Integrated Quantitative Analysis of Nitrogen Stress Response in Chlamydomonas reinhardtii Using Metabolite and Protein Profiling

Nitrogen starvation induces a global
stress response in microalgae
that results in the accumulation of lipids as a potential source of
biofuel. Using GC-MS-based metabolite and iTRAQ-labeled protein profiling,
we examined and correlated the metabolic and proteomic response of Chlamydomonas reinhardtii under nitrogen stress.
Key amino acids and metabolites involved in nitrogen sparing pathways,
methyl group transfer reactions, and energy production were decreased
in abundance, whereas certain fatty acids, citric acid, methionine,
citramalic acid, triethanolamine, nicotianamine, trehalose, and sorbitol
were increased in abundance. Proteins involved in nitrogen assimilation,
amino acid metabolism, oxidative phosphorylation, glycolysis, TCA
cycle, starch, and lipid metabolism were elevated compared with nonstressed
cultures. In contrast, the enzymes of the glyoxylate cycle, one carbon
metabolism, pentose phosphate pathway, the Calvin cycle, photosynthetic
and light harvesting complex, and ribosomes were reduced. A noteworthy
observation was that citrate accumulated during nitrogen stress coordinate
with alterations in the enzymes that produce or utilize this metabolite,
demonstrating the value of comparing protein and metabolite profiles
to understand complex patterns of metabolic flow. Thus, the current
study provides unique insight into the global metabolic adjustments
leading to lipid storage during N starvation for application toward
advanced biofuel production technologies.