## 1 Introduction  
Research studies on microalgae have increased in the last decades due to the wide range of applications associated with these photosynthetic microorganisms that can be utilized as sources of biofuels, pharmaceuticals, cosmetics, health, and dietary supplements [1; 2; 3; 4; 5; 6]. Microalgal photosynthesis is similar to that of higher plants, although characterized by a higher yield compared to terrestrial crops as it is more efficient in converting solar into chemical energy [7]. Due to their high growth rate, these organisms can be easily cultured in closed bioreactor systems reaching high biomass yields [4; 8]. Since the microalgae cultivation does not compete with that of terrestrial plants thanks to different resources use, the cultivation and exploitation of microalgae biomass represent an excellent opportunity for the development of alternative compounds for different bio-applications. The biological carbon pump is a major component of the global carbon cycle, which regulates atmospheric CO\({}_{2}\) levels and transfers both organic and inorganic carbon fixed by primary producers in terrestrial and aquatic ecosystems [9]. While terrestrial photosynthetic organisms utilize the atmospheric carbon dioxide (CO\({}_{2}\)) as a source of inorganic carbon (Ci) into Calvin-Benson cycle directly [10], aquatic photosynthetic organisms are adapted to use bicarbonate ions as an inorganic carbon source [10; 11; 12] since the carbonate/bicarbonate system converts CO2 into carbonate (CO3\({}^{2-}\)) and bicarbonate (HCO3\({}^{-}\)) ions. For this reason, the total inorganic carbon or dissolved inorganic carbon (DIC) in the aquatic environment is considered as the sum of inorganic carbon species in a solution, and it varies from less than 20 \(\upmu\)M, in acidic soft waters, to more than 5 M, in highly alkaline hard waters [13]. Both freshwater and marine microalgae are able to absorb HCO3\({}^{-}\) from the external environment into the cell through active membrane pumps [14]. The bicarbonate transferred into the cytosol is then converted by carbonic anhydrase enzyme (EC: 4.2.1.1) in CO2 usable in chloroplasts by [PERSON] (EC: 2.1.1.127) for the Calvin-Benson cycle to produce 3-phosphoglycerate [15; 16]. Microalgae have developed a carbon concentrating mechanism (CCM) which pumps extracellular bicarbonate ions into cells and raises the intracellular levels of CO2, promoting the carboxylation reaction and suppressing the oxygenase reaction of [PERSON], as well as the photorespiration [17; 18; 16]. [PERSON] et al. [19] demonstrated that HCO3\({}^{-}\) is also an important constituent of the water-oxidizing complex of photosystem II (PSII), by binding other components of PS II and influencing the molecular processes associated with the electron acceptor and electron donor sides of PS II. Generally, insufflating pure CO2 in microalgae cultivation is a system used to obtain a high level of biomass, although this process could be expensive [20] and inefficient due to the loss of CO2 in the atmosphere [21]. For this reason, the use of sodium bicarbonate (NaHCO3) as algal feedstock may be a cheaper and more sustainable alternative, considering its better solubility in aqueous solution than CO2 (9.21% (w/w) at 25 \({}^{\circ}\)C) [22; 23]. In the last years, several studies demonstrated that bicarbonate has a positive outcome on growth and metabolite accumulation as lipids, starch, and pigments in numerous microalgae [15; 22; 23; 24; 25; 26; 27; 28; 29]. _Chlorella_ spp. (Chlorophyta, Trebouxiophyceae) is a genus of green microalgae without flagella that shows spherical shape (\(\varphi\) -2 to 10 \(\upmu\)m). _Chlorella_ spp. has few and small vacuoles in the cytoplasm and a single and huge chloroplast in the shape of a cup with a starch granule. The chloroplast contains the green photosynthetic pigments chlorophyll-a and -b, \(\beta\)-carotene, and xanthophyll. Through photosynthesis, _Chlorella_ multiplies rapidly, requiring only carbon dioxide, water, sunlight, and a small amount of minerals to reproduce.