Time-resolved transcriptome analysis of Scenedesmus obliquus HTB1 under 10% CO2 condition

Abstract

Certain microalgal species can grow under high CO₂ concentrations providing potential for mitigating CO₂ pollution in flue gas produced by power plants. Microalga Scenedesmus obliquus strain HTB1 was isolated from the Chesapeake Bay and grow rapidly in a high level of CO₂ . However, little is known about the molecular responses of HTB1 to high CO₂ levels. Here, we investigated how HTB1 responds to 10% CO₂ using the time-resolved transcriptome analysis. Gene expression profiles indicated that HTB1 responds quickly (in 2 h) and becomes adaptive within 12 h when exposed to 10% CO₂ . Interestingly, certain genes of light-harvesting, chlorophyll synthesis and carbon fixation (i.e. rbcS) were up-regulated at 10% CO₂ , and these functional responses are consistent with the increased photosynthesis efficiency and algal biomass under 10% CO₂ . Nitrate assimilation was strongly enhanced, with amino acid biosynthesis and aminoacyl tRNA biosynthesis genes being markedly up-regulated, indicating that HTB1 actively takes up nitrogen and accelerates protein synthesis at 10% CO₂ . Carbon metabolism including fatty acid biosynthesis and TCA cycle was enhanced at 10% CO₂ , supporting the earlier observation of increased lipid content of Scenedesmus sp. under high CO₂ levels. Interestingly, key genes like RuBisCO (rbcL) and carbonic anhydrase in carboxysomes did not respond actively to 10% CO₂ , implying that exposure to 10% CO₂ has little impact on the carbon concentrating mechanisms and CO₂ fixation of the Calvin cycle. It appears that HTB1 can grow rapidly at 10% CO₂ without significant metabolic changes in carbon fixation and ATP synthesis.

FIGURE 1

Physiological responses of HTB1 under air and 10% CO₂ conditions. (A) Growth curves; (B) pH trends; (C) Fv/Fm; (D) Cellular ATP content. Data points represent means and error bars are standard deviation (SD), n = 3 (triplicates cultures); * indicates significant differences at 0.05 level; ** indicates significant differences at 0.01 level. Samples for transcriptomic analysis were collected at 2, 12, 24 h and day 4

FIGURE 2

Differential gene expression profile at varied time points. (A) Venn diagram of DEGs at multiple time points. (B) Significant KEGG pathway enrichment of DEGs at multiple points (Q‐value <0.05). Circle colour strength represents Q‐value. ‘Rich ratio’ is the ratio of the DEGs number to the number of genes in the specific pathway

FIGURE 3

DEGs involved in photosynthesis and carbon fixation at varied time points. CA, intracellular carbonic anhydrase; Cytb₆f, cytochrome b 6 f complex; ECA, extracellular carbonic anhydrase; ETC, electron transport chain; Fd, ferredoxin; FNR, ferredoxin–NADP⁺ reductase; Lhca, light‐harvesting complex I chlorophyll a/b binding protein; Lhcb, light‐harvesting complex II chlorophyll a/b binding protein; OEC, oxygen‐evolving complex; PC, plastocyanin; PQ, plastoquinone; PS I, photosystem I; PS II, photosystem II; rbcS, ribulose‐1,5‐bisphosphate carboxylase/oxygenase small chain; α, β, γ, δ, ε, a, b, c, F‐type H+‐transporting ATPase subunit α,β, γ, δ, ε, a, b, c

FIGURE 4

Schematic representation of biological pathway in HTB1 at varied time points. ETC, electron transport chain; Lhca, light‐harvesting complex I chlorophyll a/b binding protein; Lhcb, light‐harvesting complex II chlorophyll a/b binding protein; PS I, photosystem I; PS II, photosystem II; rbcS, ribulose‐1,5‐bisphosphate carboxylase/oxygenase small chain; I, NADH dehydrogenase; II, succinate dehydrogenase; III, cytochrome c reductase; IV, cytochrome c oxidase; glycolysis, hexokinase