To increase size or decrease density? Different Microcystis species has different choice to form blooms.

Ming Li,Huaimin Chen,Man Xiao,Wei Zhu,Jing Hu,Lili Guo

doi:10.1038/srep37056

Ming Li, Huaimin Chen + Show 4 more

Open Access

https://doi.org/10.1038/srep37056

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Abstract

The buoyancy of Microcystis colonies is a principal factor determining blooms occurrence but the knowledge of seasonal variation in buoyancy is quite poor because of challenge in analysis method. In this study, a method based on the Stokes’ Law after researching on the effects of shapes on settling velocity of Microcystis colonies, whose gas vesicles were collapsed, to accurately measure density was established. The method was used in Lake Taihu. From January to May, mean density of Microcystis colonies decreased from 995 kg m−3 to 978 kg m−3 and then increased to 992 kg m−3 in December. The density of colonies in different Microcystis species was in the order M. wesenbergii > M. aeruginosa > M. ichthyoblabe. For all the Microcystis species, the density of colonies with gas vasicles increased significantly along with the increase of colony size. Our results suggested that the main driving factor of Microcystis blooms formation in Lake Taihu was low density for M. ichthyoblabe from May to July but was large colony size for M. wesenbergii and M. aeruginosa from August to October.

Highlights

The buoyancy of Microcystis colonies is a principal factor determining blooms occurrence but the knowledge of seasonal variation in buoyancy is quite poor because of challenge in analysis method
Nakamura et al reported that the regression between floatation velocity and colony size was not a quadratic relationship but linear in most cases[15]
The floatation velocity of M. ichthyoblabe colonies was much higher than colonies of both M. aeruginosa and M. wesenbergii if all the colonies were in the same size

Summary

Introduction

The buoyancy of Microcystis colonies is a principal factor determining blooms occurrence but the knowledge of seasonal variation in buoyancy is quite poor because of challenge in analysis method. A method based on the Stokes’ Law after researching on the effects of shapes on settling velocity of Microcystis colonies, whose gas vesicles were collapsed, to accurately measure density was established. Our results suggested that the main driving factor of Microcystis blooms formation in Lake Taihu was low density for M. ichthyoblabe from May to July but was large colony size for M. wesenbergii and M. aeruginosa from August to October. Microcystis could regulate buoyancy by collapse and synthesis of gas vesicles as well as accumulation and consumption of photosynthetic products such as polysaccharide and protein[8] This function provided an essential factor for floatation and blooms formation of Microcystis. This method could only be used to measure particles whose density is larger than water This means this method could not analyze Microcystis colonies forming blooms with gas vesicles. Both of these results implied that the above two assumptions would be not appropriate

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