Metabolic and Gene Expression Changes Triggered by Nitrogen Deprivation in the Photoautotrophically Grown Microalgae Chlamydomonas reinhardtii and Coccomyxa sp. C-169

Authors

Joseph Msanne, University of Nebraska-LincolnFollow
Di Xu, University of Nebraska-LincolnFollow
Anji Reddy Konda, University of Nebraska-LincolnFollow
J. Armando Casas-Mollano, University of Nebraska-LincolnFollow
Tala Awada, University of Nebraska-LincolnFollow
Edgar B. Cahoon, University of Nebraska-LincolnFollow
Heriberto Cerutti, University of Nebraska-LincolnFollow

Date of this Version

2012

Citation

Phytochemistry 75 (2012), pp. 50–59.

doi: 10.1016/j.phytochem.2011.12.007

Comments

Copyright © 2012 Elsevier. Used by permission.

Abstract

Microalgae are emerging as suitable feedstocks for renewable biofuel production. Characterizing the metabolic pathways involved in the biosynthesis of energy-rich compounds, such as lipids and carbohydrates, and the environmental factors influencing their accumulation is necessary to realize the full potential of these organisms as energy resources. The model green alga Chlamydomonas reinhardtii accumulates significant amounts of triacylglycerols (TAGs) under nitrogen starvation or salt stress in medium containing acetate. However, since cultivation of microalgae for biofuel production may need to rely on sunlight as the main source of energy for biomass synthesis, metabolic and gene expression changes occurring in Chlamydomonas and Coccomyxa subjected to nitrogen deprivation were examined under strictly photoautotrophic conditions. Interestingly, nutrient depletion triggered a similar pattern of early synthesis of starch followed by substantial TAG accumulation in both of these fairly divergent green microalgae. A marked decrease in chlorophyll and protein contents was also observed, including reduction in ribosomal polypeptides and some key enzymes for CO₂ assimilation like ribulose-1,5-bisphosphate carboxylase/oxygenase. These results suggest that turnover of nitrogen-rich compounds such as proteins may provide carbon/energy for TAG biosynthesis in the nutrient deprived cells. In Chlamydomonas, several genes coding for diacylglycerol:acyl-CoA acyltransferases, catalyzing the acylation of diacylglycerol to TAG, displayed increased transcript abundance under nitrogen depletion but, counterintuitively, genes encoding enzymes for de novo fatty acid synthesis, such as 3-ketoacyl-ACP synthase I, were down-regulated. Understanding the interdependence of these anabolic and catabolic processes and their regulation may allow the engineering of algal strains with improved capacity to convert their biomass into useful biofuel precursors.