Effects of different aqueous DO, redox potential and salinity on cyanobacterial colonies from Lake Taihu, China

Abstract

Abstract In summer, cyanobacteria accumulate at the water surface and form surface blooms when the water column is stable. Cyanobacterial photosynthesis results in oxygen supersatuation during daytime and a great redox potential difference is created between day and night. Cyanobacteria usually exist in colonies in the field, but the microenvironment within colonies is not as easily detected. An accurate analysis of physicochemical properties within the colonies is important to a better understanding of cyanobacterial growth characters and the formation mechanism of cyanobacterial blooms. To investigate the influence of aqueous DO, redox potential and salinity on the characteristics of cyanobacterial colonies from Lake Taihu, dissolved oxygen (DO) microelectrode was used to study the physiological responses in the colonies at different initial aqueous DO, redox potential and salinity. The results showed that DO of the colonies increased with increasing initial aqueous DO level, the highest intercellular DO was achieved at an initial aqueous DO of 1000 µmol L− 1. The maximal net photosynthesis (Pn) and dark respiratory rate (Rdark) was achieved in the colonies incubated at an initial aqueous DO of 100 µmol L− 1 and 1000 µmol L− 1, respectively. DO levels of the colonies decreased with decreasing aqueous redox potential and they decreased with increasing salinity, the highest DO achieved at an redox potential of 0 mV and the lowest DO achieved at a salinity of 8 mg L − 1. The maximal Pn in the colonies was achieved at an redox potential of 0 mV. Abundant oxygen was produced within the colonies in the light and it escaped from the interior of the colonies and then elevated the aqueous DO level. Elevated aqueous DO level in turn promoted the DO production within the colonies. Eventually, a large number of oxygen bubbles were formed and attached to the colonies surface, which could provide a great extra buoyancy for the colonies. All of the physiological responses of the microenvironment within the colonies favor cyanobacteria to be the dominant bloom-forming species and eventually result in water blooms.