| Zooplankton plays an important role in marine ecosystems as a key loop between phytoplankton and fish. Marine zooplankton has a potentially regulatory effect on bloom-forming microalgae. Its feeding activity and population fluctuation directly affect the microalgal species, growth and population size, and then remarkably influence the occurrence and development of harmful algal blooms (HABs). One the other hand, bloom-forming microalgae will affect the zooplankton's survival, reproduction, growth and feeding activity, and thus exert adverse effect on its population size and development. Therefore, a research on the interaction between phytoplankton and zooplankton is of important meaning to illuminate the processes of matter cycle and energy flow of marine ecosystem. In present study, the interactions among Schmackeria inopinus (Burckhardt,1913) and four species of bloom-forming microalgae (Alexandrium tamarense, Prorocentrum donghaiense, Heterosigma akashiwo and Skeletonema costatum) were investigated under controlled laboratory conditions, and the possible effective mechanism was discussed. Results showed that:1. Feeding activities of S. inopinus on four species of bloom-forming microalgaeFeeding rates of S. inopinus differed significantly when fed on A tamarense, P. donghaiense, H. akashiwo and S. costatum in co-culture. S. inopinus was preferred to S. costatum, while presented a certain feeding rates on A. tamarense and P. donghaiense, but almost not fed on H. akashiwo. The feeding selectivity was:S. costatum> A. tamarense> P. donghaiense> H. akashiwo.Feeding rates of male and female S. inopinus were different: The feeding rates of female ones were higher than those of male when fed on A. tamarense, P. donghaiense and S. costatum, but no difference was observed between male and female when fed on H. akashiwo.2. Influences of S. inopinus on population dynamics of bloom-forming microalgae in co-cultureS. inopinus showed significantly influences on population dynamics of P. donghaiense, A. tamarense and S. costatum in co-cultures. The addition of S. inopinus not only regulated the population growth, but also decreased the microalgal population density (P< 0.05):The time at which microalgal population achieving exponential growth phase and platform phase delayed, and the maximum cell density obviously decreased (P< 0.05) in the co-culture as compared with that in the control growing in the mono-culture. The changes became more remarkable with the increaseed amount of S. inopinus. In contrast, S. inopinus had no obvious effect on the growth and cell density of H. akashiwo in the co-culture and almost no significance was observed as compared with the control group (P> 0.05). This result further demonstrated that S. inopinus almost didn't feed on H. akashiwo.3. Influences of bloom-forming microalgae on reproduction, growth and development of S. inopinus in co-cultureAll the four bloom-forming microalgae (A. tamarense, P. donghaiense, H. akashiwo and S. costatum) delayed the egg-carrying time, decrease the egg mass rate, and prolonged the duration of egg mass development and the interval of egg carrying of S. inopinus as compared to the control (P< 0.05). However, the tested microalgae had no obvious influence on the growth of S. inopinus (P> 0.05) while significantly restrained the development of S. inopinus and prolonged the duration from N II stage to adult and the duration time of each larval stage as compared to the control which cultured in normal conditions (P< 0.01). Among the microalgae, H. akashiwo showed the strongest inhibitory effect on reproduction and development of S. inopinus followed by P. donghaiense and A. tamarense. S. costatum showed little inhibition as compared to the other three.Four components of H. akashiwo (water extracts, culture medium filtrate, re-suspended cells and microalgal culture medium) not only delayed the egg-carrying time, but also led to restrain of egg carrying (P< 0.05). The cell-free water extracts had no obvious effects on prolonging the duration time of egg mass development and the interval of reproduction (P> 0.05); however, the culture medium filtrate, re-suspended cells and algal medium significantly prolonged the duration time of egg mass development and the interval of reproduction as compared to the control (P< 0.05). These four components had little effect on the growth of S. inopinus (P> 0.05), but significantly affected the development of S. inopinus (P< 0.05). On one hand, the four components presented inhibitory effects on developmental process of S. inopinus, and prolonged duration time of each developmental stage, among which microalgal culture medium showed the strongest inhibitory effects on reproduction and development, re-suspended cells and culture medium filtrate had stronger one than that of extracts from disrupted cells.4. Influences of bloom-forming microalgae on the population quantity of S. inopinusUnder controlled laboratory conditions, A. tamarense and P. donghaiense inhibited the growth of S. inopinus resulting in a decreased growth rate and the inhibitory effect, and the inhibition increased accordingly with the initial microalgal density increasing. The population quantity of S. inopinus decreased significantly with the increase of H. akashiwo density in the co-culture, and the decrease was density-dependent, inferring that H. akashiwo present a lethal effect on S. inopinus in the co-culture. S. costatum had inhibitory effect on population growth of S. inopinus. The inhibitory effect decreased with the increase when cell density of S. costatum was set at the range from 2×104 cells ml-1 to 20x10 cells ml-1, but the inhibition was obviously increased when the density was set at 30x104 cells ml-1.In microalgal co-culture systems, the population fluctuation of S. inopinus did not depend on the density ratio of A. tamarense to P. donghaiense. Along with culture time, the population quantity of S. inopinus constantly increased at three density ratios (8:5,1:1 and 5:8), but no obvious difference of population density was observed among the three density ratios (P> 0.05). Under co-culture systems of H. akashiwo and S. costatum, the population fluctuation of S. inopinus remarkably depended on the microalgal density ratio, and the growth rate of population quantity of S. inopinus increased with the increase in the ratio of S. costatum/ H. akashiwo.5. Influence of H. akashiwo on the antioxidant system of S. inopinusH. akashiwo decreased the activities of superoxide dismutase (SOD), glutathione reductase (GR) and glutathione peroxidase (Gpx), the enzymatic activities decreased with the increase of microalgae density (P< 0.05). Likewise, H. akashiwo decreased the contents of two antioxidant metabolites (CAR and GSH) (P< 0.05), and the total antioxidant activity was greatly inhibited as compared to the control (P< 0.05).The effect of H. akashiwo led to a significant accumulation of reactive oxygen species (ROS), and the generation of ROS was observed throughout the whole culture process. ROS content elevated with the increase in microalgal density, and was significantly higher than that in control groups (P< 0.05). simutaneous analysis on the malonaldehyde (MDA) showed that S. inopinus were subjected to a serious peroxidative stress of membrane lipid.6. The possible mechanism of H. akashiwo on reproduction, development and population dynamics of S. inopinusWhen considered the results as a whole, we conjectured the possible mechanism of H. akashiwo on reproduction, development and population quantity of S. inopinus. In co-culture system, H. akashiwo secreted toxic substance (allelopathy) which caused direct injuries on S. inopinus. On the other hand, the activities of antioxidant enzymes and the antioxidant metabolites contents decreased when exposed to these toxic substances, resulting in a decrease in total antioxidant activity. Furthermore, the decease led to the over-production of ROS, and the overwhelming ROS could break the ROS homeostasis in S. inopinus and initiate the membrane lipid peroxidation which could be characterized by the increase of MDA. It decreased the antioxidant system activities thereafter, which further aggravated the oxidative stress. ROS-mediated oxidative stress might be the possible mechanisms that damage the growth and reproduction of S. inopinus in the co-culture, and the allelochemicals secreted by the bloom-forming microalga was the original reason.
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