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The use of CO2 rich industrial flue gases to support cultivation of microalgae to produce lipids for biofuel and other applications is an increasingly researched option. However, this approach presents a challenge, as whilst flue gasses typically contain 6-10% CO2, excessive medium acidification can be caused by the presence of NOx and SO2. The use of acidophilic or acid-tolerant species is a possible solution, but little is known about these microalgae. In this study we investigated the growth of a bioprospected acid-tolerant mixed photosynthetic green microalgae culture (91% dominated by a single Coccomyxa sp. taxon) at pH 2.5 and fed with a simulated flue gas containing 6% CO2 and 94% N2. At the end of the exponential growth phase, lipid accumulation and profiles, and the elemental composition of biomass were analysed over one week during which biomass was exposed to either continued light-dark cycle conditions or continual dark conditions. After three days of dark stress, the biomass consisted of approximately 28% of lipids, which was 42% higher than at the end of the exponential phase and 55% higher than the maximum lipid content achieved under light/dark conditions. Oleic acid (C18:1), pentadecanoic acid (C15:0), and palmitic acid (C16:0) were the dominant fatty acids at the end of the exponential phase, and light-dark and dark-treated biomass, respectively. Dark stress conditions favoured polyunsaturated fatty acid production and showed an increase in nitrogen content. This suggests that the use of dark stress to stimulate production of desirable lipids is a no-cost alternative to other commonly used stressors.

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