| Previous studies indicated that global warming and seawater temperatures rising have a significant impact on the evolution of marine phytoplankton populations. Skeletonema costatum, Prorocentrum donghaiense and Karenia mikimotoi were mainly harmful bloom species in frequent HAB areas of the East China Sea (ECS) in recent years. Diatoms and dinoflagellates blooms have occurred alternately (diatoms-dinoflagellates-diatoms), and the succession between diatoms and dinoflagellates blooms were dominated by the changes in nutrient levels. However, the nutrient concentrations were same level when the succession occurred in dinoflagellates, which suggesting that other factors (temperature, light and salinity) are likely to play a crucial role in P. donghaiense and K.mikimotoi blooms succession. In the present study, basing on the fact that the seawater temperature rises constantly and the concentration of phosphorus was low in sweater, when the dinoflagellate blooms occurred during the late spring and early summer in ECS, changes in growth, photosynthesis and Rubisco gene expression of P. donghaiense and K. mikimotoi were investigated under different temperature gradients (from16℃to28℃, the main temperature interval of dinoflagellate blooms) and nutrient levels, and emphatically discusses the growth interactions between P. donghaiense and K. mikimotoi were investigated by using bi-algal cultures under different temperatures, namely in normal nutrient and low phosphorus conditions. In addition, allelopathic effects of P. donghaiense and K. mikimotoi were investigated by using cell-free filtrate of algal culture experiments on each other under different temperatures. Finally, the correlation between sea surface temperature over the years and the regular outbreak pattern of two species were analyzed, and as well the occurrence trend of the dinofalgellate blooms will be predicted under the different global climate changes scenarios (warming and cooling). The main contents and conclusions are as follows: (1) Through simulating the scene that the seawater temperature gradually increased when seasonal transitions, the growth, photosynthesis and Rubisco gene expression of P. donghaiense and K. mikimotoi were investigated and the results showed that these index were affected under different temperature gradients (from16℃to28℃), the maximal specific growth rate of P. donghaiense and K. mikimotoi occurred at20and24℃, and the specific growth rate of K. mikimotoi were higher than P. donghaiense. The maximum of photosynthetic activity of P. donghaiense and K. mikimotoi occurred at24and28℃, and the photosynthetic activity of K. mikimotoi was lower than others significantly when the temperature at16℃, while P. donghaiense had a higher photosynthetic capacity and higher photosynthetic efficiency than did K. mikimotoi. The maximal mRNA expression of Rubisco in P. donghaiense and K. mikimotoi occurred at20and24℃.(2) Through simulating the scene that the concentration of phosphorus was low in seawater when the dinoflagellate blooms occurred, the growth of two species were investigated and the results showed that the growth of two species were significantly different by the different nutrient levels (P<0.05), and the cell densities in high nutrient level group (The medium have been added f/2medium again when algae culture at12d) were higher than in low nutrient level group. In addition, P. donghaiense has a high absorption efficiency of phosphate and K. mikimotoi has high absorption efficiency of both phosphate and nitrate when determination of the nutrients synchronously. The growth of P. donghaiense and K. mikimotoi were suppressed remarkably (P<0.05) in low phosphorus conditions while the cell volume of K. mikimotoi increases obviously after9d, and cell volume will recovery soon when the phosphorus concentration returned to normal levels.(3) On the basis of the mono-algal cultures, by simulating of the scene that P. donghaiense and K.mikimotoi blooms have occurred together in ECS, the growth interactions between P. donghaiense and K. mikimotoi were investigated in normal nutrients and low phosphorus conditions under different temperatures. In normal nutrient conditions, P. donghaiense has an advantage at low temperature (16-20℃) and high initial cell densities over K. mikimotoi, while K. mikimotoi has an absolutely advantage at high temperature (24-28℃) and high initial cell densities (0.8×104cells ml-1). In low phosphorus conditions, the cell density of P. donghaiense both in mono-algal cultures and in bi-algal cultures increased at first time (approximately6d to8d) and then decreased or fluctuated until the end of the culturing period. Prorocentrum donghaiense has some advantage over K. mikimotoi at16℃with different initial cell density and K. mikimotoi has an absolutely advantage at high temperature (24-28℃). Furthermore, P. donghaiense cannot survive in mono-algal cultures when in phosphorus-limiting and high temperature conditions. Therefore, K. mikimotoi has more advantage when competition with P. donghaiense in low phosphorus condition.(4) Allelopathic effects of P. donghaiense and K. mikimotoi were investigated under different temperatures and the results showed that the growth of P. donghaiense in the K. mikimotoi culture filtrate was suppressed in all experiment, the temperature and the initial cell density was higher and the inhibitory effect was lighter, and the allelochemicals released by K. mikimotoi was heat-sensitive. The growth of K. mikimotoi was suppressed in the P. donghaiense culture filtrate with the highest initial cell density at four temperature treatments, but the allelochemicals released by P. donghaiense was not heat-sensitive, and the allelopathic effect of K. mikimotoi was smallest when the algae in the optimum growth temperature. In addition, there was little inhibitory effect even the stimulating effect at some treatment groups when the initial cell density was low at16℃, however, the mechanism is still unclear.(5) According to statistics (from2001to2010), the mean sea surface temperature was20.30℃when P. donghaiense blooms occurred, which belonging to the low temperature of harmful bloom species; the mean sea surface temperature was23.42℃when K. mikimotoi blooms occurred, which belonging to the low temperature of harmful bloom species; the mean sea surface temperature was20.79℃when P. donghaiense and K. mikimotoi blooms occurred together. In field data showed that the scale and duration of P. donghaiense blooms have an advantage in ECS in2005(belongs to the warm year) and K. mikimotoi blooms occurred before P. donghaiense blooms and its scale was large. There are some relationship between global climate change and the regular outbreak patterns of dinoflagellates blooms in ECS based on these phenomenon. Thus, it was proved again that the temperature play a key pole in the succession of two dinoflagellates blooms.In summary, the results of either laboratory experiments or field survey, the changes of seawater temperature have very important influence on the succession of two dinoflagellates blooms in frequent HAB areas of the ECS. The results of the present study indicate that the effects of seawater temperature on the growth and photosynthetic physiological and biochemical index of two dinoflagellate species also clarify P. donghaiense and K. mikimotoi competition mechanism to some extent. Preliminary forecast can be obtained that the scale of P. donghaiense bloom enlarges when the seawater temperature of ECS is cooler than annual year because global climate becomes cooler, and on the hand, and the scale of K. mikimotoi bloom eclarges than annual year because global climate becomes warmer. |
PDF Full Text Request