In the latter experiment, filters were attached to the upper surface of the ice so that the algae were exposed in situ to treatments of ambient levels of PAR and UV radiation, ambient radiation minus UVB, and ambient radiation minus all UV. After 16 d, significant increases in chl a and cell numbers were recorded for all treatments, but there were no significant differences Tamoxifen in vitro among the different treatments. Bottom-ice algae exposed in vitro were considerably less tolerant to UVB than those in situ, but this tolerance improved when algae were retained within a solid block of ice. In addition, algae extracted from brine channels in the upper meter of sea ice and exposed
to PAR and UVB in the laboratory were much more tolerant of high UVB doses than were any bottom-ice isolates. This finding indicates that brine algae may be better adapted to high PAR and UVB than are bottom-ice algae. The data indicate that the impact of increased levels of UVB resulting from springtime ozone depletion on Antarctic
bottom-ice communities is likely to be minimal. These algae are likely EGFR activity protected by strong UVB attenuation by the overlying ice and snow, by other inorganic and organic substances in the ice matrix, and by algal cells closer to the surface. “
“Diatoms are frequently exposed to high light (HL) levels, which can result in photoinhibition and damage to PSII. Many microalgae can photoreduce oxygen using the Mehler reaction driven by PSI, which could protect PSII. The ability of Nitzschia epithemioides Grunow and Thalassiosira pseudonana Hasle et Heimdal grown at 50 and 300 μmol photons · m−2 · s−1 to photoreduce oxygen was examined by mass spectrometric measurements of 18O2. Both species exhibited significant rates of oxygen photoreduction at saturating light levels, 上海皓元医药股份有限公司 with cells grown in HL exhibiting higher rates. HL-grown T. pseudonana
had maximum rates of oxygen photoreduction five times greater than N. epithemoides, with 49% of electrons transported through PSII being used to reduce oxygen. Exposure to excess light (1,000 μmol photons · m−2 · s−1) produced similar decreases in the operating quantum efficiency of PSII (Fq′/Fm′) of low light (LL)- and HL-grown N. epithemoides, whereas HL-grown T. pseudonana exhibited much smaller decreases in Fq′/Fm′ than LL-grown cells. HL-grown T. pseudonana and N. epithemioides exhibited greater superoxide and hydrogen peroxide production, higher activities (in T. pseudonana) of superoxide dismutase (SOD) and ascorbate peroxidase (APX), and increased expression of three SOD- and one APX-encoding genes after 60 min of excess light compared to LL-grown cells. These responses provide a mechanism that contributes to the photoprotection of PSII against photodamage. “
“Siphonous plants represent an alternate scheme to the way most macroscopic plants are constructed.