Effects Of Solar Ultraviolet Radiation On Gracilaria Lemaneiformis And Ulva Lactuca

Macroalgae, living mainly in the intertidal zone, play an important role in coastal waters asprimary producers. However, little is known about the relation between solar UV radiation(UVR) and their photosynthetic performance, growth. Therefore, responses to UVR ofGracilaria lemaneiformis Bory, an economic red alga and Ulva Lactuca Linnaeus, a commongreen alga were studied. The main results are as follows:In situ experiments showed that Gracilaria lemaneiformis grew slower with increased water depth in March when temperature and solar radiation were low, while the highest growth rate was found at about 1 .0m depth in April when temperature and solar radiation were higher. The content of Phycoerythrin and Chla increased with increased depth, but the content of UV-absorbing substances (MAAs) decreased with increased depth. Further experiments with solar radiation treatments by exposing the alga to solar radiation with or filtered out of UV-B or UV-A + UV-B, showed that UVR reduced its total daily net photosynthetic production. UV-A was the major factor that contributed to the inhibition on sunny days in April, but UV-B contributed most to this effect on cloudy days in May. UV-A was found to enhance the photosynthetic O₂ evolution on cloudy days when the solar radiation was low. The accumulation of MAAs was associated with solar radiation and wavelength, high levels of UV-B enhanced the accumulation of MAAs.The photochemical efficiency of the green macroalga Ulva lactuca decreased when the thalli were exposed to high solar radiation in the late morning and recovered when the solar radiation decreased in the late afternoon, showing a typical pattern, the lowest during noon period with the higher values in the early morning and later afternoon. This noontime inhibition was caused by not only the photosynthetic active radiation but also UV radiation. UV-B significantly affected the photochemical efficiency. Long-term experiments showed that the photochemical efficiency was most inhibited on the second day, however, it became less and less suppressed by UVR while the thalli became acclimated. But UVR had no apparent effects on the growth and pigment of the thalli.Solar UVR affectd the phtotsynthesis of the red alga Gracilaria lemaneiformis and green alga Ulva lactuca, UVR inhibited the growth of the former but did not affect that of the latter. These results were related to changes of solar radiation during study periods of different seasonsand different mechanisms against UVR of the two species.