Microalgal production conducted outdoors using low-cost cultivation media is important for the consolidation of this bioprocess on a large scale. In this context, the use of a chemically defined medium with reduced costs can not only provide nutritional security but also contribute to an increase in the concentration of biomolecules without a loss of microalgal biomass productivity. Thus, this study aimed to evaluate the biomass production and biomolecule concentrations of Spirulina sp. LEB 18 and Synechococcus nidulans LEB 115 in outdoor cultivation using media containing reduced nutrients. Algal performance was assessed in open raceway bioreactors, using three different culture media, namely, BG-11, standard Zarrouk, and a modified Zarrouk [reduction in the sources of carbon (83%), phosphorus (94%), nitrogen (40%), and magnesium (63%) concerning the standard Zarrouk medium]. Cultivation of Spirulina sp. LEB 18 in the modified Zarrouk medium promoted the shortest generation time (2.10 days), with the concentration of carbohydrate produced (37.1%) being 346% higher than that produced using the standard Zarrouk medium (8.3%). Similarly, compared with the Zarrouk medium, the modified Zarrouk medium promoted higher biomass productivity of S. nidulans LEB 115 (0.19 g L−1 d−1) along with a 160% increase in the concentration of carbohydrates (21.6%) produced by this strain. Thus, for the examined microalgal strains, the use of the modified Zarrouk medium enhanced the potential carbohydrate production and maintained the protein concentration above 39%, thereby indicating that this medium would be a promising candidate for the cost-effective large-scale production of microalgal biomass.