**5. Conclusion**

*Microalgae - From Physiology to Application*

s<sup>−</sup><sup>1</sup>

s<sup>−</sup><sup>1</sup>

Chl *b*, which is expected to absorb higher light excitation energy.

tom of serious damage to the photosynthetic apparatus.

**4.2 Differential SOD response**

subcellular compartments [48].

light and temperature stresses was more complicated than expected.

photons m<sup>−</sup><sup>2</sup>

s<sup>−</sup><sup>1</sup>

under 120 μmol photon m<sup>−</sup><sup>2</sup>

under 240 μmol photon m<sup>−</sup><sup>2</sup>

irradiance of 240 μmol photons m<sup>−</sup><sup>2</sup>

2.3-fold at 20°C, of 3.3-fold at 15°C, and so on were calculated relative to the control (onefold at 32°C) (**Table 1**). The diminished activities caused by theoretical excessive excitation pressure were plotted as a function of acclimation temperature (**Figure 8**). Subsequently, another plot was obtained for the doubled irradiance of 240 μmol

follow the theoretical ones, implying that regulation of the response to the combined

8b at temperatures below 20°C. This suggests that DT might possess a more efficient energy dissipation system against the combined stress of low temperatures and high irradiation than 8b. These results are in agreement with reports that the impact from photoinhibition due to low temperature and high light varies greatly across different green algal species [41, 43–45]. Although a greater specific growth rate was obtained

irradiance, neither DT nor 8b favored high irradiance because a smaller Chl content

In order to control light energy absorption and transfer, the LHC must modify the pigment composition of the Chl *a*/*b* ratio, and this is related to alterations in the photosynthetic apparatus under various conditions [16–18]. In the present study, decreases in both the Chl content and the Chl *a*/*b* ratio under low temperatures and high lights occurred simultaneously, suggesting a degradation of Chl molecules or the rearrangement of the LHCII complex [12]. A Chl *a/b* ratio of about 2.5 was obtained in both DT and 8b, which was similar to the green alga *Dunaliella salina* (2.3) [16], smaller than in *Chlorella vulgaris* (7.2) [2], and larger than in *Bryopsis maxima* (1.5) [38]. The lowering of Chl *a*/*b* ratios in DT and 8b is likely a mechanism to avoid absorbing too much light during acclimation [17]. The restoration of the Chl *a*/*b* ratio to 2.6 during 20°C acclimation might derive from the bleaching of

Despite the apparent decrease in the Fv/Fm ratios in our 10 and 7°C acclimation experiments, an initial increase and then a quenching of Fo was observed (data not shown). This phenomenon has been found in *C. vulgaris* and is suggested as being due to a rise in the xanthophyll cycle for dissipating excessive energy [43]. The reduction in both Fm and Fo implied changes in antenna size, thereby minimizing the absorbance of incident light [43]. Because Fo originates from the Chl *a* of the PSII-associated antenna, an increase in Fo is indicative of decreased energy transfer from LHCII to PSII. A large reduction in Fo has generally been regarded as a symp-

Since SOD is the first line of cellular defense against oxidative stress to remove

<sup>−</sup>, monitoring how SOD responds to photoinhibition during acclimation may provide more information about photoprotection [20]. It is known that SOD activity increases in cells in response to diverse environmental stresses including high light intensities and low temperatures and that SOD isoforms are expressed differently to protect against a subset of oxidative stresses under various environmental conditions [46, 47]. In particular, each of the SOD isoforms is independently regulated according to the degree of oxidative stress experienced in the respective

In our experiments, under a moderate irradiance of 120 μmol photons m<sup>−</sup><sup>2</sup>

DT and 8b showed no significant differences in growth rates and photochemical efficiency when subjected to various low temperatures. However, under a doubled

s<sup>−</sup><sup>1</sup>

was found during the stationary phase, that is, less biomass was generated.

, assuming that the excessive excitation pressure was twice the value

irradiance. However, the experimental curves did not

, DT had a slightly higher growth rate than

irradiance compared to 120 μmol photon m<sup>−</sup><sup>2</sup>

 s<sup>−</sup><sup>1</sup> ,

s<sup>−</sup><sup>1</sup>

**184**

O2·

The green algae *Chlorella* species DT (DT) and *Chlorella pyrenoidosa* 211-8b (8b) were very alike in their cell growth rate (total Chl), light energy absorption regulation (Chl *a*/*b* ratio), and photochemical efficiency (Fv/Fm) under optimal conditions of 120 μmol photons m<sup>−</sup><sup>2</sup> s<sup>−</sup><sup>1</sup> and as temperatures decreased from 32 to 7°C. Upon exposure of the cultures to a doubled irradiance of 240 μmol photons m<sup>−</sup><sup>2</sup> s<sup>−</sup><sup>1</sup> , DT exhibited higher cell growth rates than 8b at chilling temperatures of 20°C and 15°C. It was also found that under the combined stresses of chilling temperature and relatively high irradiance, DT possessed higher SOD activity and more new SOD isoforms for removing free radicals than 8b.
