## 1 Introduction
### Research Question: Spirulina for CO\({}_{2}\) Removal and NaHCO\({}_{3}\)-Reduced Cultivation  
At present, microalgae species have been explored for their direct carbon assimilating potential [37; 38]. Among the myriad 70,000 algae species, one of the most studied is _Spirulina_ because they are part of the largest taxonomic groups and the massive biomass is industrially producible even in inorganic/heterotrophic culture media [39; 40]. _Spirulina_ (Arthospira), a genus of photosynthetic cyanobacteria, are a filamentous and free-floating biomass that can photoautotrophically thrive in mineral-rich aquatic surfaces with high pH (8-10) and carbonate substances. _Spirulina_ vary in their CO\({}_{2}\) capture performance according to their species and culture conditions, but they generally have excellent photosynthetic efficiency, high tolerance against environmental perturbation, and the ability to absorb substantial amounts of CO\({}_{2}\) from the atmosphere. In [PERSON] and [PERSON]'s study [41], 415 mg of CO\({}_{2}\)/L/d was processed by _Spirulina_ sp. at \(\sim\)12% of CO\({}_{2}\) concentration. In [PERSON] al.'s study [42], _Spirulina_ sp. absorbed up to 27 to 38% CO\({}_{2}\). Some researchers suggest that _Chorella_ sp. is the most appropriate for CO\({}_{2}\) mitigation [28; 43], but the choice of the best algae species depends on the purpose of use and growing conditions. For example, the biomass of _C. vulgaris_ and _S. plantensis_ is 251.64 and 318.16 mg/L/day, respectively [44], and _S. maxima_ carries a very high amount of protein (60-71% [38]). Considering that algae can fix about two times greater CO\({}_{2}\) than its biomass, it indicates that _Spirulina_ may outperform other species in growth and bio-productivity [45]. Indeed, [PERSON] et al. [46] revealed that the CO\({}_{2}\) sequestration rate of _S. plantensis_ was about three times greater (0.49 g/L/d) than other microalgae in natural water at 10% CO\({}_{2}\) supply.  
These advantages make _Spirulina_ bioreactors a strongly viable option to support/replace the conventional building systems. However, the biochemical complexity of the cultivation medium and complicated growth kinetics thereof become a major barrier to public spread of the microalgal photobioenactors. The _Spirulina_ cultures generally need alkalinity (pH), inorganic salts, nitrogen, and especially a high amount of carbon dioxide sources (CO\({}_{2}\), HCO\({}_{3}\)\({}^{-}\), etc.), and it often increases cost in enclosed culture systems, since NaHCO\({}_{3}\) takes the largest portion in the standard Society of Toxicology (SOT) medium (-76%) [47]. The need for regular replenishment and adjustment of the chemical composition to meet the optimal nutritional requirements becomes a limiting factor in their public use. To settle this challenge, a few researchers [13; 48] demonstrate that CO\({}_{2}\) added by external aeration substitutes for the inorganic carbon supply from sodium bicarbonate (NaHCO\({}_{3}\)). Although NaHCO\({}_{3}\) is a readily available form of CO\({}_{2}\) for _Spirulina_'s photosynthesis, exploiting CO\({}_{2}\) through mechanical aeration can be a more renewable and controlled alternative for the bioreactors, reducing cost and potential contamination associated with the sodium content.