**1. Introduction**

The unicellular green alga *Chlorella* is a popular nutraceutical that is produced industrially in Taiwan. *Chlorella* requires a moderate climate, including ample sunshine and high temperatures of about 25–38°C for optimal large-scale outdoor growth. However, in winter, the temperature can range from 4 to 15°C, which is unsuitable for algal growth. In order to maintain productivity, it would be helpful to understand how green algae overcome chilling temperatures and a mimicking high irradiance resulted from chilling temperature [1–3].

Photosynthesis is the energy source for the growth and development of photosynthetic organisms. Photosynthetic efficiency is reliant on environmental conditions such as light and temperature. At low temperatures, algae experience reduced photosynthetic efficiency, whereas in high-light environments, they absorb more energy than they can consume in the photosynthetic processes [4]. The absorption of too much energy can lead to an increase in the production of reactive

oxygen species (ROS), which can damage the photosynthetic apparatus and further decrease photosynthetic efficiency [5]. Therefore, in response to wide daily and seasonal fluctuations in temperature and light, algae must possess some protective and regulatory systems to avoid this "energy excess" [6–9].

Upon initial exposure to low temperature or high irradiation, excessive excitation pressure may be induced between the rate of energy absorbed via the photosynthetic antenna and energy utilization [4, 5, 10–12]. One of the protection mechanisms that algae and higher plants employ to avoid receiving too much light energy is to adjust their chlorophyll (Chl) a/b ratios and the structure of the photosystem I and II (PSI and PSII) antenna complexes in response to different combinations of light intensity and temperature [2, 13–15]. Light-harvesting complexes (LHCs) with modified Chl composition have the ability to absorb different levels of light energy depending on the environmental conditions [16–18]. Another protective mechanism of algae and plants after receiving too much light energy is to adjust the antioxidant response of the scavenging system such that any excess excitation pressure is transferred to the superoxide radical (O2· <sup>−</sup>) pathway and other derived reactive oxygen species (ROS) [19]. Superoxide dismutase (SOD, EC 1.15.1.1) is known as the first line of cellular defense against oxidative stress, and it catalyzes the dismutation of O2· <sup>−</sup> to H2O2 and O2. There are three distinct types of SOD classified on the basis of their metal cofactors: the copper/zinc (CuZnSOD), iron (FeSOD), and manganese (MnSOD) isoenzymes [20]. SOD activity increases in cells in response to diverse environmental stresses including high light and chilling temperatures [21–23].

The primary objective of this present work was to explore combinations of light and temperature in algal cultures that may inform optimization of production system in manufacturing [24, 25]. The two warm-climate green algae, *Chlorella* sp. DT (DT) and *Chlorella pyrenoidosa* 211-8b (8b), were compared in their photosynthetic activity and antioxidant enzymatic responses under relatively high irradiance and various chilling temperatures [26, 27]. To determine the capacity of these algae to absorb light, their Chl contents and Chl a/b ratios were measured. Photochemical efficiency and the extent of photodamage were assessed by quantifying the chlorophyll fluorescence emission of PSII [28, 29]. The responses of SOD antioxidant enzymes to chilling and high-light acclimation were also examined because they enabled correlation with cell growth and photosynthetic activity [30].
