Photobiological Studies On The Arthrospira Plantensis (Cyanophyta) And Phaeodactylum Tricornutum (Bacillariophyceae)

Environmental changes in global scales, such as increase of UVR, elevated CO₂ concentration, ocean acidification and global warming, due to human activities can affect physiological processes and primary production in aquatic environments, and eventually influence ecosystems. Cyanobacteria are the oldest photosynthetic autotropic inhabitants of the planet (3.3-3.5 billion years ago), and they may have been exposed to high UV radiation levels, including shorter wavelengths UV-C. Therefore, they must have developed effective mechanisms to counteract detrimental effects of UVR. However, diatoms did not appear until 0.15-0.2 billion years ago, when the ozone layer already existed. UV-protection mechanisms in diatoms may differ from that in cyanobacteria. Therefore, in this paper an economically important prokaryotic cyanobacterium, Arthrospira (Spirulina) platensis and a eukaryotic diatom, Phaeodactylum tricornutum, were chosen to study their photobiological responses to solar UV radiation.A. platensis was insensitive to UVR at higher levels of temperature. At 30±2?C, the spiral structure of both the indoor (439) and the outdoor (D-0083) strains was mainly affected by PAR. However, the spirals became more compressed in the outdoor strain but more loosened in the indoor strain under high levels of PAR. UVR led to filament breakage at lower temperature (15 & 22?C), however, no filament breakage was found when exposed to both PAR and PAR+UVR at 30?C. Furthermore, UVR induced DNA damage in A. platensis increased at lower temperature levels. Spiral orientation, which is one of the most important morphological characters, altered from the right to left-handed only in the outdoor strain. Presence of UVR enhanced such an orientation change, and re-growing under indoor low PAR (UVR-free condition) reversed the orientation to right-handed.For Phaeodactylum tricornutum, the specific growth rate was suppressed when the cells were transfer from indoor to outdoor in the first 2 days, and then increased to a higher rate from day 4. Exposure to UVR decreased the growth rate. UV-tolerance became induced under outdoor conditions, and presence of UVR enhanced such ability, though UV-absorbing compounds (UVACs) did not increase. Such tolerating capability decreased when the cells were re-grown under low levels of PAR or under UVR alone. In the presence of chloramphenicol, a protein synthesis inhibitor, photoinhibition increased, reflecting the roles played by de novo synthesis of D1,D2. When cultured under high CO₂ (800 ppm) and altered seawater carbon system, its CCM was down-regulated, photosynthetic CO₂ affinity decreased. However, no significant difference in specific growth rate (μ) was found. On the other hand, elevated CO₂ (decrease of pH) and solar UVR decreased the activity of alkaline phosphatase, implying negative impacts of ocean acidification on the diatom under Pi-limited waters.Based on the responses of prokaryotic A. platensis and eukaryotic P. tricornutum to solar UVR, it is clear that both organisms have low contents of UVACs, but possess high tolerance to UVR. A. platensis can reduce the harms via changing its spiral structure, while P. tricornutum can protect itself with efficient repair of UV-induced damages.