Effects of different initial pH and irradiance levels on cyanobacterial colonies from Lake Taihu, China

Cyanobacteria usually appear in colonies on the surface of lakes, but the microenvironment inside colonies is not as easily detected. An accurate analysis of microenvironment properties within the colonies is key to a better understanding of the formation mechanism of cyanobacterial blooms. To understand the influence of irradiance and pH on the characteristics of cyanobacterial colonies from Lake Taihu, dissolved oxygen (DO) and pH microelectrodes were used to investigate physiological responses within these colonies and in the motionless water blooms at different irradiances and initial pH levels. The results showed that DO and pH increase with increasing irradiance, causing a dynamic alkaline environment to form inside these colonies. The maximum pH varies from 9 to 9.5 at all initial pH readings and the highest DO was achieved in the colonies incubated at an initial pH of 9. The maximum net photosynthesis (P (n)) and dark respiratory rate (R (dark)) were achieved in the colonies incubated at an initial pH of 8 and 6, respectively. The maximum pH differences were lower in colonies incubated at an initial pH of 10 compared with those incubated at a pH of 6 to 9. Photosynthesis of the colonies raised the aqueous pH to about 10.5, which is similar to the value found inside the colonies. In the motionless water bloom layer, the maximum pH varies from 10 to 10.5 at all initial pH levels and both the highest DO and pH values were achieved at an initial pH of 10. Cyanobacterial photosynthesis first created an alkaline microenvironment in the colonies and then increased the aqueous pH. This elevated aqueous pH promotes photosynthesis of the colonies and further increases the aqueous pH until it is higher than 10. Abundant oxygen bubbles attached at the colonies surface provide extra buoyancy for the colonies. An anaerobic environment forms at 3 to 4 cm depth under the bloom surface, aggravating the outbreak of cyanobacterial bloom. All these physiological characters of microenvironment in cyanobacterial colonies and water blooms favor cyanobacteria as the dominant water bloom species in eutrophic water.