Effect of SOx from simulated flue gas on growth and gene expression of S-compounds in the microalgae Desmodesmus abundans

Desmodesmus abundans high CO2 acclimated strain (HCA) is a microalga adapted for 13 years to high CO2 atmospheres. Previous studies have shown overexpression of genes associated with purines and folate biosynthesis under model flue gas (250 000 ppm CO2, 700 ppm NO, and 100 ppm SO2). Therefore, it is hypothesized that a high S concentration in the growth medium generates biomass with desirable concentrations of S-containing compounds reflected in the overexpression of key genes involved in the biosynthesis of purines and folate, and other associated pathways such as cysteine, methionine, and S metabolism. D. abundans was cultured with different S concentrations in Erlenmeyer flasks to ¾ log phase (4 d) under 25% v/v CO2/air (continuous flow at 0.05 vvm), 300 mL BG11 medium, 25 ± 2 °C, 100 rpm, and 85 µmol PAR-photons m-2 s-1 of continuous light). Three experimental conditions in triplicate, and duplicated biologically, were tested: i) low S (BG11-3/4S, 7 mg L-1 SO42-), ii) high S (BG11-S+SOx, 100 ppm SO2 continuous supply), and iii) control (BG11, 29.2 mg L-1 SO42-). Gene expression analysis of pathways of interest was done using RNA-Seq. Analysis of differential expression of genes (DEGs) compared S treatments to the control (|Log2FC > 1| padj < 0.05). DEGs were obtained using CDS information of D. abundans HCA and filtered genes were mapped against KEGG pathways. Also, free amino acids were analyzed using a Waters ACQUITY UPLC® System with a fluorescence detector. Neither culture was inhibited by the growth condition (no lag phase) and exponential growth was similar among cultures; however, final biomass production was highest in the control (0.8 ± 0.1 g L-1 d.w.), followed by the high S and low S conditions (0.6 ± 0.06 and 0.51 ± 0.06 g L-1 d.w. respectively). Therefore, the presence of SOx slightly reduced by 19% final biomass production. A similar trend was observed for total protein content with a greater reduction of 38% under high S (56.2 ± 1.6%, 34.6 ± 0.41%, and 26.5 ± 3.7% d.w. for the control, high S, and limited S, respectively). DEGs showed that S-limitation resulted in up-regulation of sulfate assimilation- related genes but did not correlate to a greater quantity of S-containing amino acids and only the serine O-acetyltransferase (SAT) gene was upregulated in the cysteine and methionine pathways under this condition. Although upregulation in the folate biosynthesis pathway could be observed in both conditions, these genes were not related to the interconversion of folate derivatives. Given that gene expression of reactions of interest was not clearly observed through transcriptomic analysis, neither synthesis of S-containing amino acids, implementation of direct approaches based on sequence homology of specific genes and quantification of other S related compounds as folate should be pursue.