Effects Of Natural Organic Matter On Colony Formation Of Microcystis

Bloom of the toxin-producing cyanobacterium Microcystis is a serious ecological and environmental problem worldwide.Colony formation is a key process for the occurrence of Microcystis blooms.Restraining colony formation of Microcystis and reducing its colony size are more practical to control algal blooms.Natural organic matter,as a kind of organic matter without serious ecological harm,is widely distributed in nature.Recently,some researchers have accidentally discovered that some natural organic matter can promote colony formation of Microcystis.Because natural organic matter is widespread in lakes and reservoirs and can be indirectly controlled by watershed management,it is necessary to carry out systematic research on the influence of natural organic matter on colony formation of Microcystis.Inducing single-celled Microcystis to form colony in the laboratory is the basis for the study of natural organic matter inhibiting colony formation of Microcystis.This study first established an experimental system for increasing the iron concentration to induce colony formation of Microcystis,and analyzed the effect of increased iron concentration on the EPS synthesis and gene expression of Microcystis through the transcriptome.In this experimental system,the effects of humic acid,plant allelochemicals（gramine,catechol,berberine and sanguinarine）,and terrestrial dissolved organic matter（gallic acid,proline and tea polyphenols）on colony formation of Microcystis were studied.The effect and mechanism of natural organic matter on colony formation of Microcystis were explored by measuring the number of Microcystis cells,Fv/Fm,colony size and extracellular polysaccharide content,combined with transcriptome analysis.The main conclusion of this study are as follows:（1）The results showed that the extracellular polysaccharides（EPS）content and average colony size reached 0.93 pg cells^-1 and 26.1μm,respectively,while iron concentration increased to 6.8 mg L^-1,suggesting that high level of iron stimulated EPS secretion and induced unicellular Microcystis to form colonies.Transcriptome analysis showed that two genes described as glycosyltransferases（BH695-2217 and BH695-3696）were significantly up-regulated while EPS content increased with increasing iron concentration indicating that iron may regulate the expression of genes involved in polysaccharide synthesis to promote polysaccharide synthesis.（2）When treated with 10 mg C L^-1 humic acid at high level of iron,the EPS content and average colony size decreased by 35.48%and 56.31%,respectively,revealing that humic acid inhibited EPS secretion under high level of iron condition,and ultimately inhibited colony formation of Microcystis.Transcriptome analysis showed that two genes described as glycosyltransferases（BH695-2217 and BH695-3696）were significantly down-regulated while treated with 10 mg C L^-1 humic acid at high level of iron indicating that the expression of genes involved in polysaccharide synthesis may be inhibited.Therefore,humic acid could be used as an agentia inhibiting large colony formation of Microcystis and thereby reducing the occurrence of Microcystis blooms.（3）Gramine,catechol,berberine and sanguinarine could significantly inhibit the growth of Microcystis aeruginosa.When treated with 0.6 mg C L^-1 gramine,2 mg C L^-1 catechol and 0.2 mg C L^-1 berberine at high level of iron,the average colony size decreased from16.70μm to 5.30μm,7.29μm and 3.95μm,respectively.The average colony size increased from 16.70μm to 19.72μm while treated with 0.004 mg C L^-1 sanguinarine.In addition,a significant positive relationship was found between EPS content and D50 of M.aeruginosa in the group with high level of iron,suggesting EPS are the basic material for colony formation of Microcystis and play an important role in colony formation of Microcystis.（4）Gallic acid and tea polyphenols could significantly inhibit the growth of Microcystis aeruginosa.When treated with 2 mg C L^-1 gallic acid and tea polyphenols at high level of iron,the average colony size decreased from 23.88μm to 4.22μm and 4.29μm,respectively.Proline promoted the growth of Microcystis and had no obvious effect on the colony formation of Microcystis.Previous studies mainly focused on the inhibitory effect of terrestrial dissolved organic matter on the growth of Microcystis,while ignoring the influence on the colony formation of Microcystis.This study found that terrestrial dissolved organic matter significantly inhibited the growth and colony formation of Microcystis,which provided a new idea for the management of cyanobacteria blooms.