Abstract
Microalgae can be cultivated on waste dark fermentation effluents containing volatile fatty acids (VFA) such as acetate or butyrate. These VFA can however inhibit microalgae growth at concentrations above 0.5-1 gC.L–1. This study used the model strain Chlorella sorokiniana to investigate the effects of acetate or butyrate concentration on biomass growth rates and yields alongside C:N:P ratios and pH control. Decreasing undissociated acid levels by raising the initial pH to 8.0 allowed growth without inhibition up to 5 gC.L–1 VFAs. However, VFA concentration strongly affected biomass yields irrespective of pH control or C:N:P ratios. Biomass yields on 1.0 gC.L–1 acetate were around 1.3-1.5 gC.gC–1 but decreased by 26-48% when increasing initial acetate to 2.0 gC.L–1. This was also observed for butyrate with yields decreasing up to 25%. This decrease in yield in suggested to be due to the prevalence of heterotrophic metabolism at high organic acid concentration, which reduced the amount of carbon fixed by autotrophy. Finally, the effects of C:N:P on biomass, lipids and carbohydrates production dynamics were assessed using a mixture of both substrates. In nutrient replete conditions, C. sorokiniana accumulated up to 20.5% carbohydrates and 16.4% lipids while nutrient limitation triggered carbohydrates accumulation up to 45.3%.
Highlights
Dark fermentation (DF) has gained interest over the past 20 years since it enables waste treatment alongside hydrogen (H2) production, which is projected to be a sustainable vector for the transportation sector (Hosseini and Wahid, 2016)
These results show that C. sorokiniana can consume VFAs entirely as long as pH is controlled and nutrients provided in adequate amounts
This study aimed at providing insights in C. sorokiniana physiology in presence of acetate and butyrate, the two main components of DF effluent (DFE)
Summary
Dark fermentation (DF) has gained interest over the past 20 years since it enables waste treatment alongside hydrogen (H2) production, which is projected to be a sustainable vector for the transportation sector (Hosseini and Wahid, 2016). Mixotrophic growth occurs when microalgae grow by simultaneously using inorganic carbon (CO2) using photosynthesis as well as organic carbon sources (Chew et al, 2018) This cultivation mode allows increasing productivity compared to pure autotrophy thanks to the heterotrophic metabolism as well as reduction in CO2 emission compared to pure heterotrophy thanks to photosynthesis (Smith et al, 2015).The coupling is limited partly by the low VFA concentration that microalgae can tolerate. An alternative is to increase the proportion of acetate in DFE to promote microalgae growth (Fei et al, 2015; Baroukh et al, 2017), resulting in an acetate:butyrate (A:B) mass ratio equal or above 1 This composition is unrepresentative of an average DFE composition (Moscoviz et al, 2018). Gathering knowledge about the effect of higher butyrate concentration on algal physiology in axenic mixotrophic condition remains necessary to better understand microalgae behavior in real processes
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