| Large-scale green tides of Ulva prolifera occurred consecutively after 2007, which have become the normal ecological disasters in the Yellow Sea(YS), China. In this study, first, some species of microalgae, which share similar niche with the bloom-forming U. prolifera, were selected to study their interactions with the propagules of U. prolifera under the different nutrient levels in laboratory. Second, the competition relations between microalgae and differeent development stages of U. prolifera were observed, to compare and analysis the relations with phytoplankton in the whole life process of U. prolifera. A better understanding on these potential interactions will provide some theoretical references in comprehending the ecological consequences of green tides in the YS.To comprehend the whole development stages of U. prolifera, their life history process were continuous observed. This study observed four strains of bloom-forming U. prolifera isolated from the YS for multiple successive generations in the laboratory for approximately 2 years. Among the four strains we examined, two life history process were observed: the gametes of two strains were first observed to develop into parthenosporophytes(PS), followed by alternative generations of gametophytes(G) and parthenosporophytes. The other two strains, however, exhibited repeating generations of gametophyte, and then reverted to the isomorphic PS/G life cycle after 7 gametophyte generations nearly at the same time.Under the sufficient nutrient co-culture conditions, this study found the interactions between microscopic propagules of U. prolifera and nine microalgal species were different at the settlement and germination stage, both of them took the dominate position in turn. The competitive advantage of microalgae was mainly shown at the settlement stage, while U. prolifera propagules dominated the position at the germination stage. The results indicated that the settlement of gametes could be inhibited by three toxin-producing microalgae, Alexandrium tamarense, Prorocentrum lima and Karenia mikimotoi, at the cell density of blooming(102-103 cells·mL–1), while the gametes had no apparent effects on the growth of microalgae. Inversely, the microalgae didn’t affect the germination of gametes, while the germlings germinated from the gametes of U. prolifera had negative effects on the growth of five HAB-forming species of microalgae(A.tamarense, K.mikimotoi, Skeletonema costatum, Prorocentrum donghaiense and Aureococcus anophagefferens) and two microalgae that could serve as food for zooplankton(Phaeodactylum tricornutum and Chlorella sp.), the inhibition rate on the 7th day ranged from 28% to 66%. Among them, A. tamarense showed most significant inhibitory even lethal effects on the settlement of gametes, the 24 h ID50 were only 12 cells·m L-1. Further analysis implied that the paralytic shellfish toxins(PST) produced by Alexandrium did not play a major role in affecting gametes, and this phenomenon was resulted from other allelopathic substances secreted by Alexandrium. Meanwhile, the competitive advantage of U. prolifera, at the germination stage, which may indicate the allelochemicals secreted by microscopic propagules caused this effect. Compared with the gametes, the spores of U. prolifera showed the similar interaction with microalgae. They could be suppressed by Alexandrium at the settlement stage, the settled rate was only 2% of the control groups; while at the subsequent germination stage they caused negative effects on some microalgal strains, the growth inhibition rates reached 15%-42%, respectively.Compared with the sufficient nutrient culture conditions, there existed some difference in the interactions between microalgae and U. prolifera propagules under the nutrient limited levels. The results showed that microalgae also took the competitive advantage at the settlement stage, while U. prolifera germlings took the dominate position at the subsequent germination stage. Whether under the sufficient or limited nutrient levels, A.tamarense significantly decreased the settled numbers of U. prolifera gametes. The settled rate were only 5% and 3% compared to the control groups, but no significant difference between these two treatments was observed; while the germinated gametes co-cultured with A.tamarense and Isochrysis galbana under the resource competition seawaters, the growth inhibition rates of these two microalgae were 49% and 31%, significantly exceed the 30% and 19% under the sufficient nutrient culture conditions.In this study, it’s also found that from the early germination stage of propagules, the subsequent four development stages of U. prolifera(germling stage, seedling stage, mature stage and death stage) also significantly inhibited the growth of microalgae. However, the microalgae didn’t affect the germination and growth of U. prolifera in these stages. Further analysis found that U. prolifera in these four stages showed different inhibitory effects on microalgae: U. prolifera germlings and seedlings reduced the density of A.tamarense, the growth inhibition rates reached 20% and 30%, respectively. The consumption rate of NO3- and PO43- were 49%-91% during the experiment process, and the remaining nutrient concentrations were also enough for the growth of algae. Meanwhile, the DO and pH content were 11.8 mg·L-1 and 8.4, which in the oxygen saturation state and normal pH level; The mature U. prolifera thalli also showed significant inhibitory effects on the growth of microalgae, the inhibitory effect reached 78%. The PO43- was almost depleted, which is likely to be the limiting factor at the end of experiment. The DO and pH showed no significant difference compared to the control groups; U. prolifera at the death stage had lethal effects on the microalgae, the live cells were hardly observed during the experiment. But the nutrients in the co-culture systems were nearly unconsumed, the remaining concentrations were still sufficient for the growth of algae when the experiment ended. The DO and pH were significantly reduced, the concentration of DO was only 46% of the controls(less than 4.0 mg·L-1), which has reached hypoxia level. The pH of the culture media was measured 7.7, which has caused water acidification.The above results demonstrated that some microalgae could only inhibit the naked U. prolifera propagules at the settlement stage. Once the U. prolifera propagules germinated, they could show strong and non-selective inhibitory effects on the growth of microalgae in multiple ways including allelopathy, nutrient competition even the changed culture environment. Therefore, the complex interactions between microalgae and bloom-forming U. prolifera, which may influence the phytoplankton community composition, even closely related with the occurrence dynamics of green tides and red tides in the YS. |
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