Investigating the effects of surfactants on Chlorella pyrenoidosa in the hydrolysis acidification liquor by integrated physiochemical and transcriptome analyses

In our two recently published articles, we investigated the effects of Tween 80, sodium lignosulfonate (LS), sodium dodecyl sulfate (SDS), and rhamnolipid (RL) on the co-conversion of endogenous carbon (C) and nitrogen (N) of dairy wastewater in the hydrolytic acidification coupled microalgae culture system. Although the effects of surfactants on the microalgae (Chlorella pyrenoidosa) growth from the perspective of enhancing the accumulation of the volatile fatty acids (VFAs) were investigated, the question regarding the effect of surfactants themselves on microalgae growth and mechanism remains to be addressed. In this work, we extend our discussion to clearly demonstrate the effects of these four surfactants on microalgae growth, nutrient removal, and the output of the culture system in artificial hydrolysis-acidification liquor (HAL). Furthermore, de novo transcriptome profiling and the metabolic pathway analysis of C. pyrenoidosa cells in response to these four surfactants were also per-formed. The results showed that SDS inhibited while the other three surfactants promoted microalgae growth (promotion order of dry weight: Tween 80 > RL > LS). The maximum ammonia nitrogen removal rate followed Tween 80 80.90 % > RL 78.32 % > LS 75.23 % > control 67.08 % > SDS 34.74 %. Surfactants enhanced the C assimilation followed Tween 80 > RL approximate to LS > control. Tween 80, RL, and LS treatment groups achieved higher yield of protein, carbohydrates, and lipid. The differential transcriptome analysis revealed that the number of down-regulated differential expression of unigenes (DEUs) more than that of up-regulated in SDS group, while the opposite was true for the other 3 surfactants. KEGG enrichment analysis revealed that Tween 80 mainly affects lipid metabolism in the early stage of microalgae growth, RL mainly affects lipid metabolism and membrane transport, SDS mainly affects energy metabolism, lipid metabolism, and transport and catabolism, while no DEUs significant enriched KEGG in LS group.