Co-culture of Chlorella vulgaris with nitrogen-fixing Azotobacter chroococcum in nitrogen-deplete medium to increase algal lipid content

Reliance on fossil fuels significantly contributes to climate change. Sustainable biofuels are a promising alternative. The high lipid content and environmental benefits of microalgae lend themselves strongly to biofuel production. This study focuses on taking an industrially relevant, lipid-rich microalgae strain and engineering a unique solution to further increase the lipid content. Although lipid yield is greater than terrestrial biofuel feedstocks, it remains a key limiting factor preventing microalgal biofuel from becoming a positive investment. Both microalgal-bacterial co-culture and nitrogen deprivation have individually increased lipid yields in prior art. Nitrogen depletion forces microalgae into a stressed state in which lipids accumulate rapidly, while certain bacteria have been shown to have symbiotic relationships with algae increasing lipids or biomass. Co-culture with a nitrogen-fixer supplies low levels of bioavailable nitrogen to the algae, allowing for higher photosynthetic efficiency while keeping the algae in a stressed state. These effects enhance lipid production. However, the combination of nitrogen-fixing Azotobacter chroococcum and lipid-rich Chlorella vulgaris under nitrogen-deplete conditions has yet to be tested. This study combined co-culture of C. vulgaris with nitrogen-fixing A. chroococcum under nitrogen-deplete conditions and found a significant 8.3-fold increase in lipid content per cell compared to nitrogen-replete monoculture algae. This was paired with a nearly 2-fold increase in algal lipids compared to a nitrogen-deprived control as well as an Escherichia coli co-culture nitrogen-deprived control.