Alkaliphilic microalgae and cyanobacteria have gained significant importance due to their robustness, high biomass productivity, and ability to efficiently capture carbon dioxide directly from the atmosphere. To grow these alkaliphiles under high pH (pH >10.4) and high alkalinity conditions (0.1 - 0.5 M), substantial amounts of nutrients are required, which could potentially increase the operating costs of cultivation and adversely affect its environmental footprint. One conceivable way of tackling this issue is by re-using the spent medium and supplementing only the depleted nutrients. To effectively re-use the spent medium, first it is important to understand the nutrient bioavailability and uptake by alkaliphiles. In this study, we have determined the bioavailability of nutrients (e.g. C, N, P, S, Mg, S, Ca, Fe, etc.) in a high pH (> 10.4) and alkalinity (0.5 M) medium. Our results show that –with the exception of Mg, Ca, and Fe– all the nutrients are in bioavailable form for microbial growth. The availability of Mg, Ca, and Fe is limited because of precipitate formation with carbonates and hydroxides. Additionally, we have also carried out cultivation experiments to determine biomass productivity, elemental composition, and stoichiometric formula based on nutrient uptake. The cyanobacterial cultures grew well without any inhibition and a maximum productivity of 153 mg-AFDW L^-1 d^-1 was achieved. The elemental composition of biomass suggested that Mg and Ca content in biomass is low, consistent with the limited availability of these elements during the growth. Finally, the derived stoichiometric equation resulted in the following chemical formula CH_1.75N_0.17O_0.41P_0.003.