|Other Abstract||Cyanobacterial blooms have been a worldwide serious environmental problem in water bodies due to the increasing eutrophication for decades. Severe cyanobacterial blooms reduced the availability of water resource, causing serious ecological damage and gigantic economic losing. Continuous efforts for cyanobacterial bloom control have been made in the past in China and other countries of the world. Many factors associated with cyanobacterial bloom occurrence in lakes have been investigated. However, we are still faced with the hard task to control the harmful cyanobacterial blooms in freshwater bodies under the changing environmental conditions in association with global warming. Bicarbonate (HCO3-) is an important external source of inorganic carbon for photosynthesis in cells of planktonic algae. Several reports have indicated that high concentrations of HCO3- inhibit uptake of other nutrient ions and decrease plant growth. It is well known that HCO3- is the prominent dissolved inorganic carbon (DIC) formed during algal blooming. Therefore, it is important to evaluate the effect of HCO3- on the growth and physiological activities of the dominant bloom forming cyanobacterial species such as M. aeruginosa and Aphanizomenon flos-aquae. In this study, ecophysiological effects of DIC on the bloom-forming cyanobacteria were studied. The main results are showing as the followings:
1） Aphanizomenon flos-aquae bloom appeared regularly in January in the recent years in Lake Dianchi. In order to investigate the response of A. flos-aquae to elevated atmospheric CO2 concentration, an experiment was carried out. Effects of elevated atmospheric CO2 on the physiological and biochemical characteristics of A. flos-aquae were determined. Results indicated: ○1A.flos-aquae could growth well at 200 ppmv CO2 with CO2 concentrating mechanism (CCM). Low concentration of CO2 decreased the intensities of photosynthesis and respiration. ○2The growth rate of A. flos-aquaedid not increased when culture medium was bubbled with enriched CO2. ○3Salt stresses caused decrease the growth of A. flos-aquae by disrupting the cell physiological processes, especially the photosynthesis. Malondialdehyde (MDA) content increased following the salinity enhancement. Growth rate, photosynthetic activity and the ratio of photosynthetic pigment chlorophyll a to phycocyanin (Chla/PC) could be enhanced by elevated CO2 concentration. ○4Increased CO2 levels could also play a crucial role in A. flos-aquae’s response to oxidative stress．
2） Under elevated CO2 concentration, the growth and photosynthesis of A. flos-aquae, Scenedesmus obliquus, and Peridinium pusillum were characteristically and obviously different. When CO2 concentration increased, S. obliquu and A. flos-aquae displayed higher growth rates. Elevated CO2 concentration had no significant effect on the growth of P. Pusillum.
3） The influence of bicarbonate (HCO3-) on Microcystis aeruginosa FACHB 905 was assessed. M. aeruginosa could grow well in 0.6~11.9 mM HCO3-. No dose-effect relationship between the content of HCO3- and the growth rate of M. aeruginosa above 3.5 mM HCO3- was found.
4） In order to access the effects of HCO3- on M. aeruginosa, investigations were performed at 2.3 mM and 12.4 mM HCO3-; treatments with sodium chloride (NaCl), variant pH, and K+ were conducted alongside. It was found that upon treatment with elevated HCO3- concentrations of 2.3mM and 12.4mM, cell densities were 13% and 27% higher than the control respectively. In the photosynthetic performance, elevated HCO3- concentration initially stimulated Fv/Fm at the prophase of culture and then subsequently inhibited it. The inhibition of 2.3mM was higher than that of 12.4mM HCO3-. The maximum relative electron transport rate (ETRmax) displayed an inhibition at elevated HCO3- concentrations. DI0/CS decreased at 2.3 mM and increased at 12.4mM. In both treatments, ABS/CS, TR0/CS, ET0/CS, RC/CS0 and RC/CSm decreased by elevated HCO3- concentrations, which indicated damages in the photosynthetic apparats and/or an inactivation in a fraction of the reaction center. This point of results was also proven by ultrastructural observation. Cells in high HCO3- exposure lost their characteristic arrangement of photosynthetic membrane, in comparision with the control and the high salinity treated samples. At 2.3mM concentration of HCO3-, photosynthetic activity decreased due to the damage of photosynthetic apparats. These findings suggested that elevated HCO3- concentration stimulated the growth and photosynthesis of M. aeruginosa in a short time. Exposure to high HCO3- concentrations for a longer period of time will damage photosynthetic apparatus. In addition, results from the ultrastructure observation indicated that elevated HCO3- concentration led to photosynthetic apparati damage. According to the observation of that, the inhibition effect of 2.3mM HCO3- was higher than that of 12.4mM HCO3-. We hypothesized that M. aeruginosa induced a protective mechanism under high concentrations of HCO3-.
5） Effects of HCO3- on the photosynthetic performance of Chlorella pyrenoidosa, a green alga, were investigated. Photosynthetic activity, pigments, MDA and ultrastructure of C. pyrenoidosa were measured in cultures under conditions of two HCO3- concentration, 2.3 mM and 12.4 mM, respectively. At 2.3 mM, photosynthetic activity was stimulated at the prophase of culture and then was inhibited at the anaphase. At 12.4 mM, the same phenomenon was observed, but the inhibition effects were lower than those at 2.3 mM. Ratios of Chlb/Chla increased, but caro/TChl ratios decreased at elevated DIC. The contents of MDA increased at 2.3 mM and decreased at 12.4 mM. In addition, secretion of glycocalix-like layer was remarkably stimulated at elevated DIC. These results showed that elevated DIC promoted the carbon assimilation and stimulated secretion of glycocalix-like layer of C. pyrenoidosa. Carbohydrate could alleviate stresses at elevated DIC. Secretion of glycocalix-like layer could reduce the synthesis of excess photosynthetic products and relieve feedback-inhibition, which will increase the stress tolerance of C. pyenoidosa at elevated DIC. Compared with M. aeruginosa, elevated HCO3- is more detrimental to M. aeruginosa than to C. pyrenoidosa. This evidence supported the hypothesis that HCO3- plays an important role in collapse of the algal blooms and in the species succession of algal bloom.
6） Lake Dianchi is about 300 square kilometers (74,132 acres) in area. It is the largest freshwater lake in Yunnan Province and the so-called sixth largest one in China, famous due to its picturesque scenery and its location on the Yungui Plateau. Cyanobacterial bloom occurred heavily and annually in Lake Dianchi. Foe investigation on the relationship between DIC in situ and the occurrence of cyanobacterial blooms, experiments were conducted in lake Dianchi through January to July 2007. Results showed that the HCO3- and Chl.a concentration all varied with seasonal changes and significant correlations between HCO3- and Chl.a were found (r=0.84, p<0.01). During the period of investigation, the most significant environment change was wind, which resulted in an accumulation of the superficial algael biomass on the leeward area and windward lakeshore. In combination with the historical records, we considered that DIC played a role of regulating the succession of the dominant phytoplanktonic cyanobacterial species.|