The Experimental Ecology Studies On Responses Of Marine Microalgae To The CO₂ Enrichment

The steady enrichment of CO₂ at unprecedented speed is one of the seriously global environment problems. The enrichment of greenhouse gas, such as CO₂, has led to an increase in global temperature, and consequent sea level rising, biodiversity loss and desertification aggravation. All these things described above have obviously influenced terrestrial ecosystem. CO₂ enrichment will directly affect green plants in ecosystem, and then caused changes in different levels of organization including ecosystem, community, population, individual, cell and molecule. In the entire marine food web, marine microalgae are the most direct responders to CO₂ enrichment. They not only drive the energy flow and matter cycle in marine ecosystem, fostering million tons of marine animals, but also play an important role in regulating global changes. In present study, experimental ecology studies were conducted on responses of six species of marine microalgae (Chlorella sp., Platymanas subcordiformis, Isochrysisgalbana Parke 8701, Skeletonema costatum, Dunaliella salina, Heterosigma akashiwo) to CO₂ enrichment with respect to interspecies, population, biochemical, physiological, molecule levels. Results showed that:1. Response of population growth of marine microalgae to CO₂ enrichmentAll the six species of microalgae represent responses to CO₂ enrichment. CO₂ enrichment (5000μl/L) could significantly promote population growth of marine microalgae (Chlorella sp., P. subcordiformis, I. galbana, S. costatum, D. salina and H. akashiwo), and shorten the time of entering the stationary phase. And maximum population densities increased remarkably (p < 0.05).Differences in sensitivity of microalgal population growth to CO₂ enrichment (5000μl/L) were observed among different microalgae, and the sensitivity was Chlorella sp. < D. salina < I. galbana < S. costatum < P. subcordiformis < H. akashiwo. H. akashiwo was the most sensitive, while Chlorella sp. was the most insensitive.2. Response of interspecies competition to CO₂ enrichment between microalgae in co-culture systemsCo-cultured H. akashiwo and S. costatum were researched to investigate the effect of CO₂ enrichment on interspecies between these. Results showed that the initiative inoculum density had significant effects on population growth of H. akashiwo and S. costatum. And time advances of entering exponential growth phase and stationary phase were observed with increasing in inoculum density for both microalgae. Correspondingly, the maximum population density decreased obviously.In co-culture system, S. costatum was always in predominance in the competition. And the dominance was promoted by increasing inoculum density. CO₂ enrichment could change the relationship of competition between H. akashiwo and S. costatum, and resulted in a reduction of competitive ability of H. akashiwo, while remarkably promoted the competition dominance of S. costatum.3. The responses of population competition to CO₂ enrichment in co-culture systemsCo-cultured P. subcordiformis, S. costatum and I. galbana were researched to investigate competitive balance among these and the effect of CO₂ enrichment on it. Results showed that under normal condition (air inflation), I. galbana was weak in competition and S. costatum was strong in competition, while P. subcordiformi showed a constant increase in population density. Therefore, in co-culture system, S. costatum took an advantage in competition, and finally became the dominant species. While P. subcordiformi became subdominant species and I. galbana were taken at a disadvantage.CO₂ enrichment treatment (5000μl/L CO₂ inflation) influenced the population dynamics of these three microalgae, and simultaneously influenced the competitive balance. Under CO₂ enrichment conditions, S. costatum showed a constant increase in population density and P. subcordiformi was strong in competition, while I. galbana was weak in competition. Therefore, in co-culture system, P. subcordiformi took an advantage in competition, and finally became the dominant species. While S. costatum became subdominant species and I. galbana were taken at a disadvantage.4. The response of macromolecular substance synthesis in marine microalgae to CO₂ enrichmentCo-cultured P. subcordiformis, Heterosigma akashiwo and Chlorella sp. were researched to investigate the effect of CO₂ enrichment on macromolecular substance synthesis. Results showed that the DNA synthesis represented the response to CO₂ enrichment representing as increase in synthesis speed and content of DNA. For these three microalgae, DNA synthesis in H. akashiwo was the most sensitive to CO₂ enrichment, and it more sensitive in P. subcordiformis than that in Chlorella sp.RNA synthesis represented a similar response to CO₂ enrichment. Protein synthesis in P. subcordiformis was the most sensitive to CO₂ enrichment representing as increase in synthesis speed and content of RNA. And RNA synthesis in H. akashiwo was more sensitive than that in Chlorella sp.Protein synthesis in microalgae showed different response to CO₂ enrichment. Protein synthesis in H. akashiwo was the most sensitive to CO₂ enrichment representing as increase in synthesis speed and content of protein. And protein synthesis in S. costatum was less sensitive to CO₂ enrichment, while that in Chlorella sp. had no obvious response.5. The biochemical and physiological responses of microalgae to CO₂ enrichmentCo-cultured S. costatum, H. akashiwo and Chlorella sp. were researched to investigate the effect of CO₂ enrichment on responses of physiological and biochemical activity. Results showed that photosynthetic rate, carbon fixation and carbonic anhydrase activity represented obvious response to CO₂ enrichment, compared with control groups (p < 0.05). CO₂ enrichment stimulated an increase in the activity of carbonic anhydrase, so increased the photosynthetic carbon fixation and photosynthetic efficiency and therefore indirectly promoted the increase of microalgae. The response of H. akashiwo was the most sensitive to CO₂ enrichment, and S. costatum was more sensitive than Chlorella sp.Nitrate reductase activity and absorption rate of N, P in microalgae represented obvious response to CO₂ enrichment, and were significantly different compared to control (p< 0.01). CO₂ enrichment stimulated an increase in the activity of nitrate reductase, so increased the N, P-absorption rate and therefore indirectly promoted the increase of microalgae. The sensitivity of P absorption rate was: H. akashiwo > S. costatum > Chlorella sp. And the sensitivity of N absorption rate was: S. costatum > Chlorella sp. > H. akashiwo. The sensitivity of nitrate reductase was: H. akashiwo > Chlorella sp. > S. costatum.Chlorophyll a contents in microalgae did not represent significant response to CO₂ enrichment (p > 0.05). Under CO₂ enrichment conditions, the content of chlorophyll showed no obvious difference compared with control groups (p > 0.05).