Comparative transcriptome analyses of oleaginous Botryococcus braunii race A reveal significant differences in gene expression upon cobalt enrichment

Abstract

BackgroundBotryococcus braunii is known for its high hydrocarbon content, thus making it a strong candidate feedstock for biofuel production. Previous study has revealed that a high cobalt concentration can promote hydrocarbon synthesis and it has little effect on growth of B. braunii cells. However, mechanisms beyond the cobalt enrichment remain unknown. This study seeks to explore the physiological and transcriptional response and the metabolic pathways involved in cobalt-induced hydrocarbon synthesis in algae cells.ResultsGrowth curves were similar at either normal or high cobalt concentration (4.5 mg/L), suggesting the absence of obvious deleterious effects on growth introduced by cobalt. Photosynthesis indicators (decline in Fv/Fm ratio and chlorophyll content) and reactive oxygen species parameters revealed an increase in physiological stress in the high cobalt concentration. Moreover, cobalt enrichment treatment resulted in higher crude hydrocarbon content (51.3% on day 8) compared with the control (43.4% on day 8) throughout the experiment (with 18.2% improvement finally). Through the de novo assembly and functional annotation of the B. braunii race A SAG 807-1 transcriptome, we retrieved 196,276 non-redundant unigenes with an average length of 1086 bp. Of the assembled unigenes, 89,654 (45.7%), 42,209 (21.5%), and 32,318 (16.5%) were found to be associated with at least one KOG, GO, or KEGG ortholog function. In the early treatment (day 2), the most strongly upregulated genes were those involved in the fatty acid biosynthesis and metabolism and oxidative phosphorylation, whereas the most downregulated genes were those involved in carbohydrate metabolism and photosynthesis. Genes that produce terpenoid liquid hydrocarbons were also well identified and annotated, and 21 (or 29.2%) were differentially expressed along the cobalt treatment.ConclusionsBotryococcus braunii SAG 807-1 can tolerate high cobalt concentration and benefit from hydrocarbon accumulation. The time-course expression profiles for fatty acid biosynthesis, metabolism, and TAG assembly were obtained through different approaches but had equally satisfactory results with the redirection of free long-chain fatty acid and VLCFA away from TAG assembly and oxidation. These molecules served as precursors and backbone supply for the fatty acid-derived hydrocarbon accumulation. These findings provide a foundation for exploiting the regulation mechanisms in B. braunii race A for improved photosynthetic production of hydrocarbons.Electronic supplementary materialThe online version of this article (10.1186/s13068-018-1331-5) contains supplementary material, which is available to authorized users.

A bioprocess engineering approach for the production of hydrocarbons and fatty acids from green microalga under high cobalt concentration as the feedstock of high-grade biofuels

“…Excess cobalt can be harmful to cells, resulting in diminished growth and lipids accumulation. Furthermore, the response of microalgae to cobalt stress varies by strain, with certain strains showing no substantial increase in hydrocarbon or lipid synthesis under cobalt stress conditions [66]. While cobalt stress can accelerate the synthesis of hydrocarbons and lipids in specific microalgae strains, cobalt levels must be carefully In this study, we investigated the effect of different concentrations of cobalt nitrate (0.49, 1, 5, 10, and 50 mg/L) on the production of biomass, lipids, and hydrocarbons by B. braunii cultivated in BBM 3N within a mini-photobioreactor.…”

“…The transcriptome of B. braunii over four time points during the cobalt treatment, as proposed by Cheng et al, showed that the most strongly expressed genes under high concentration were those related to fatty acid biosynthesis and metabolism, as well as oxidative phosphorylation; conversely, the most downregulated genes were related to carbohydrate metabolism, photosynthesis, and amino acid metabolism. The global gene transcript patterns indicated that different phenotypes under cobalt enrichment are likely caused by different expression dynamics [66].…”

“…Expression profiles of genes involved in fat metabolisms lipids synthesis, and TAG assembly indicated a redirection of free long-chain fatty acids and very longchain fatty acids away from TAG assembly and oxidation. As a replacement, they functioned as precursors and the foundation for fatty acid-derived hydrocarbons [66]. Moreover, transcriptome analysis revealed the presence of prokaryotic pathways, along with 17 unigenes known as fatty acid synthase (animal type, EC 2.…”

“…This shows that fatty acid elongation can occur in the cytosol as well as the chloroplasts. Previous studies suggested an alternate TAG synthesis mechanism in yeast, plants, and green microalgae, involving phospholipid:diacylglycerol acyltransferase (PDAT) and the use of phospholipids as donors [66]. Interestingly, the SAG 807-1 transcriptome contained nine PDAT unigenes [66].…”