# Indole-3-Acetic Acid Action in Outdoor and Indoor Cultures of _Spirulina_ in Open Raceway Reactors
## 1 Introduction  
The pursuit of renewable sources of food and biofuels has emerged as a top priority in response to global challenges, such as population growth and the scarcity of natural resources [1]. Within this context, the microalgae _Spirulina_ (_Arthrospin_) holds promising potential to address these demands [2]_Spirulina_ biomass contains iron, essential unsaturated fatty acids (such as \(\gamma\)-linoleic acid), vitamin B12, and a variety of B vitamins and natural carotenes, making them valuable for food applications and health foods. Additionally, _Spirulina_ is seen as a sustainable feedstock for bioenergy and biofuels, offering potential as a carbon-neutral source for biodiesel, biofhanol, and biogas production [3]. _Spirulina_ is one of the most extensively researched and cultivated microalgal genera worldwide [4] due to its adaptability to various cultivation conditions, diverse environments, culture media, and reactor types [3]. Among the factors impacting microalgal growth and productivity, light, temperature, and nutrients are the most influential [5]. Light is pivotal in photosynthesis, furnishing the energy needed for carbon fixation and microalgae growth. The intensity, spectral quality, and photoperiod represent critical factors influencing photosynthetic activity and, consequently, culture productivity [6]. The microalgae _Spirulina_ boasts a wide rangeof growth temperatures, demonstrating its ability to adapt to tropical and subtropical conditions [7]. Furthermore, light availability and exposure to varying temperatures can regulate the synthesis of specific biomolecules, including photosynthetic pigments [8], proteins, and carbohydrates [9]. These molecules hold significance for applications in the food [10], cosmetics [11], pharmaceutical [12], biofuel [13], and environmental solution [14] sectors. Specifically, concerning the microalgae _Spirulina_, recent studies have unveiled the presence of its phytohormones [9] and their influence on cultivation modulation [15; 16]. Among phytohormones, the best known are auxins, with indole-3-acetic acid (IAA) standing out for showing more significant activity in plants and algae [17] and stimulating _Spirulina_ biomass and biomolecules production [14; 15]. Phytohormones encompass natural chemical compounds that play a fundamental role in the development and metabolism of plants, algae, and microalgae. Phytohormones are effective in enhancing the microalgae growth and production of biomolecules by promoting the expression of genes involved in DNA replication and repair and energy metabolism (glycolysis, TCA cycle, and oxidative phosphorylation), as well as signaling and protective synthesis pathways [18]. In addition, phytohormones have advantages such as being efficient at minimal concentrations, natural, environmentally safe, and degrading quickly, leaving no environmental remnants [19]. However, the mechanism needs to be clarified. Therefore, further investigations should be conducted to identify the genes that control the effects of phytohormones. Despite the extensive literature documenting the impact of phytohormone stimulation on microalgae biomass and biomolecule productivity [15; 16; 18; 20; 21; 22; 23; 24], no research to date has explored the role of phytohormones within outdoor microalgae cultivation systems employing nonsterile media and operating on a larger scale [25]. In this study, we investigate the application of phytohormones in outdoor microalgae cultivation using raceway-type reactors and a nonsterile medium, focusing on the supplementation of indole-3-acetic acid (IAA). Our outdoor experimentation reveals promising enhancements compared to indoor studies. These findings highlight the potential of phytohormone supplementation as a cost-effective and environmentally friendly strategy for large-scale microalgae production, addressing a crucial research gap in the field.