Isolation and Cultivation of Microalgae Species from Cattle Manure Wastewater and the Use of Its Biomass as a Biostimulant for Chili (Capsicum annuum L.) Germination

The use of cattle manure wastewater for microalgae cultivation is beneficial, as it contains nutrients such as N, P, K, Mg, Zn, and Fe. Therefore, different species of algae were isolated and cultivated, and nutrient analyses were conducted, including microalgae Watanabea sp., which resulted in higher biomass production (1.5 g L−1), 36% protein, and 40% carbohydrates, although the ash fraction was only 8%, the lowest. In a TOPSIS (Technique for Order Preference by Similarity to Ideal Solution) analysis, this species was selected from among the isolates because it demonstrated the best performance for use as a biostimulant for chili germination. After that, optimal conditions were established for cultivating Watanabea sp. microalgae in six polypropylene photobioreactors with a capacity of 2.9 L were used and tested at pH values of 7.0, 7.5, 8.0, 8.5, 9.0, and 9.5, at T = 25 °C, under constant light at 850 µE∙m−2∙s−1, and different dilution rates from 0.1 until 0.9%. The highest biomass production was achieved at pH 7.5 and a dilution rate of 0.1%, indicating that slightly alkaline conditions favored biomass accumulation. These conditions were associated with enhanced nitrate and phosphate assimilation. Furthermore, the effect of Watanabea sp. biomass on stimulating the germination of agricultural chili (Capsicum annuum L.) seeds was evaluated. Three methods of application (grounding and ungrounding freeze-lyophilized and fresh biomass) and concentrations of microalgal biomass (0.5, 1.0, and 1.5 g L−1) and a control (distilled water) were used at two temperatures (25 and 35 °C). The microalgae Watanabea sp. grounding freeze-lyophilized and treated chili seeds increased the final germination percentage and slightly earlier with better vigor and root growth at both temperatures. Further studies are needed to evaluate its effects on seedling establishment and field performance.