# Indole-3-Acetic Acid Action in Outdoor and Indoor Cultures of _Spirulina_ in Open Raceway Reactors
## 4 Discussion
This can be compared to the findings of [PERSON] et al. [50], who observed similar trends when cultivating _Spirulina_ sp. LEB 18 in raceways augmented with magnetic fields. Notably, their indoor tests yielded a maximum pH of 10.75, while the outdoor cultures recorded a pH peak of 12.77. Microalgae can harness both CO\({}_{2}\) and HCO\({}_{3}\) as substrates for their CO\({}_{2}\) metabolism. Through active transport of inorganic carbon, these microorganisms accumulate intracellular bicarbonate and activate internal carbonic anhydrase [51]. This biological process underpins the mechanism for the observed elevation in carbohydrate content resulting from IAA supplementation. Phytohormones likely contribute to carbon fixation during photosynthesis [19; 52], stimulating a cascade of reactions within photosynthetic metabolism. Consequently, this stimulation culminates in an amplified synthesis of biomolecules, exemplified by heightened carbohydrate production. Another significant determinant in fostering carbohydrate synthesis is the choice of reactor configuration. Raceways or analogous open systems facilitate enhanced cellular exposure to atmospheric CO\({}_{2}\), consequently promoting biomolecule synthesis [53]. Moreover, outdoor cultivation settings exhibit an augmented carbohydrate yield. This phenomenon can be attributed to increased light exposure and elevated temperatures, which directly influence the synthesis of this macromolecule [54; 55]. A contrasting trend was observed in protein content, with the elevation in carbohydrate concentration evident in the supplemented and outdoor experimental conditions compared to controls and indoor setups. This counterintuitive effect can be attributed to the phenomenon of nitrogen limitation because of cultivation duration, which prompts microalgae cells to shift their protein synthesis toward the production of carbohydrates or lipids [56]. The decline in protein synthesis and the concurrent rise in carbohydrate content align with findings reported by [PERSON] and [PERSON] [18] and [PERSON] et al. [57] in their investigations involving the supplementation of IAA in cultures of _Chlorella pyrenoidosis_ FACHB-9 and _Scenedesmus obliquus_ GU732418, respectively. Another fact that may have influenced the increase in carbohydrate production and the decrease in protein synthesis in outdoor cultivation is the unfavorable conditions that stress the microalgae cells. This is in line with [PERSON] et al. [58], who described that an increase in light intensity is correlated with a reduction in protein content and an increase in carbohydrate content. The same was observed when the duration of light (photoperiod) increased. Higher carbohydrate content at the expense of lower protein content occurs consistently with higher daily irradiances. Firstly, higher light encourages photosynthesis processes that produce carbohydrates. Secondly, the higher irradiance also increases culture temperature, which induces cyanobacterial cell stress and reduces the enzymatic capacity for protein synthesis so that the first-order photosynthetic products of simple carbohydrates are accumulated in cells [59]. Moreover, analogous to the impact observed due to the type of reactor employed, elevated temperatures and increased illuminance further augmented carbohydrate production while concurrently exerting a deleterious effect on protein content within the biomass. Maintaining the cultivation temperature at approximately 30 \({}^{\circ}\)C and ensuring a pH range of 9.0-10.5 creates the optimal conditions for fostering heightened protein synthesis in _Spirulina_ sp. [56; 60; 61]. Fluctuations in pH and temperature can disrupt the bioavailability of essential nutrients and influence the activity of microalgae cells [62]. Given that the outdoor conditions employed in this study encompassed a broad spectrum of temperatures and illuminance levels and that the resultant pH of the cultures surpassed the ideal range, the observed decline in protein synthesis can be understood.