Microalgae have long been used for the commercial production of natural colorants such as carotenoids and chlorophyll. Due to the rising demand for carotenoids and other natural products from microalgae, strategies to increase production efficiency are urgently needed. The production of microalgal biorefineries has been limited to countries with moderate climates. For countries with cooler climates and less daylight, methodologies for the efficient production of microalgal biorefineries need to be investigated. Algal strains that can be safely consumed as whole cells are also attractive alternatives for developing as carotenoid supplements, which can also contain other compounds with health benefits. Using such strains helps to eliminate the need for hazardous solvents for extraction and several other complicated steps. In this study, the mesophilic green alga Chlamydomonas reinhardtii was employed to study the effects of cold stress on cell physiology and the production of pigments and storage compounds. The results showed that temperatures between 10 and 20 °C induced carotenoid and chlorophyll accumulation in the wild-type strain of C. reinhardtii. Interestingly, the increased level of carotenoids suggested that they might play a crucial role in cold stress acclimation. A temperature of 15 °C resulted in the highest carotenoid and chlorophyll productivity. At this temperature, carotenoid and chlorophyll productivity was 2 times and 1.3 times higher than at 25 °C, respectively. Subjecting a mutant defective in lutein and zeaxanthin accumulation to cold stress revealed that these two carotenoids are not essential for cold stress survival. Therefore, cold temperature could be used as a strategy to induce and increase the productivity of pigments in C. reinhardtii.

Introduction Biodiesel from microalgae is still not available commercially due to the high cost of production. Most commercial cultivation of microalgae utilize open pond systems as they are easy and inexpensive to construct 1 , but one of the disadvantages is uncontrollable light intensity that could lead to oxidative stress and eventually biomass reduction. In outdoor conditions, light intensity gradually increases from sunrise to midday with around 2,000 μmol photons m 2 s 1 and then gradually decreases towards sunset. Furthermore, light intensity is highest around the equator and decreases with increasing distance from the equator to the poles. Light is absolutely essential for algal growth, and different strains require different amounts of light for maximal growth, whereas excessive light is harmful. When excess light is absorbed, damage to the photosynthetic apparatus may be induced, which results in the reduction of photosynthetic capacity and changes in pigment concentrations 2, 3. Exposure of photosynthetic organisms to