Relationship between extracellular polysaccharide (EPS) content and colony size of Microcystis is colonial morphology dependent

https://doi.org/10.1016/j.bse.2014.04.009Get rights and content

Highlights

  • The relationship between EPS content and Microcystis colony size in Lake Taihu was analyzed.

  • Relation between EPS content and colony size of Microcystis is colonial morphology dependent.

  • The maximum EPS content of Microcystis wesenbergii and Microcystis aeruginosa appeared in the size class 100–150 μm.

  • The EPS content of Microcystis flos-aquae increased along with an increase in colony size.

Introduction

Large colony formation is important to the occurrence of blooms of Microcystis (Yamamoto et al., 2011). Large colonies migrate more quickly than small ones (Nakamura et al., 1993); and are better able to float to the surface and form blooms (Wu and Kong, 2009). Colony formation is also a strategy to resist predation by zooplankton (Cyr and Curtis, 1999, Yang et al., 2006, Yang et al., 2009). Furthermore, colony morphology was also helpful in reducing the growth inhibition of Microcystis from Microcystis inhibiting substances (Wu et al., 2007, Park et al., 2009) and high light intensity (Wu et al., 2011, Zhang et al., 2011). The mechanisms behind Microcystis colony formation are important in understanding the formation of Microcystis blooms.

Colonial Microcystis lose their colonial morphology and exist as free-living cells in axenic laboratory cultures following long-term cultivation (Zhang et al., 2007). To reveal the mechanisms of Microcystis colony formation, the effects of different environmental factors on colony formation of Microcystis have been extensively investigated. Previous research has demonstrated that both biotic and abiotic factors and their combined effects could induce colony formation in Microcystis (Wang et al., 2010). These factors included predation by zooplankton (Yang et al., 2008), stimulation by heterotrophic bacteria (Shen et al., 2011) and Cylindrospermopsis raciborskii (Mello et al., 2012), temperature, light intensity (Li et al., 2013a), nutrient concentration (Yang and Kong, 2013), calcium level (Wang et al., 2011) and heavy metals (Bi et al., 2013). However, the morphology of the colonies formed in the above studies still differed from that of the colonies existed under natural conditions.

Extracellular polysaccharides (EPS) cohere single algal cells into colonies (Plude et al., 1991) (Kessel and Eloff, 1975). The production of EPS was related to the colony formation in algae, such as Chlorella pyrenoidosa (Yang et al., 2010) and Scenedesmus obliquus (Liu et al., 2010). An increase in EPS content was reported to be important to Microcystis colony formation (Yang et al., 2008). A significant relationship between EPS content and Microcystis colony size was also observed (Liu et al., 2011, Li et al., 2013a). Many scientists believed that the cellular EPS content of large colonies would be much higher than that of smaller ones (Xu et al., 2013). Since colonies obtained in the laboratories differ from those under natural conditions, it is important to determine whether a significant positive relationship between EPS content and Microcystis colony size exists in the environment. To answer this question, we analyzed the relationship between EPS content and Microcystis colony size in Lake Taihu, a well-studied eutrophic shallow lake in China.

Section snippets

Sample collection

Sampling was carried out in Microcystis bloom areas in Meiliang Bay of Lake Taihu, China, from July to November 2012. This area (coordinates: 31° 24′–31° 28′N; 120° 10′–120° 12′E) was previously described by Li et al. (2014a). The samples were collected at a depth of 30 cm below the lake surface into 1 L plastic bottles, and immediately fixed with formalin [2% (v/v)] for the laboratory analysis.

Microscopic examination

Samples were shaken thoroughly, and photomicrographs of the samples were taken using a digital camera

Results

A total of ten samples were collected and divided into three groups depending on Microcystis colonial morphology. Table 1 shows the proportion of colonies in each size class and in different colonial morphologies for each sample. Sample A was dominated by M. wesenbergii with most colonies larger than 150 μm. Colony size in sample B was similar to sample A, but M. flos-aquae made up 98.3% of the sample. Sample C was evenly split between M. flos-aquae and M. aeruginosa.

Fig. 1 shows the EPS

Discussion

Our previous investigation illustrated that Microcystis accounted for more than 90% of algal biomass in Lake Taihu from July to November (Li et al., 2013b, Li et al., 2014a). Consequently, TOC is considered as an appropriate indicator for Microcystis biomass in the current study and the diversity of EPS content of Microcystis in Lake Taihu can be interpreted by the value of EPS/TOC.

Many researchers found a positive correlation between EPS content and Microcystis colony size in cultures (Liu

Acknowledgments

This study was sponsored by the National Program on Key Basic Research Project of China (2012CB719804), the Natural Science Foundation of Jiangsu Province (BK2011025), and the Hydraulic Science & Technology Project of Jiangsu Province (2012012).

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