Competition between toxic Microcystis aeruginosa and nontoxic Microcystis wesenbergii with Anabaena PCC7120

To elucidate the changes in the proportions of microcystin (MC)-producing Microcystis, non-MC-producing Microcystis and Anabaena strains during cyanobacteria blooms, we compared their fitness under different initial biomass ratios. Culture experiments were carried out with three cyanobacterial strains: single-celled toxic Microcystis aeruginosa PCC7806 (Ma7806), single-celled nontoxic Microcystis wesenbergii FACHB-929 (Mw929) and filamentous Anabaena PCC7120 (An7120). Growth curves expressed as biovolume, Ma7806 microcystin-LR (MC-LR) content (detected with HPLC and ELISA), and the culture medium dissolved total nitrogen and dissolved total phosphorous (DTP) were measured to monitor nutrient uptake. Results suggest that the dominant strain in competition experiments between Ma7806 and An7120 was mainly controlled by the initial biomass ratio of the two strains, but there was also evidence for allelopathic interactions, where MC-LR produced by Ma7806 played an important role in the competition process. However, Mw929 was always less competitive when co-cultured with An7120 regardless of initial biomass ratio. Culture medium DTP showed significant differences between competition experiments in all sets, suggesting that Mw929 could be more suited to low phosphorus environments than Ma7806 and An7120. Overall, the competitive ability of Ma7806 was stronger than Mw929 when co-cultured with An7120 in the case of excess nutrients and the results could well unravel the seasonal succession process of cyanobacteria blooms.To elucidate the changes in the proportions of microcystin (MC)-producing Microcystis, non-MC-producing Microcystis and Anabaena strains during cyanobacteria blooms, we compared their fitness under different initial biomass ratios. Culture experiments were carried out with three cyanobacterial strains: single-celled toxic Microcystis aeruginosa PCC7806 (Ma7806), single-celled nontoxic Microcystis wesenbergii FACHB-929 (Mw929) and filamentous Anabaena PCC7120 (An7120). Growth curves expressed as biovolume, Ma7806 microcystin-LR (MC-LR) content (detected with HPLC and ELISA), and the culture medium dissolved total nitrogen and dissolved total phosphorous (DTP) were measured to monitor nutrient uptake. Results suggest that the dominant strain in competition experiments between Ma7806 and An7120 was mainly controlled by the initial biomass ratio of the two strains, but there was also evidence for allelopathic interactions, where MC-LR produced by Ma7806 played an important role in the competition process. However, Mw929 was always less competitive when co-cultured with An7120 regardless of initial biomass ratio. Culture medium DTP showed significant differences between competition experiments in all sets, suggesting that Mw929 could be more suited to low phosphorus environments than Ma7806 and An7120. Overall, the competitive ability of Ma7806 was stronger than Mw929 when co-cultured with An7120 in the case of excess nutrients and the results could well unravel the seasonal succession process of cyanobacteria blooms

RAINFALLS ACCELERATE THE DECLINE PROCESS OF MICROCYSTIS (CYANOPHYCEAE) BLOOMS

In Lake Chaohu, China, a vertical stratification of water temperature and dissolved oxygen (DO) was observed on sunny days, whereas it was not obvious on rainy days and DO was low in the whole water body. This indicates that rainfall can lead to a vertical convection and a low DO. In this study, we found that the density of small Microcystis colonies (&lt;100 mu m) was higher on rainy day than on sunny days, implying that rainfall had a mechanical impact on large colonies and broke them into small colonies. The measured results of algae collected on sunny day showed that oxygen removal in mucilaginous envelope, mechanical damage, and destruction of gas vesicles could significantly decrease the buoyancy of Microcystis colonies by 27.36 +/- 4.85% (p &lt; 0.01), 45.57 +/- 5.29% (p &lt; 0.01) and 18.23 +/- 3.24% (p &lt; 0.05) in one-way ANOVA, respectively. However, when algae sampled on rainy day were treated with above mentioned three methods, the buoyancy of Microcystis colonies only decreased by 6.72 +/- 2.77% (p &gt; 0.05), 33.37 +/- 4.02% (p &lt; 0.01), and 13.34 +/- 3.58% (p &lt; 0.05) in one-way ANOVA, respectively. These results suggested the rapid decrease in colony buoyancy on rainy days was not only due to the breakage of colonies but also due to the loss of oxygen bubble entrapped in mucilaginous envelope. Under simulated rainy day condition, the low DO in the water significantly decreased the photosynthetic activity, the up-floating velocity, and the growth rate of Microcystis colonies, which suggested that low-oxygen in some areas of lake could damage the photo-system of Microcystis and cause a bloom decline. The reculture of settled Microcystis colonies collected on rainy days showed that the recovery of photosynthetic activity and growth rate were very low, suggesting that most of the settled colonies could not act as seed source for the next bloom even though they were stirred and suspended in the water column after the weather conditions became suitable. Therefore, the physiological inactivity and the settling of Microcystis colonies under frequent heavy rainfall condition are the important reasons for the seasonal, rapid, irreversible collapse of severe algal blooms until the following year.In Lake Chaohu, China, a vertical stratification of water temperature and dissolved oxygen (DO) was observed on sunny days, whereas it was not obvious on rainy days and DO was low in the whole water body. This indicates that rainfall can lead to a vertical convection and a low DO. In this study, we found that the density of small Microcystis colonies ( 0.05), 33.37 +/- 4.02% (p < 0.01), and 13.34 +/- 3.58% (p < 0.05) in one-way ANOVA, respectively. These results suggested the rapid decrease in colony buoyancy on rainy days was not only due to the breakage of colonies but also due to the loss of oxygen bubble entrapped in mucilaginous envelope. Under simulated rainy day condition, the low DO in the water significantly decreased the photosynthetic activity, the up-floating velocity, and the growth rate of Microcystis colonies, which suggested that low-oxygen in some areas of lake could damage the photo-system of Microcystis and cause a bloom decline. The reculture of settled Microcystis colonies collected on rainy days showed that the recovery of photosynthetic activity and growth rate were very low, suggesting that most of the settled colonies could not act as seed source for the next bloom even though they were stirred and suspended in the water column after the weather conditions became suitable. Therefore, the physiological inactivity and the settling of Microcystis colonies under frequent heavy rainfall condition are the important reasons for the seasonal, rapid, irreversible collapse of severe algal blooms until the following year

DYNAMICS OF CHLOROPHYLL FLUORESCENCE AND ECO-MORPHOLOGICAL PROPERTIES OF Microcystis BLOOM IN MEILIANG BAY OF LAKE TAIHU, CHINA

Eutrophication has become a serious concern in many lakes, resulting in cyanobacterial blooms. Some variations of physiological activity and eco-morphological property in Microcystis bloom, especially in field experiment, are currently less studied. This study aimed to describe what changes of physiological activity and eco-morphological property had happened, and identify what is the key factor resulting in the decline of Microcystis bloom. The chlorophyll fluorescence, buoyancy and colony sizes of Microcystis and variation of physico-chemical data in the Meiliang Bay estuary of Lake Taihu, China, was investigated everyday from July 18, 2009 to July 29, 2009. In field investigation, F(v)/F(m) Phi(PSII) and qP can regarded as effective indicators to evaluated physiological activity and the growth state of Microcystis bloom. We found on sunny days, photoinhibition occurred 2 hours later than that on cloudy days, but photodamage indicated by unrecoverable decreasing F(v)/F(m), was observed after exposure to high light for a long time. While on cloudy days, the photoinhibition of Microcystis seemly occurred more easily and recovered quickly at weak light. Controlling experiment showed that nutrient depletion could lead to decreased buoyancy, dis-aggregated colonies and dropping photosynthesis of Microcystis. Based on these results of high light, rich DO, a negative correlation between DO and F(v)/F(m), and a lagged photoinhibition on sunny days, we inferred Mehler's reaction was a protective mechanism of photosystem for a short time, but results in photodamage when exposure is long enough. In all, we conclude the long-term illumination of high light companied with nutrient depletion caused by rapid growth of Microcystis might be the main reasons for the bloom decline