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Nannochloropsis metabolism during transition from quiescence to autotrophy.

ABSTRACT: Purpose: The marine microalgae Nannochloropsis oceanica (CCMP1779) is a prolific producer of oil and is considered a viable and sustainable resource for biofuel feedstocks. Nitrogen (N) availability has a strong impact on the physiological status and metabolism of microalgal cells. However the exact nature of this response is poorly understood. To fill this gap we performed transcriptomic profiling combined with cellular and molecular analyses of N. oceanica CCMP1779 during the transition from quiescence to autotrophy. Methods: Nannochloropsis cells (50 ml per sample) were harvested for RNA isolation during nitrogen deprivation (N-) and after nitrogen resupply (NR) at the following time points: 6, 12, 24, 48 and 72 h. For each time point two biological replicates were harvested. RNA was extracted using the E.Z.N.A® Plant RNA kit (OMEGA) according to the manufacturer’s instructions. RNA quality was verified using the Bioanalyzer (Agilent). Single-end, 50 bp nucleotide sequences were acquired for each sample using an Illumina HiSeq 2500 (MSU-Research Technology Support Facility). RNA-Seq reads were trimmed using Trimmomatic (v0.32) with removal of leading and trailing low quality bases (below quality score 10 with a 4-base wide sliding window) as well as TruSeq Single End adaptors. The resulting reads were aligned to the N. oceanica CCMP1779 genome assembly (GSE36959) using STAR (2.3.0e) (Dobin et al., 2013) by only allowing unique mapping, a maximum of 4 mismatches per pair and a maximum intron length of 10,000 bp. The HTSeq (0.6.1) was used to generate a count table for all genes using the exon as the feature type as well as the intersect-nonempty mode so that only reads that were completely aligned to two genes are considered ambiguous. HTSeq count tables were used for differential expression analysis using DEseq2. Differential gene expression was examined using DESeq2. A generalized linear model was fit, and we tested on condition (deprivation or recovery) and the time:condition interaction with log2-fold changes greater than 1. Significant differentially expressed genes with p-values less than 0.01 were identified after Benjamini-Hochberg (BH) multiple testing correction. Differentially expressed genes were clustered based on expression using K-means clustering (k = 4) with the Pearsons correlation distance metric within MeV (4.8.1). Results: We showed that N deprivation-induced quiescence is accompanied by a strong reorganization of the photosynthetic apparatus and changes in the lipid homeostasis leading to accumulation of triacylglycerol (TAG). Cell cycle activation and re-establishment of photosynthetic activity observed in response to resupply of the growth medium with N were accompanied by a rapid degradation of TAG stored in lipid droplets (LDs). Beside observing LD translocation into vacuoles we also provide evidence for direct interaction between the lipid droplet surface protein (NoLDSP) and autophagy-related 8 (NoATG8) protein and show a role of microlipophagy in LD turnover in N. oceanica CCMP1779. Conclusions: This knowledge is crucial not only for understanding the fundamental mechanisms controlling the cellular energy homeostasis in microalgal cells but also for the development of efficient strategies to achieve higher algal biomass and better microalgal lipid productivity.

ORGANISM(S): Nannochloropsis oceanica

PROVIDER: GSE129245 | GEO | 2019/10/01

REPOSITORIES: GEO

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Project description:Limited systems-level understanding of oil synthesis in wild oleaginous algae has hindered the development of microalgal feedstock. Nannochloropsis is a small unicellular microalgae widely distributed in oceans and fresh water. In many large-scale and pilot-scale outdoor cultivation facilities, Nannochloropsis strains have been found to be capable of robust growth when supplied with flue gases, naturally accumulating large quantities of oils in a stationary phase, and exhibiting resistance to environmental contaminants. The rich genomic resources, compact genomes, resistance to foreign DNA invasion, wide ecological adaptation, large collections of natural strains and the demonstrated ability to grow on a large scale suggested Nannochloropsis can serve as research models and platform strains for economical and scalable photosynthetic production of fuels and chemicals. To untangle the intricate genome-wide networks underlying the robust biomass accumulation and oil production in Nannochloropsis, we applied high-throughput mRNA-sequencing and reconstructed the structure and dynamics of the genome-wide functional network underlying robust microalgal triacylglycerol (TAG) production in Nannochloropsis oceanica, by tracking the genome-wide, single-base-resolutiontranscript change for the complete time-courses of nitrogen-depletion-induced TAG synthesis. Nannochloropsis oceanica IMET1 cells were grown in liquid cultures under continuous light (approximately 50 M-BM-5mol photons m-2 s-1) at 25M-bM-^DM-^C and aerated by bubbling with a mixture of 1.5% CO2 in air. Mid-logarithmic phase algal cells were collected and washed three times with axenic seawater. Equal numbers of cells were re-inoculated in nitrogen replete medium (Control condition or C, i.e. N+) and nitrogen deprived medium (N deficiency or N, i.e. N-) with 50M-BM-5mol m-2 s-1 light intensity, respectively. Cell aliquots were collected for RNA isolation after being transferred to the designated conditions for 3h, 4h, 6h, 12h, 24h and 48h. Three biological replicates of algal cultures were established under each of the above M-bM-^@M-^\CM-bM-^@M-^] (i.e. N+) and M-bM-^@M-^\NM-bM-^@M-^] (i.e. N-) conditions, respectively. In total, 36 samples collected at six time points (3h,4h,6h,12h,24h and 48h) were used for mRNA-Seq library preparation and then submitted to Illumina HiSeq 2000 for sequencing.

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Nannochloropsis metabolism during transition from quiescence to autotrophy.

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