# Indole-3-Acetic Acid Action in Outdoor and Indoor Cultures of _Spirulina_ in Open Raceway Reactors
## 4 Discussion  
The findings from this study present significant implications for the scale-up of microalgae cultivation, particularly in outdoor settings and under uncontrolled conditions, with the supplementation of IAA showing promising potential. The observed outcomes pave the way for the industrial adoption of this strategy, given the substantial enhancements in productivity and reduction in biomass and biomolecule production costs achieved through the synergistic interaction of IAA and environmental factors. The results regarding biomass growth and productivity are consistent with the previous literature indicating the positive impact of phytohormone supplementation on microalgae under adverse conditions [39; 40; 41; 42; 43]. Despite the extreme fluctuations in abiotic factors, outdoor cultivation supplemented with IAA exhibited a higher biomass concentration and productivity than controlled indoor conditions. Phytochromores are suggested to confer cellular stress resistance by inhibiting chloroplast structure degradation [19]. Similar to the synergistic effects observed between phytohormones and different light spectra to enhance biomass and biomolecule production [44; 45], optimal light intensity is also pertinent for the efficacy of phytohormone performance, as evidenced by growth parameter evaluations. Notably, the outdoor experiments experienced higher light intensities, with a minimum of 160 umol photons m\({}^{-2}\) s\({}^{-1}\), twice that of indoor cultivation (80 umol photons m\({}^{-2}\) s\({}^{-1}\)). Furthermore, the maximum illuminance in outdoor conditions was 15-fold higher (1200 umol photons m\({}^{-2}\) s\({}^{-1}\)) than indoors. The elevation in biomass concentration is attributed to the concurrent substantial augmentation in light intensity. Exposing _Spirulina_ sp. LEB 18 to these intensified environmental conditions proved efficient in promoting its growth. Moreover, it amplified the phytohormone's effectiveness on the microalgae, culminating in a synergistic growth effect. The maximum quantum efficiency (Fv/Fm) indicates Photosystem II's photosynthetic efficiency. A decline in the Fv/Fm value signifies a reduction in photosynthetic efficiency and can be attributed to the cellular stress experienced by microalgae [46]. Remarkably, the  
Figure 5: Cost of biomass production and biomolecules produced in indoor and outdoor cultures of _Spirulina_ sp. LEB 18 supplemented with IAA. C-I = indoor control culture; IAA-I = indoor culture supplemented with IAA; C-O = outdoor control culture; IAA-O = outdoor cultivation supplemented with IAA. outdoor experiments displayed no disparity between initial and final values, signifying the consistent maintenance of maximum quantum efficiency throughout the cultivation period and an absence of noteworthy stress exposure. Conversely, the indoor experiments exhibited a decrease in this parameter after 30 days of cultivation. This phenomenon can be elucidated by microalgae's diminished photosynthesis and energy generation when subjected to low-light indoor conditions [47]. Both indoor and outdoor experiments exhibited an elevation in pH levels. This phenomenon can be attributed to the bicarbonate assimilation mechanism inherent to microalgae. Initially, the Zarrouk medium contains sodium bicarbonate (NaHCO\({}_{3}\)) as the primary form of carbon, serving as the favored substrate for carbon assimilation by microalgae [48]. This process is reflected in the pH range of 6 to 10.5. Over time, the microalgae consume two bicarbonate ions: one is internalized as carbon dioxide (CO\({}_{2}\)), and the other is released as carbonate (CO\({}_{3}\)\({}^{-2}\)). This intricate interplay contributes to a pH elevation in the medium, resulting in values surpassing 10.5 [49]. Consequently, the outcomes derived from this study point toward a greater assimilation of carbon molecules in outdoor cultures. This assertion is supported by a pH level exceeding 11 at the culmination of the outdoor experiment, whereas indoor conditions maintained a pH of approximately 10.