First outdoor cultivation of the N 2 -fixing cyanobacterium Tolypothrix sp. in low-cost suspension and biofilm systems in tropical Australia

Tropical N ₂ -fixing cyanobacteria offer an attractive alternative for production of biomass and bioproducts with potentially low cultivation and harvesting costs. The present study evaluated the biomass productivity of the N ₂ -fixing cyanobacterium Tolypothrix sp. NQAIF319 grown in nitrogen-free medium in outdoor suspension and biofilm prototype cultivation systems in tropical Australia (Queensland). One-week cycles yielded maximum biomass productivities—estimated based on ground area occupied by single systems—of 45–49 g dry weight m ⁻²  day ⁻¹ (suspension) and 1.0–1.2 g dry weight m ⁻²  day ⁻¹ (biofilm) with minimal biological contamination (Tolypothrix sp. biomass representing 94–98 % of the photosynthetic community). Moderate productivities of the pigments phycocyanin/phycoerythrin (0.1–2.8 g m ⁻²  day ⁻¹ ), fatty acids (0.1–2.0 g m ⁻²  day ⁻¹ ), and nitrogen stored in the biomass (0.1–5.9 g m ⁻²  day ⁻¹ ) were reached in biofilm and suspension systems, respectively, opening avenues for production of low-value commodities with potentially big markets (nitrogen-rich biofertilizers and aquaculture feed) and higher-value chemicals (phycobiliproteins and fatty acids). Simulated multi-system arrangements yielded theoretical overall areal productivities four to six times lower than those in single systems thus highlighting the need for future tests fine-tuning inter-system separation to minimize shadowing while maximizing the efficiency in land use in larger-scale production plants. Biofilm and self-flocculated biomass showed 80-fold and 53-fold reduced extracellular-water contents compared to suspension cultures, respectively, which will need to be considered for techno-economic and water/carbon footprint evaluation of each of the possible bioproduct synthesis pathways. In conclusion, the flexible and simple prototypes developed together with the good properties of Tolypothrix sp. represent a promising platform for low-cost production of cyanobacterial bioproducts in tropical regions using low nitrogen-containing water sources.