Membrane Glycerolipid Remodeling Triggered by Nitrogen and Phosphorus Starvation inPhaeodactylum tricornutum

Abstract

Diatoms constitute a major phylum of phytoplankton biodiversity in ocean water and freshwater ecosystems. They are known to respond to some chemical variations of the environment by the accumulation of triacylglycerol, but the relative changes occurring in membrane glycerolipids have not yet been studied. Our goal was first to define a reference for the glycerolipidome of the marine model diatom Phaeodactylum tricornutum, a necessary prerequisite to characterize and dissect the lipid metabolic routes that are orchestrated and regulated to build up each subcellular membrane compartment. By combining multiple analytical techniques, we determined the glycerolipid profile of P. tricornutum grown with various levels of nitrogen or phosphorus supplies. In different P. tricornutum accessions collected worldwide, a deprivation of either nutrient triggered an accumulation of triacylglycerol, but with different time scales and magnitudes. We investigated in depth the effect of nutrient starvation on the Pt1 strain (Culture Collection of Algae and Protozoa no. 1055/3). Nitrogen deprivation was the more severe stress, triggering thylakoid senescence and growth arrest. By contrast, phosphorus deprivation induced a stepwise adaptive response. The time scale of the glycerolipidome changes and the comparison with large-scale transcriptome studies were consistent with an exhaustion of unknown primary phosphorusstorage molecules (possibly polyphosphate) and a transcriptional control of some genes coding for specific lipid synthesis enzymes. We propose that phospholipids are secondary phosphorus-storage molecules broken down upon phosphorus deprivation, while nonphosphorus lipids are synthesized consistently with a phosphatidylglycerol-to-sulfolipid and a phosphatidycholine-to-betaine lipid replacement followed by a late accumulation of triacylglycerol.

Nitrate Reductase Knockout Uncouples Nitrate Transport from Nitrate Assimilation and Drives Repartitioning of Carbon Flux in a Model Pennate Diatom

“…Differences in accumulation of FAME were significant for the two genotypes through 60 h as determined by a two-way ANOVA (Supplemental File 1). FAME composition in both wild-type and NR-KO14 cells was dominated by C16:0, C16:1, and C20:5 FAs (Supplemental Figures 7A to 7E), consistent with previous reports for diatom FAME composition (Abida et al, 2015;Shen et al, 2016;Yu et al, 2009). As previous studies have established (Bender et al, 2014;Hockin et al 2012;Li et al, 2014;Rismani-Yazdi et al, 2012;Shifrin and Chisholm, 1981;Yang et al 2013;Yoon et al, 2012), N-limited conditions in photosynthetic microalgae induce accumulation of lipids, predominantly TAG, which is stored in lipid droplets.…”

“…By 36 h, with available NO 3 2 assimilated, wild-type cells redirect carbon resources to TAG biosynthesis ( Figures 9B and 9C), a well described phenomenon for various microalgae. Conversely, in NR-KO14, the NO 3 2 addition elicits the signals for N-stress, which results in dramatic increases in transcript abundance for ;35% of the cluster (Supplemental Figure 7I) and drives initiation of biosynthesis of FAs C16:0, C16:1, the two major components of TAG in P. tricornutum (Supplemental Figures 7A, 7C, and 7E) (Abida et al, 2015;Shen et al, 2016). In the wild type, TAG accumulates through the end of the experiment, but in NR-KO14 cells, C16 FA accumulation and total lipid content subsides by 36 h. We surmise that the end of TAG biosynthesis in NR-KO14 cells is directly related to rapid downregulation of transcription at 1 h of the genes for the major photosynthesis complexes, specifically PET (Supplemental Figure 9A).…”

Transcript level coordination of carbon pathways during silicon starvation‐induced lipid accumulation in the diatom Thalassiosira pseudonana

“…Nutrient stress in diatoms induces changes in transcript abundances and can cause dramatic shifts in cellular health and physiology (Allen et al ., 2008; Mock et al ., 2008; Dyhrman et al ., 2012; Hockin et al ., 2012; Bender et al ., 2014; Abida et al ., 2015). Studies aimed at elucidating changes in transcript abundance during nutrient‐limited lipid accumulation often use nitrogen as the limiting nutrient, which is a stressful condition and induces massive cellular changes such as chlorosis and an associated down‐regulation of photosynthesis (Sun et al ., 2013; Abida et al ., 2015; Levitan et al ., 2015).…”

“…Studies aimed at elucidating changes in transcript abundance during nutrient‐limited lipid accumulation often use nitrogen as the limiting nutrient, which is a stressful condition and induces massive cellular changes such as chlorosis and an associated down‐regulation of photosynthesis (Sun et al ., 2013; Abida et al ., 2015; Levitan et al ., 2015). In contrast, short‐term (24 h) silicon starvation has been used as a method to synchronize diatom cultures, and is not generally considered a stressful condition (Coombs et al ., 1967; Hildebrand et al ., 2007; Shrestha et al ., 2012).…”

“…This is in strong contrast to the response of nitrogen limitation in which many photosynthetic taxa become chlorotic (Abida et al ., 2015). Here, we show chloroplasts also became more heavily pigmented, the absorption cross‐section ( σ PSII ) decreased, and pigment composition shifted towards a higher proportion of xanthophyll pigments.…”