Physiological Response Of Several Intertidal Marine Macroalgae To UV-B Irradiation And Its Preliminary Mechanism Of Interspecific Competition

Enhanced UV-B (280nm-320nm) irradiation resulting from ozone depletion is one of global environmental problems. Marine ecosystem cannot but be affected by enhanced UV-B radiation. Tideland is interlock region of marine ecosystems and dry land ecosystems, which is one of the most susceptible zones of the biosphere. Intertidal macroalgae live in this transition zone, which inevitably expose to amphibious surroundings because of the tide. During low tide, marine macroalgae are potentially exposed to full sunlight and thus higher UV-B. When submerged, macroalgae could also be obviously affected by UV-B irradiation because it is capable of penetrating the water column to an ecologically significant depth. As a result, macroalgae was sure to be influenced by the negative impacts from UV-B irradiation. Exposure to UV-B leads to the generation and accumulation of reactive oxygen species (ROS). However, an efficient antioxidant defense system have developed in plants to counteract oxidative stress, which need to be further studied. In addition, many studies on effects of allelopathic effect and interspecific competition concentrated on the interaction between marine microalgae and microalgae, especially on description of inhibitory effects of macroalgae on microalgae. However, interspecific competition between intertidal macroalgae has received little attention. Unfortunately, less was know about the effect of UV-B irradiation enhancement on the growth and allelopathic effect of intertidal macroalgae. It could be imagined that macroalgae competed for resources at the same or similar ecological niche. Moreover, some algae possibly inhibited the growth of other algae by secreting allelochemicals, and the situation might change when exposed to UV-B radiation. Therefore, the present study was designed to investigate the response of macroalgae to three doses of UV-B irradiation as low dosage (Luv), medium dosage(Muv) and high dosage(Huv) and clarify the mechanisms associated with reactive oxygen species. Four species of common macroalgae at the seaside of Qingdao including Ulva pertusa (Chlorophyta), Grateloupia filicina (Rhodophyta), Corallina officinalis L. (Rhodophyta) and Sargassum thunbergii (Mert .) O. Kuntze.(Phaeophyta). The aim of this study was to investigated the intensity and number changes of isozyme bands of SOD, POX, CAT and APX to further identify the biochemically expression regular of different species of algae and isozymes relevant to UV-B exposure. In addition, the interspecific competition, allelopathy effect and nutrient competition between U.pertusa and G. filicina and it's response to the UV-B irradiation enhancement were analyzed using mono-culture and co-culture methods. This study adopted took biomass as the main examined index. Furthermore, effect of different initial weights on interspecific competition of two species of macroalgae in co-culture, and whose trends subjected to special dosage of UV-B irradiation (9.6kJm-2d-1) were studied in this experiment.The main results were presented as following:1,Response of activities of antioxidant enzymes of four species of marine macroalgae to UV-B irradiationThe activity of SOD and CAT in U. pertusa reacted rapidly to UV-B irradiation, which increased significantly at the initial stage when exposed to UV-B treatments. Subsequently, the activity of the two enzymes declined in varying degrees. POX activity rose in early days under Luv treatment and decreased significantly under Muv and Huv treatments. APX activity has always been maintained at a more stable level. Responses of GR and GPX to UV-B stress started up a litter later and activity peak appeared on the day 6, whereafter the activity of these enzymes were higher under luv or Muv treatments than that in Huv treatment. The content of antioxidant such as AsA and GSH were similar with control in the early medium-term but reduced at later period, which decreased significantly in Muv and Huv tissues. H2O2 and TBARS content in U. pertusa have not accumulated rapidly under UV-B treatment, until the day 6 to 12, H2O2 and TBARS contents increased significantly compared to the control under Muv and Huv stress of UV-B. The difference of H2O2 content between the value under the low dosage of UV-B radiation and the control value was not obvious all the time.The change of activity of SOD,CAT,APX,GPX,GR in G.filicina showed dissimilarly when exposed to different dosage of UV-B treatments. Their activities increased at varying degrees in early phase and decreased in later phase compared with control under luv and Muv treatments. Under high UV-B radiation treatment, these enzymes had no significant difference with control in the early, medium-term apart from GR activity, which increased compared with control. However, they showed decreasing trend in later phase. POX activity changed rapidly and acutely induced by UV-B irradiation, which increased notably under three dosage of UV-B treatments. AsA contents enhanced promptly under luv and Muv treatments except high dosage treatment, which declined under all the three dosage of UV-B treatments. GSH content had transient increase in early phase subsequently with decreasing trend later under high dose of UV-B radiation; GSH content mainted at a fairly high level under luv and Muv treatments in the early stage then fell to the control level in the end. Consistent with the loss of the activities of antioxidation system, H2O2 and TBARS content increased significantly under high dose of UV-B radiation treatment and which were also accumulated effectively under medium UV-B treatment in the latter half of time. TBARS content increased significantly until 9-12d compared with control under Luv treatment.The activity of SOD and POX in C. officinalis maintained at higher level under low dose of UV-B irradiation, which were similar to the control level under Muv and Huv treatments. The activity of CAT showed corresponding changes with the increasing dose and time of UV-B irradiation. Their activities increased under Luv and Muv treatments in varying degrees in early medium days and showed decreasing trend in later phase, which reduced more obviously under Huv treatment in mid and late days. APX activity was similar to the control level under Luv and Muv treatments, while which increased significantly under Huv treatments. The changes of activity of GR and GPX were partly similar, with the exception of the initial transient rise of GR activity, which had no significant difference between the control value and Luv treatment. Under Muv and Huv treatments the activity of those two enzymes were stable, then showed a downward trend and Huv treatment tissues decreased more prominent. The content of AsA showed a more stable trend and without significant change corresponds to the stress dose and time. GSH content increased slowly under Luv and showed fluctuant change under Muv treatment. However, GSH content decreased significantly under Huv treatment. H2O2 and TBARS content in C. officinalis had not significant accumulation under low dose of UV-B radiation, which gradually increased under the Muv treatment. Under Huv treatment, both of H2O2 and TBARS increased more rapidly, which was significantly higher than the control in medium-term.SOD and CAT activities in S. thunbergii were close related to the dosage and time of UV-B radiation, which increased in varying degrees compared with control under luv and Muv treatments in the early medium-term. They decreased significantly under high dosage of UV-B radiation from the mid term. GR and GPX activities were relatively stable and responded slowly to UV-B radiation, which decreased under Muv or Huv treatments at the end of exposure. UV-B irradiation failed to influence the POX activity in S. thunbergii. POX activity did not show any regular changes correlated to different doses of UV-B irradiation. APX activity was especially low in the control and all the UV-B treatments. Therefore, no effective activity was detected in S. thunbergii. AsA content increased especially in Luv and Muv treatments. GSH content was significantly lower than control in Huv tissues, but slightly higher than control under Luv and Muv treatments at beginning of exposure. H2O2 and TBARS contents accumulated fastly especially under Muv and Huv treatments.Overall, AsA and GSH content and antioxidant enzymes including SOD, POX, CAT, APX, GR and GPX showed varying basic activity and different response to UV-B radiation in different species of macroalgae, which reflected the obvious interspecific differences in characteristics. Green algae U. pertusa possessed higher activity of antioxidant system under UV-B radiation than that in G. filicina and C. officinalis (Rhodophyta). However, S. thunbergii (Phaeophyta) antioxidase system activity was relative lower and it's sensitivity to UV-B stress was lowest.2,Effects of UV-B irradiation on several isozymes of four species of marine macroalgaeSOD,POX,CAT and APX isozymes of different species of marine macroalgae showed various expression nature and intensity, which expressed species-specific. When exposed to UV-B irradiation, SOD,POX,CAT and APX isoenzymes changes mainly in the number and activity of isozymes, which suggested that UV-B irradiation could result in changes of these isozymes of all the four species of marine macroalgae. Moreover, different dosage caused different effects and which were consistent with the total avtivity of corresponding enzymes. According to the overall performance of basic activity and changes of isozymes, we speculated that SOD and CAT in U. pertusa, and SOD, POX and CAT in G. filicina, and SOD, POX, CAT and APX in C. officinalis L. were more sensitive to UV-B irradiation. SOD and CAT isoenzymes were also visibly influenced by UV-B in S. thunbergii. However, no APX isoenzyme of S. thunbergii was detected in this study.3,Response of interspecific competition between U. pertusa and G.filicina to UV-B irradiation enhancementThe relation of interspecific competition included both allelopathy effect and nutrient competition. Specific growth rates of U. pertusa under treatment with abundant nutrition and limited nutrition was 2.54 and 2.47 times of those of G. filicina. Thus, compared to U. pertusa, G. filicina was in inferior position. UV-B irradiation could inhibit the growth of U. pertusa and G. filicina under the condition of mono-culture. The higher the dosage and the longer exposure of UV-B irradiation were, the more significant the inhibitive effect was. When they were cultured together, Luv and Muv irradiation reduced the competitive ability of U. pertusa. The relation of interspecific competition tended to be at a balanced level even though U. pertusa was still the dominant algae. However, high dosage of UV-B irradiation had more serious inhibitive effect on G. filicina, and competitive dominant position of U. pertusa tended to be more obvious. Thus, UV-B changed the relation of competitive balance of U. pertusa and G. filicina, which changed along with the dosage of UV-B. Moreover, UV-B irradiation might influence the metabolism of the allelochemicals produced by U. pertusa and G. filicina in a long time.4,Effect of UV-B irradiation enhancement on the interspecific competition of different initial weights of U.pertusa and G.filicinaThe growth of both U. pertusa and G. filicina was restrained by special dosage of UV-B irradiation (9.6kJm-2d-1) in mono-culture and specific growth rates decreased obviously. The relationship of interspecific competition between U. pertusa and G. filicina was closely related to the initial weights when they were cultured together. When the initial proportion of U. pertusa (U) and G. filicina (G) was U:G = 1.2:1 and U:G = 1:1, U. pertusa was the dominant algae. When initial proportion was U:G = 1:1.2, G. filicina held the competitive dominant position in earlier stage, but U. pertusa grew faster which could take the place of G. filicina in later stage. When the initial proportion was U:G = 1:1.4, more weights provided competitive dominance for G. filicina. Under UV-B irradiation, the competitive ability of G. filicina became feeble although it was still the dominant algae and the competitive balance was in favor of U. pertusa.