Microalgae accumulate a considerable amount of lipids and carbohydrate under nutrient-deficient conditions, which makes them one of the promising sustainable resources for biofuel production. In the present study, to obtain the biomass with higher lipid and carbohydrate contents, we implemented a short-term nitrogen starvation of 1, 2, and 3 days in a green microalga Acutodesmus dimorphus. Few recent reports suggest that oxidative stress-tolerant microalgae are highly efficient for biofuel production. To study the role of oxidative stress due to nitrogen deficiency, responses of various stress biomarkers like reactive oxygen species (ROS), cellular enzymatic antioxidants superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX), and non-enzymatic scavengers proline and polyphenols were also evaluated. Further, the endogenous levels of phytohormones abscisic acid (ABA) and indole-3-acetic acid (IAA) were also determined to study their response to nitrogen deficiency. We observed that nitrogen starvation of 2 days is effective to produce biomass containing 29.92% of lipid (comprising about 75% of neutral lipid) and 34.80% of carbohydrate, which is significantly higher (about 23 and 64%, respectively) than that of the control culture. Among all nitrogen-starved cultures, the accumulations of ROS were lower in 2 days starved culture, which can be linked with the several folds higher activities of SOD and CAT in this culture. The accumulations of proline and total polyphenols were also significantly higher (about 4.7- and 1.7-folds, respectively, than that of the control) in 2 days nitrogen-starved culture. The levels of phytohormones once decreased significantly after 1 day, increased continuously up to 3 days of nitrogen starvation. The findings of the present study highlight the interaction of nitrogen starvation-induced oxidative stress with the signaling involved in the growth and development of microalga. The study presents a comprehensive picture of the adaptive mechanisms of the cells from a physiological perspective along with providing the strategy to improve the biofuel potential of A. dimorphus through a short-term nitrogen starvation.

Microalgae are considered one of the best resources for the production of biofuels and industrially important compounds. Various models have been developed to understand the fundamental mechanism underlying the accumulation of triacylglycerols (TAGs)/starch and to enhance its content in cells. Among various algae, the red alga Cyanidioschyzonmerolae has been considered an excellent model system to understand the fundamental mechanisms behind the accumulation of TAG/starch in the microalga, as it has a smaller genome size and various biotechnological methods are available for it. Furthermore, C. merolae can grow and survive under high temperature (40 °C) and low pH (2–3) conditions, where most other organisms would die, thus making it a choice alga for large-scale production. Investigations using this alga has revealed that the target of rapamycin (TOR) kinase is involved in the accumulation of carbon-reserved molecules, TAGs, and starch. Furthermore, detailed molecular mechanisms of the role of TOR in controlling the accumulation of TAGs and starch were uncovered via omics analyses. Based on these findings, genetic engineering of the key gene and proteins resulted in a drastic increment of the amount of TAGs and starch. In addition to these studies, other trials that attempted to achieve the TAG increment in C. merolae have been summarized in this article.