Differential Transcriptome and Lipidome Analysis of the Microalga Desmodesmus abundans Under a Continuous Flow of Model Cement Flue Gas in a Photobioreactor

Abstract

Microalgae represent a potential strategy for flue gas mitigation as they capture CO2 at high rates. Flue gases can also supply certain nutrients and, as a result, it can be valorized through biomass conversion into value-added compounds. The objective of this study was to characterize growth and analyze transcriptome, lipidome and cellular structure and composition of Desmodesmus abundans under continuous flow of cement model flue gas (MFG) in a 1 L photobioreactor using two strains adapted for nine years to atmospheres of 50% CO2 and air, referred to as HCA (high CO2 acclimated) and LCA (low CO2 acclimated), respectively. Controls with the LCA strain were also evaluated in air, CO2 and CO2+cement kiln dust (CKD). Higher initial growth rates were observed with strain HCA; however, at the end of the run (5 days) similar biomass productivity was reached by the two strains (0.30-0.34 g d.w. L-1 d-1). As expected, the CO2 control presented the highest growth rate (1.7-fold higher than under MFG), and when CKD was incorporated a slightly decreased (14 %) in growth was observed. Transcriptome analysis by RNA-seq, performed at day 4, resulted in a de novo assembly of 70 458 contigs with a N50 of 1 677 bp. Strain comparison under MFG resulted in 16 435 up-regulated and 4 219 down-regulated genes for strain HCA. Most of these genes were related with nucleotide, amino acid and carbon metabolisms; specifically, C3 and C4 cycle, glycolysis and gluconeogenesis, and TCA cycle, where almost all the contigs were up-regulated. In accordance, cell component GO terms up-regulated were in cell wall, chloroplast and photosystems. Likewise, starch and TAG metabolism were up-regulated. Cell structure analysis by SEM and TEM showed that most cells of both strains under MFG were unicellular contrary to typical Desmodesmus morphology; under air, some cells still preserved a grouping morphology. Strains cell size under MFG was similar (17-37 μm2), while under air cells were significant smaller (7-13 μm2). Both strains under MFG possessed high content of starch granules, a disorganized chloroplast and several lipid bodies, while a thicker cell wall was only observed in strain HCA. Biomass composition at the end of the run (day 5) showed no differences in proximate analysis between strains under MFG. A 1.8 to 2-fold higher protein content in strain LCA was found in complete medium (BG-11) than under MFG (BG-11-N-S). Under MFG, LCA presented the highest starch content (47.2 ± 22.3 % d.w) followed by HCA (23.1 ± 4.5 % d.w). On the contrary, HCA showed a higher content of pigments compared to LCA but the highest values were found in the control with only CO2. Lipidome analysis resulted in 663 detected features. Under MFG no many differences were found between strains by day 5; however, clear differences were observed at day 4 when both strains were in exponential growth. Particularly, 12 glycerolipids (GL) and 18 glycerophospholipids (GP) increased, and 27 GL and 3 GP decreased in HCA compared LCA. Still, most differences were found when strain LCA under MFG was compared with CO2+CKD (incomplete vs complete culture medium) that showed changes in GL (42 increased and 27 decreased) and GP (58 increased and 42 decreased), possibly attributed to low N in MFG. The results presented in this study show significant differences between strains HCA and LCA under MFG. However, most differences were observed at the transcriptome level (day 4), while biomass production was comparable at the end of the experimental period (day 5). Morphological changes appeared to be induced by the high CO2 condition at the moment of growth, with no significant differences between acclimated strains, except for a thicker cell wall in the HCA strain. Overall, both strains presented a high content of starch that represents a high value compound under MFG. Further studies could contemplate continuous cultures under MFG with longer experimental periods (>5 days) to validate differences between strains. Also, explore differences between strains at the genome level such as synonymous mutation rates by sequencing and studies of epigenetic changes. Additionally, metabolome and proteome analysis to better understand differences under the different control conditions.