Synthetic culture media, such as Zarrouk’s medium (ZM), are widely used in industrial Arthrospira cultivation but rely heavily on chemical fertilizers, raising concerns over cost and environmental impact. In natural habitats where Arthrospira blooms, the macronutrient concentrations are much lower than those provided by synthetic media. We hypothesized that natural growth may be facilitated by a microbial consortium. To test this, we developed a lab-scale Arthrospira platensis H53 cultivation system using a newly developed organic compost medium (OCM), designed to mimic the natural nutrient composition and microbial interactions. Compared to ZM, A. platensis H53 grown in OCM exhibited elevated growth by day 7. The specific growth rate in OCM was 0.20 day−1, higher than that of 0.17 day−1 in ZM, with optical density values reaching 1.57, compared to 1.13 in ZM. A 1.63-fold increase in biomass was observed in OCM, despite lower initial macronutrient concentrations. Nutrient use efficiency (NUE) in OCM was significantly improved, with nitrate (NO3−) and phosphate (PO43−) utilization up to 5.8-fold higher. Additionally, A. platensis H53 filaments in OCM were more tightly coiled, indicating a physiological change in response to lowered macronutrient concentrations. Microbial composition analysis using 16S rRNA gene amplicon sequencing revealed the presence of growth-promoting bacteria, including Pontibacter spp., Brevundimonas spp., and Aliihoeflea spp., likely contributing to nutrient cycling and enhanced growth. These findings suggest potential symbiotic interactions between cyanobacteria and non-cyanobacteria in the OCM system, promoting increased growth and productivity. This study is the first to propose such symbiosis in an extremely alkalophilic environment, offering another sustainable alternative to traditional chemical-based Arthrospira cultivation methods.
