Introduction

Algae are the organisms with the ability for photosynthesis, and widely distributed in the waters. In the response to of stresses, reactive oxygen species (ROS) will largely generated, which can be scavenged by antioxidants and antioxidant enzymes, such as ascorbate (AsA) and ascorbate peroxidase (APX; EC 1.11.1.11). The photosynthetic electron transport components will be overreduced upon exposure to high light (HL), and in turn, O2 will be photoreduced through photosystem I (PSI) and photosystem II (PSII) and lead to ROS formation. Accumulated ROS generally causes the oxidization of macromolecules such as lipids, proteins and nucleic acids (Asada 2006). To avoid oxidative stress, ROS such as superoxide anion (O2) are first converted to hydrogen peroxide (H2O2) by superoxide dismutase (SOD; EC 1.15.1.1), and then APX scavenges H2O2 using AsA as the substrate, which can be oxidized to monodehydroascorbate (MDA). MDA is recycled back to AsA via catalysis of monodehydroascorbate reductase (MDAR, EC 1.6.5.4) using NADPH as the reductant (Gill and Tuteja 2010). Therefore, MDAR plays an important role in the response of plants to oxidative stress by maintaining the intracellular ascorbate redox state mainly in the reduced state.

Increase in carbon dioxide (CO2) emissions derived from fossil fuel combustion is one of the main causes of global warming [1]. Microalgae are among the most promising solution for carbon sequestration and also an ideal feedstock for the production of biofuels [2]. Furthermore, microalgae are also capable of biosynthesizing high value products like pigments and omega-3 fatty acids [3].