Research on Growth Characteristics of Green-Tide-Forming Green Algae under Stress Conditions

The cytological characteristics of major green-tide-forming green algae Ulva prolifera collected from Yellow Sea were studied through cutting segments, long time low temperature or dark treatments. After being dried in the shade and preserved at -20°C for 30 days, the U. prolifera was cultured at 4°C in sterilized seawater under 40 μmol photons m-2·s-1 light intensity for 120 days, results indicated that the plastid of U. prolifera continuously shrank with the extension of treatment, and most cells turned white and died, only a small amount of cells still contained a few of visible inclusions at the 120d of treatment. Then those samples were transferred to 20°C and 40 μmol photons m-2·s-1 condition for recovery cultivation, after about 10 days, some recovery cells were observed in the thallus, and those cells developed to young thallus gradually and released germ cells almost in the same time. After about 60 days of recovery cultivation, the newly-grown green thallus broke through the original dead thallus, and the germ cells also grew to new individual thallus. Before dark treatment, the U. prolifera cells were filled with plastid, contained visible starch grain and discernible cell outlines, while after 120 days of dark treatment, the plastid shrank and degraded together with the disappearance of cell inclusions, and the cell outlines also blurred, then those samples were transferred to optimal culture conditions at 20°C in 40 μmol photons m-2·s-1 light intensity, and 15 days later, newly-grown cells appeared on the almost dead thallus, these cells divided continuously and grew to young thallus, and those newly-grown thallus also generated active germ cells, which developed to new thallus that cytologically identical to the original thallus. Observation of chopped tissue of U. prolifera cultivated at 20°C, 40 μmol m-2·s-1 showed that the morphological upper part cells turned to germ cells first, those germ cells including gametophyte and sporophyte, which released later and grew to new individual thallus. These findings provided cytological evidences for how U. prolifera live through stress conditions such as low temperature, darkness, and also useful for understanding the mechanism of the occurrence of green tide.