Harmful algal blooms of the coastal Prorocentrum donghaiense and Skeletonema costatum have been the focus of many studies, primarily due to the devastating effects this alga has had on the aquaculture industry. Prorocentrum donghaiense and Skeletonema costatum are widely distributed in the world. Prorocentrum donghaiense and Skeletonema costatum have caused great economic loss in many countries. Allelopathy is a prevalent natural phenomenon in aquatic ecosystems. The phenomenon of a large number of marine macroalgal proliferation is called 'Green Tide'. Generally, green tide occurs in spring or summer and end in summer when temperature is high, but sometimes extended to the fall. In recent years, frequency and biomass of green tide are rising totally, it has become a major concern of the international community to marine environmental problems and ecological disasters. Green tide occurs in the south Yellow Sea of China from 2008 to 2012, which bring about significant losses to coastal fisheries, aquaculture and tourism, causing significant impact on the natural landscape too. In recently years, Ulva prolifera is the primary peacebreaker algae of green tide.
At present, Ulva prolifera has been researched more, but mostly focus on ecological characteristics of Ulva prolifera, algae composition and Ulva competion with microalgae, less focus on the influence of Ulva prolifera on water-quality. This paper studies the allelopathic effects of the macroalga Ulva prolifera on the microalga Prorocentrum donghaiense and Skeletonema costatum, in order to provide some theoretical proof for biological control of harmful algae blooms.
Allelopathic effects of different solvent extracts of Ulva prolifera dry powder on the harmful algae (Prorocentrum donghaiense and Skeletonema costatum) were studied in laboratory. The results showed that growth inhibitory effects by different concentrations of distilled water extracts and three organic solvent (methanol, ethyl acetate and n-hexane) extracts from Ulva prolifera dry powder on two microalgaes are obviously. The inhibition rate becomes higher when the concentration increases, while it may promote the growth of microalgae when the concentration is low, which are similar with the effects to other microalgaes. The distilled water extracts are of the weakest growth inhibitory activity on the microalgae. Methanol and ethyl acetate extracts are of the highest growth inhibitory effect on two microalgaes. So it can be deduced that the extracts from the chemical with higher polarity have the higher inhibition activity. The sensibility of the microalgaes are different that Prorocentrum donghaiense is more easily affected than Skeletonema costatum, which may be caused by the characteristic of the cell structure. The growth of all tested microalgae was restrained by the distilled water, methanol, ethyl acetate, as well as n-hexane extracts from Ulva prolifera dry powder, and the lethal threshold of Prorocentrum donghaiense by the extracts above were 5.00, 0.50, 0.50, 0.63 g/L (equal to fresh tissue were 22.00, 2.20, 2.20,2.75 g-wet/L) respectively in the first three days. In addition the lethal threshold of Skeletonema costatum by the four extracts were 20.00, 1.25, 1.25,2.50 g/L (equal to fresh tissue were 88.00, 5.50, 5.50, 11.00 g-wet/L) in the first three days.