The Regulation of Photosynthetic Structure and Function during Nitrogen Deprivation in Chlamydomonas reinhardtii    

Abstract

The accumulation of carbon storage compounds by many unicellular algae after nutrient deprivation occurs despite declines in their photosynthetic apparatus. To understand the regulation and roles of photosynthesis during this potentially bioenergetically valuable process, we analyzed photosynthetic structure and function after nitrogen deprivation in the model alga Chlamydomonas reinhardtii. Transcriptomic, proteomic, metabolite, and lipid profiling and microscopic time course data were combined with multiple measures of photosynthetic function. Levels of transcripts and proteins of photosystems I and II and most antenna genes fell with differing trajectories; thylakoid membrane lipid levels decreased, while their proportions remained similar and thylakoid membrane organization appeared to be preserved. Cellular chlorophyll (Chl) content decreased more than 2-fold within 24 h, and we conclude from transcript protein and 13 C labeling rates that Chl synthesis was down-regulated both pre-and posttranslationally and that Chl levels fell because of a rapid cessation in synthesis and dilution by cellular growth rather than because of degradation. Photosynthetically driven oxygen production and the efficiency of photosystem II as well as P700 + reduction and electrochromic shift kinetics all decreased over the time course, without evidence of substantial energy overflow. The results also indicate that linear electron flow fell approximately 15% more than cyclic flow over the first 24 h. Comparing Calvin-Benson cycle transcript and enzyme levels with changes in photosynthetic 13 CO 2 incorporation rates also pointed to a coordinated multilevel down-regulation of photosynthetic fluxes during starch synthesis before the induction of high triacylglycerol accumulation rates.

Carbon and Acyl Chain Flux during Stress-induced Triglyceride Accumulation by Stable Isotopic Labeling of the Polar Microalga Coccomyxa subellipsoidea C169

“…6C). This second-phase decrease in the FA synthetic rate was linear from days 2 through 14 of nitrogen starvation (5% per day (R 2 ϭ 0.99)) and coincided with previously published reductions in both chlorophyll and MGDG content, which lead to a diminished photosynthetic capacity (22,34,42). Following the readdition of nitrogen on day 14, de novo FA synthesis was not detectable by deuterium labeling on days 14 -16 but returned to the pre-starvation rates within 4 days of nitrogen readdition (Fig.…”

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

“…Shortly after NO 3 2 is introduced to NR-KO14 cells, expression levels of key functional and metabolic gene clusters indirectly related to N-assimilation are sharply downregulated. Despite ample extra-and intracellular NO 3 2 , transcript profiles for these clusters respond similarly to those observed in wild-type cells under N-stress (Alipanah et al 2015;Bender et al, 2014;Juergens et al, 2015;Levitan et al, 2015;Schmollinger at al., 2014). However, in contrast to wild-type cells which gradually acclimate to a dwindling supply of N-product, the response in NR-KO14 cells is quite rapid.…”

“…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). N deprivation has been reported to affect photosynthetic output in P. tricornutum and other microalgae through a process that results in a gradual, managed deconstruction of the photosynthetic apparatus (Juergens et al, 2015;Levitan, et al, 2015;Simionato et al, 2013). Here, the immediate transcriptomic response of NR-KO14 cells presumably interrupts production of ATP and NAPDH, causing FA biosynthesis and FA desaturation to shut down.…”

Optimizing light regimes on growth and lipid accumulation in Ankistrodesmus fusiformis H1 for biodiesel production

“…Therefore, they were chosen to determine the optimal nitrogen source for the culture of A. fusiformis H1 in current study. In a range of microalge, nitrogen deprivation stimulated high accumulation of carbohydrate and TAG but decreased the carbon assimilates (Li et al, 2011;Msanne et al, 2012;Juergens et al, 2015). Inspired by the above-mentioned literatures, obtaining the optimal nitrogen concentration which combine growth and lipid accumulation together is of great importance in microalge cultivation.…”