Nitrogen starvation is known to cause drastic alterations in physiology and metabolism leading to the accumulation of lipid bodies in many microalgae, and it thus presents an important alternative for
biofuel production. However, despite the importance of this process, the molecular mechanisms that mediate the metabolic remodeling induced by N
starvation and especially by stress recovery are still poorly understood, and new candidates for bioengineering are needed to make this process useful for
biofuel production.
RESULTS: We have studied the molecular changes involved in the adaptive mechanisms to N
starvation and full recovery of the vegetative cells in the microalga Chlamydomonas reinhardtii during a four-day time course. High throughput mass spectrometry was employed to integrate the
proteome and the metabolome with physiological changes. N
starvation led to an accumulation of oil bodies and reduced Fv/Fm.. Distinct
enzymes potentially participating in the
carbon-concentrating mechanism (CAH7, CAH8, PEPC1) are strongly accumulated. The membrane composition is changed, as indicated by quantitative
lipid profiles. A reprogramming of protein biosynthesis was observed by increased levels of cytosolic ribosomes, while chloroplastidic were dramatically reduced. Readdition of N led to, the identification of early responsive
proteins mediating stress recovery, indicating their key role in regaining and sustaining normal vegetative growth. Analysis of the data with multivariate correlation analysis, Granger causality, and sparse partial least square (sPLS) provided a functional network perspective of the molecular processes. Cell growth and N metabolism were clearly linked by the
branched chain amino acids, suggesting an important role in this stress.
Lipid accumulation was also tightly correlated to the COP II
protein, involved in vesicle and lysosome coating, and a major lipid droplet
protein. This
protein, together with other key
proteins mediating signal transduction and adaption (BRI1, snRKs), constitute a series of new metabolic and regulatory targets.
CONCLUSIONS: This work not only provides new insights and corrects previous models by analyzing a complex dataset, but also increases our biochemical understanding of the adaptive mechanisms to N
starvation in Chlamydomonas, pointing to new bioengineering targets for increased
lipid accumulation, a key step for a sustainable and profitable microalgae-based
biofuel production.