The Interaction Between Ulva Pertusa (Chlorophyta) And Heterosigma Akashiwo (Xanthophyta) And Its Responses To UV-B Radiation

Globally, the problem of red tide has expanded considerably over the lastseveral decades and seriously threatens the marine sustainability. It not only causesserious destruction to marine ecosystem and the recreational activities, but also leadsto massive mortality of the wild and farmed fisheries. Various methods have beenproposed on red tide controlling or mitigation, among which biological strategies arethe most potential one. Marine macroalga, for instance, is one of the potentialcandidates in red tide mitigation. It could inhibit the red tide micrialgal growth eitherby nutrient competition ability or by allelopathy. Ozone depletion is another seriousenvironmental problem that results in UV-B (280nm~320nm) radiation enhancement,and the enhancement would seriously damage the sustainability of marineecosystem.A macroalga Ulva pertusa Kjellm and a red tide microalga Heterosigmaakashiwo (hada) hada were selected to identify their interactions under controlledlaboratory conditions with and without UV-B radiation in the present study. Resultsshowed that:1,Interaction between U. pertusa and H. akashiwo with absence of UV-B radiation(1) The interspecific competition between U. pertusa and H. akashiwo wereestimated under different initial nutrient and biomass conditions. Results showedthat: Both exploitation competition and interference competition were found in theinteraction between U. pertusa and H. akashiwo. Interference competition playdominant role in the growth of U. pertusa and H. akashiwo when the initial biomassof was set at 0.05g/150mL for U. pertusa and 1×104cell·mL-1 for H. akashiwo ;however U. pertusa was affected by exploitation competition and H. akashiwo wasaffected by both exploitation and interference competition when the initial biomass of U. pertusa increased to 0.40g/150mL. When fixed the initial biomass of U. pertusa at 0.05g/150mL and changed the intial cell density of H. akashiwo we found that H. akashiwo was affected by interference competition under lower initial cell density (0.5×104cell·mL-1 and 1.0×104cell·mL-1) while affected by both exploitation and interference competition under higher initial cell density (2.0×104cell·mL-1and 4.0×104cell·mL-1). Result suggested that the initial inoculated biomass was responsible for different interspecific competition in mixed culture.(2) The simultaneous assay on culture was carried out in mixed culture. Results showed that: U. pertusa could inhibit the growth of H. akashiwo, H. akashiwo could inhibit the growth of U. pertusa simultaneously. Results suggested an allelopathic effect between their interactions: U. pertusa could release extracellular products to inhibit the growth of H. akashiwo, while direct cell-cell contact and releasing extracellular products were responsible for inhibitory effect of H. akashiwo on U. pertusa. U. pertusa and H. akashiwo could release intracellular products into the culture medium filtrates to affect the other's growth.2,The responses of interaction between U. pertusa and H. akashiwo to UV-B radiation(1) Effects of UV-B radiation on the growth, Chl-a contents, pheapigment-a contents and sporulation of U. pertusa and H. akashiwo were studied. Results showed that: The allelopathic effect existed between U. pertusa and H. akashiwo. UV-B radiation could inhibit the growth of U. pertusa and H. akashiwo. However, UV-B radiation exhibited inhibition that was insignificant to the growth of U. pertusa but significant to the growth of H. akashiwo. The response of UV-B radiation on Chl-a and pheapigment-a contents were significant, and the changes of Chl-a and pheapigment-a contents showed comparability. UV-B radiation could inhibit the sporulation of U. pertusa. H. akashiwo could also inhibit the sporulation of U. pertusa, but it was insignificant.(2) The antioxidant enzymatic activies of U. pertusa and H. akashiwo changed greatly when exposed to UV-B radiation. Malondine (MDA) contents, tota1 antioxide capacity (T-AOC) and total superoxide dismutase (T-SOD) activities in U. pertusa increased significantly while just the opposite changes were observed in the soluble protein contents and glutathione peroxidase (GPX) activity in U. pertusa. MDA contents, T-AOC and catalase (CAT) activities in H. akashiwo increased significantly, but its T-SOD activity, soluble protein and carotenoid (Car) contents in H. akashiwo decreased obviously when exposed to the UV-B radiation. As to mixed culture, MDA contents, T-AOC, CAT and T-SOD activities in U. pertusa increased significantly, but the GPX activity in U. pertusa decreased significantly. CAT activity in H. akashiwo increased significantly, but MDA contents, T-AOC, T-SOD and GPX activities in H. akashiwo decreased obviously in mixed culture. MDA contents, T-AOC, CAT and T-SOD activities in U. pertusa increased significantly, but GPX activity and Car contents in U. pertusa decreased significantly in mixed culture exposed to UV-B radiation. CAT activity in H. akashiwo increased significantly, but MDA and Car contents, T-AOC, T-SOD and GPX activities in H. akashiwo decreased significantly in mixed culture under UV-B radiation.(3) Furthermore, biochemical method and the vertical polyacrylamide gel electrophoresis (PAGE) were used to identify the isozymes differentiation of peroxidase (PRX) and polyphenol peroxidase (PPO) in U. pertusa and H. akashiwo. Results showed that: PRX activity in U. pertusa and H. akashiwo decreased significantly when exposed to UV-B radiation. PPO activity in U. pertusa increased obviously while just the opposite changes were observed in H. akashiwo when exposed to UV-B radiation. PPO activity in U. pertusa decreased significantly, but PPO activity in H. akashiwo increased significantly in mixed culture. PRX activity in U. pertusa and PRX and PPO activities in H. akashiwo decreased significantly in mixed culture under UV-B radiation. The isozymes of PRX and PPO in U. pertusa and H. akashiwo presented differences respectively in forms of numbers of bands and staining intensity. Therefore, UV-B radiation and the interaction between U. pertusa and H. akashiwo could result in changes of PRX and PPO isozymes.(4) The extracellular products of U. pertusa and H. akashiwo were studied. Results showed that: Effects of extracellular microbe on the interaction between U. pertusa and H. akashiwo was insignificant. The thermo-sensitive products were excreted by U. pertusa which could inhibit the growth of H. akashiwo. UV-B radiation could not alleviate the deleterious products from H. akashiwo, but the deleterious products from H. akashiwo could be alleviated in mixed culture with U. pertusa.(5) Effects of N and P limitation on the interaction between U. pertusa and H. akashiwo were significant. Effects of UV-B radiation stress and allelopathic effect from H. akashiwo on the growth of U. pertusa were decreased under the condition of N and P limitation. Nevertheless effects of UV-B radiation stress and allelopathic effect from U. pertusa on the growth of H. akashiwo were increased under the condition of N and P limitation.