Algal Interactions And Their Responses To CO₂ Enrichment

Species interactions between bloom microalga and other alga play essential roles in affecting phytoplankton sequence, and it is important to study responses of species interactions to CO2 enrichment for illustrating the influence of global climate changes on marine ecosystem. Macroalga has long been suspected of suppressing phytoplankton growth through the excretion of chemical substances that inhibit phytoplankton growth. The production and excretion of allelochemcias by aquatic macroalga could be an effective defense strategy against other photosynthetic organisms competing for light and nutrients and therefore, macroalga might be a potentially biological method in bloom mitigating and controlling. However, the potential for this strategy to be successful in practice is exceedingly difficult mainly due to the lack of the information about the possible macroalgal species involved in and lack of the experimental data supporting its practical application. Moreover, the growth interaction between the phytoplankton species mediated by extracellular organic substances that are released by one or both interacting species has been considered to be essential to the development of phytoplankton community, and the increased population of one phytoplankton species might affect the growth of the other one or several species, influence bloom, pulses and seasonal succession. We carry out a serious of experiments under controlled conditions, and the purpose of the study is to identify if Ulva pertusa Kjellm (Chlorophyta) and Gracilaria lemaneiformis (Bory) Dawson (Rhodophyta) are the potential candidates in bloom controlling and mitigating, to investigate the response of algal interaction to CO2 enrichment, and to study the possibly effective mechanism of the interaction and the response. Moreover, the interactions between different microalgae are also studied and their potential role in bloom development has been discussed. Results show that: 1. Influence of U. pertusa and G. lemaneiformis on the growth of Prorocentrum donghaiense, Alexandrium tamarense, Scrippsiella trochoide and Amphidinium carteraeThe fresh tissues of either U. pertusa or G. lemaneiformis significantly suppressed the microalgal growth, even killed them by the end of the experiment. However, their culture filtrates exhibited different effects (inhibitory, stimulatory or none) on the co-cultured microalgae according to different microalgal species. It seemed that the macroalgae could release some allelopathic compounds into culture medium to affect the co-cultured microalgae.2. Influence of U. pertusa or G. lemaneiformis on the growth of Heterosigma akashiwoH. akashiwo was taken as an example to study the possible effect of macroalga in the co-culture. Both U. pertusa and G. lemaneiformis, and especially their fresh tissues, significantly impeded the growth of H. akashiwo, and quickly outcompeted co-cultured cells of this microalga. Carbonate limitations and the presence of bacteria are not necessary for the negative effects of U. pertusa and G. lemaneiformis on H. akashiwo. Nutrient assays showed that nitrate and phosphate were almost exhausted in the G. lemaneiformis co-culture system, but remained at acceptable levels in the U. pertusa system, when all cells of H. akashiwo were dead. When f/2 medium is supplied daily to G. lemaneiformis culture, the growth of H. akashiwo was greatly inhibited but not completely terminated. The allelopathic effects of U. pertusa may be essential for negative effects on H. akashiwo; however, the combined roles of allelopathy and nutrient competition may be responsible for the negative effect of G. lemaneiformis. Different amounts of fresh seaweed tissue, homogenate, and culture medium filtrate prepared from different macroalgal concentrations were analyzed to determine their effects on the growth of H. akashiwo. The results further suggested the release of allelochemicals by U. pertusa.3. Response of the interaction between macroalgae and microalgae to CO2 enrichmentUnder CO2 enrichment condition, the fresh tissues of either U. pertusa or G. lemaneiformis showed different effects on the five microalgae, such as no significant effect, enhanced effect of suppressing or killing, or decreased effect of suppressing or killing. The sensitivity of microalgae to the fresh tissue of U. pertusa was P. donghaiense > H. akashiwo > A. tamarense > S. trochoide > A. carterae. Under CO2 enrichment condition, the sensitivity of microalgae to the fresh tissue of U. pertusa was S. trochoide > H. akashiwo > A. tamarense > P. donghaiense > A. carterae. The sensitivity of microalgae to fresh tissue of G. lemaneiformis was H. akashiwo > P. donghaiense > A. tamarense and S. trochoide > A. carterae. Under CO2 enrichment condition, the sensitivity of microalgae to the fresh tissue of G. lemaneiformis was S. trochoide > H. akashiwo, A. tamarense and P. donghaiense > A. carterae.The five microalgae showed different response to culture filtrate of U. pertusa and G. lemaneiformis with and without CO2 enrichment. The change law of the response of culture filtrate was similar to the response of the fresh tissue with and without CO2 enrichment. This result confirmed that both U. pertusa and G. lemaneiformis could release some allelopathic compounds into culture medium to affect the co-cultured microalgae. Without CO2 enrichment, the sensitivity of microalgae to the culture filtrate of U. pertusa was A. tamarense and S. trochoide > P. donghaiense and H. akashiwo > A. carterae, while under CO2 enrichment condition, the sensitivity was S. trochoide > A. tamarense > A. carterae and H. akashiwo > P. donghaiense. And without CO2 enrichment, the sensitivity of microalgae to the culture filtrate of G. lemaneiformis was S. trochoide and A. carterae > H. akashiwo, P. donghaiense and A. tamarense, while under CO2 enrichment condition, the sensitivity was S. trochoide and A. carterae > A. tamarense > H. akashiwo and P. donghaiense.4. Interaction between P. donghaiense and A. tamarenseInteractions between P. donghaiense and A. tamarense were investigated using bialgal cultures. P. donghaiense was completely killed but A. tamarense was almost not affected by the end of the experiment when the initial density/size ratio was set at 1:1. However, significant growth suppression occurred on either species when the ratio was increased to 10:1, but no outcompetement was observed. The simultaneous assay on the culture filtrates showed that P. donghaiense filtrate prepared at a lower initial density (1.0×104 cells mL-1) induced stimulatory effect on growth of the co-cultured A. tamarense, but filtrate at a higher initial density (1.0×105cells mL-1) depressed its growth. The filtrate of A. tamarense at a density of 0.28×104 cells mL-1 completed killed P. donghaiense at a lower density, but only exhibited inhibitory effect on it at a higher density. It was likely that these two species of microalgae interfered with each other mainly by releasing allelochemical substance(s) into the culture medium, and the direct cell-cell contact was not necessary for their mutual interaction. Moreover, the strain of A. tramarense we used in this experiment was non-toxic species and therefore, there might another allelochemical(s) except the PSP toxin that affected the co-cultured microalga. The allelopathic test further proved that A. tamarense could affect the growth of co-cultured P. donghaiense by producing allelochemical(s); moreover, A. tamarense culture filtrate at the stationary growth phase (SP) had strongly inhibitory effect on P. donghaiense compared to that at the exponential phase (EP). The growth of P. donghaiense and A. tamarense in the bialgal cultures was simulated using a mathematical model to quantify the interaction. The estimated parameters from the model showed that the degree that P. donghaiense inhibited by A. tamarense was respectively about 17 and 8 times stronger than that P. donghaiense exerted on A. tamarense, when the initial density/size ratio were set at 1:1 and 10:1, respective. A. tamarense seemed to have a superior survival strategy to P. donghaiense in the bialgal cultures under controlled laboratory conditions. The results also indicate that A. tamarense can interfere other microalgae by releasing allelochemical substance(s) into the culture medium.5. Interactions between P. donghaiense and A. tamarense with CO2 enrichmentThe competition relation between P. donghaiense and A. tamarense changed with CO2 enrichment in co-culture. Under CO2 enrichment condition, the relative competitive ability of P. donghaiense enhanced, while the relative competitive ability of A. tamarense was weakened. The result from culture experiment in filtrate confirmed the conclusion above.Culture medium filtrate of P. donghaiense and A. tamarense could affected each other, and therefore confirmed that these microalgae could release some allelopathic compounds into culture medium to affect the another co-cultured microalgae. Under CO2 enrichment conditions, effects of culture filtrate of P. donghaiense and A. tamarense changed, and hence these phenomenon confirmed existences of alleopathic compounds and their effect, and indicated that the change of allopathic activity of the two microalgae with CO2 enrichment was the main reason why their relative competitive ability changed.