## 1 Introduction  
Alkaliphilic microalgae and cyanobacteria (inclusively referred to as algae here) have been of great interest in a commercial application. Their selective growth conditions (i.e., high pH and alkalinity) prevent overgrowth of other organisms even in outdoor open-pond cultivations and allows relatively easy quality control. Among alkaliophilic algae, _Arthrospira_ (also known as _Spirulina_) is the most commercially produced genus because of its high nutritional value (protein up to 77%dwt, minerals, and vitamins) [1] and application for blue natural color pigment production [2]. The global production of _Arthrospira_ has been estimated to be approximately 10,000 metric tons in a report in 2013 [2] and is still expanding. Its emerging bio-active compounds have also been well studied [3; 4; 5; 6]. The challenge in mass-cultivation of alkaliophilic microalgae, however, is the large requirement of alkaline agents. For example, the conventional Zarrouk medium requires 16.8 kg of sodium bicarbonate (NaHCO\({}_{3}\)) for every cubic meter of the medium [7]. Thus, medium recycles after biomass harvest is a preferred operation [8; 9]. In a used medium, photosynthetic utilization of bicarbonate (HCO\({}_{3}\)-) and carbonate (CO\({}_{3}\)\({}^{2-}\)) results in increased pH (OH\({}^{-}\) production), but CO\({}_{2}\) supply regenerates HCO\({}_{3}\)\({}^{-}\) and CO\({}_{3}\)\({}^{2-}\) back from OH\({}^{-}\)[10]. As such, medium recycling allows sustainable use of alkaline agent if combined with appropriate CO\({}_{2}\) supply. Due to the CO\({}_{2}\) requirement, algae mass-cultivation is considered as a key technology for CO\({}_{2}\) bio-capture from CO\({}_{2}\) containing gas sources such as flue gas, steel mills, and biogas [11; 12], meriting both mass-culture cost reduction and environmental impact. With the recent demands for a reduction in CO\({}_{2}\) emission to prevent global climate change, algae are considered as one of the key organisms for CO\({}_{2}\) capture and utilization (CCU) processes [13; 14]. In these processes, captured CO\({}_{2}\) is utilized for algal photosynthesis to produce bioproducts such as biofuel, bioplastics, and other materials [13]. Although the carbon footprint differs depending on the use of algal biomass [14], carbon utilization for microalgal biodiesel production is one of the best options compared to chemical production. Even as protein sources, algal protein production has approximately 20 times lower carbon footprint compared to beef [15]. To capture CO\({}_{2}\) in the gas, chemical/physical absorption, membrane separation, and chemical looping techniques have been attempted [16]. The alkaline medium of _Arthospira_ is suitable for CO\({}_{2}\) recovery through chemical absorption [17]. The biggest advantage is the high alkalinity that could trap CO\({}_{2}\). For example, dissolved inorganic carbon (DIC) optimum concentration is in the range of 0.01-0.1 mol L\({}^{-1}\) in freshwater species like _Chlorella_[18; 19; 20], while it is 0.1-0.4 mol L\({}^{-1}\) for _Arthospira platensis_[17]. With this high alkalinity, the alkaliophilic algal medium can absorb much more CO\({}_{2}\) into the same amount of liquid without pH change due to the buffer function of DIC. In the practical CO\({}_{2}\) recovery process, CO\({}_{2}\) supply and/or algal productivity may fluctuate depending on the seasons or operational variations. In such cases, CO\({}_{2}\) carbon supply may exceed the photosynthetic carbon demand, which may interfere with CO\({}_{2}\) recovery rate and algal growth owing to DIC accumulation and pH reduction. Although the alkaline media of _Arthospira_ has a high buffer capacity and carbon absorptivity, continued high CO\({}_{2}\) supply may have effects. Feasibility needs to be tested through carbon absorption modeling and carbon mass balance analysis.