The effect of gradient cooling on bloom decline in Microcystis (Cyanophyceae, Chroococcales)

Abstract:

Microcystis blooms occur frequently in many eutrophic lakes during periods of warm temperature and disappear when the temperature becomes lower. Seasonal decreases in water temperature might be a key factor leading to the decline of Microcystis blooms. However, to date, there have been few studies investigating the effects of decreasing temperature on Microcystis in lakes. Our present study found that the gradually decreased temperature could result in a decline in Microcystis biomass. In the initial experimental phase (> 22°C), Microcystis colonies could acclimate to the decreasing temperature through regulation of pigment content and photosynthetic activity. When the temperature reached 18°C, the photosynthetic activity (photochemical quenching, maximum electron transport rate and half-saturated light intensity) significantly decreased, and excess excitation energy, as represented by nonphotosynthetic quenching, gradually increased. Photodamage to the Microcystis photosystem might be alleviated through accumulation of carotenoids. In addition, as temperature decreased, Microcystis colonies gradually disaggregated, and flotation velocity decreased rapidly. Our results indicated that temperature decrease did significantly inhibit the photosynthetic activity of Microcystis, which could lead to decreased algal biomass. Since decreasing temperature in later autumn and/or winter is a common phenomenon, we conclude this seasonal cooling is an important reason for the disappearance of Microcystis blooms.

Key Words::

• Bloom decline
• Chlorophyll fluorescence
• Gradient cooling
• Microcystis

ACKNOWLEDGEMENTS

This study was jointly supported by the National Science Foundation of China (41230748; 31300391), the State Key Laboratory of Freshwater Ecology and Biotechnology (2014FBZ01) and the Major Science and Technology Program for Water Pollution Control and Treatment (2012ZX07103003-02).