, similarly biorefinery concept refers to the conversion of biomass into multiple commercially valuable products and fuels (Pérez et al., 2017).