Mechanism Of Recruitment-promoting Of Dormant Microcystis Aeruginosa By Three Benthic Bacterial Species

Microcystis aeruginosa(M.aeruginosa)is a cyanobacteria that is easy to form blooms in eutrophic water.Frequent outbreak of M.aeruginosa blooms is closely related to its life history and physiological and ecological characteristics.When the environment is stressed,M.aeruginosacells will form nutrient dormancy and sink to the surface of the sediment and pass through the harsh environment.When the environment is suitable,the dormant M.aeruginosacell starts to recruit from the sediment into the water body again.The recruitment of dormant M.aeruginosacell is the key link of the outbreak of blooms.It is necessary to study the mechanism of recruitmentand control the outbreak of blooms from the source.This paper focuses on Lake Chongtian(West Dongting Lake waters),which forming frequently spring microalgae bloom.In this lake,the mechanism of the dominant Microcystis,the dominant bacteria groups,the bacteria groups on the recruitment of the dormant Microcystisand recruitment model were studied by way of the field investigation and the indoor recruitment simulation experiments.The mechanism of the frequent outbreak of the spring M.aeruginosablooms in Lake Chongtian was discussed.The main results are as follows:(1)Dominant Microcystis in Lake Chongtian were Microcystis aeruginosa,Microcystis flos-aqua,and Microcystis wesenbergii.Dominant bacterial flora in sediment and in overlying water column mainly were Exiguobacterium,Pseudomonas,and Bacillus,which all can promote to some extent recruitment of dormant Microcystis aeruginosa,Microcystis flos-aqua and Microcystis wesenbergii from upper sediment.(2)One strain of Exiguobacterium(E.sp013)and two strains of Bacillus(Ba.spD06 and Ba.spD24)were used to perform recruitment simulation experiments.Three sediment strains all increased gradually during the first 12 days and consumed the dissolved oxygen in the sediment-water interface(SWI),andSWI was in a low dissolved oxygen station.The concentration of dissolvable inorganic nitrogen(DIN)and soluble reactive phosphorus(SRP)was reduced,and the decrease rate of soluble inorganic nitrogen was faster than that of active phosphorus,and the ratio of nitrogen and phosphorus(N:P= DIN: SRP)decreased.The results showed that the three strain promoted significantly the recruitment of dormant Microcystis aeruginosacells by decreasing the dissolved oxygen concentration and nitrogen and phosphorus ratio(N:P)in SWI water,even at low temperatures.The results of gene differential expression analysis showed that the decrease of N:P in SWI water for bacteria groupstrainedabout 7d could stimulate the expression of ntcA gene in the dormant M.aeruginosa cell and enhance the nitrogen(N)metabolic capacity of the dormant M.aeruginosa cell.(3)At lower temperature(10°C-15°C)three strains E.sp013,Ba.spD06,Ba.spD24 and mixed bacteria can significantly improve the Fv / Fm of the photosynthetic system of Microcystis aeruginosa,and the enhancement ability of the mixed bacteria group is stronger than that of the single bacteria group Ba.spD06,Ba.spD24 and the control group.There was no significant difference between the treatment group and the control group at higher temperature(20°C-25°C).By KEGG enrichment analysis of differentially expressed genes,it was found that the psaB and psbA protein gene,whichwas related photosynthesis genes of dormant M.aeruginosa was significantly up-regulated(UP)at 15 °C.This may also be the main reason for frequent outbreak of M.aeruginosablooms in spring(at low water temperature)for Lake Chongtian.(4)The metabolic products were detected by gas chromatography-mass spectrometry(GC-MS),and the differences of the metabolites in the treatment group and the control group were investigated.The effects of the metabolites on the recruitment of M.aeruginosa in the treatment group were analyzed.35 kinds of metabolites were detected from the culture of the recruitment experiment,of which 10 kinds of metabolites could only be detected from treatment groups,and 8 were verified to promote the recruitment of dormant Microcystis aeruginosa.They are bromoacetic acid ethyl ester;3-methyl-pentadecane;1-octadecanol;4-chloro-2-nitrobenzyl alcohol;butylated hydroxytoluene;2-butylated hydroxytoluene;2,4-bis(1,1-dimethylethyl)-phenol;and 1,2-benzenedicarboxylic acid,diisooctyl ester.Most of the metabolites could promote the recruitment of Microcystis aeruginosa dormant under low concentration.(5)In this study,Temperature,light and dominant bacteria are considered as the main factors affecting the recruitment of dormant M.aeruginosa,and the fuzzy sets of T-S fuzzy theory are used to describe the inaccurate and uncertain factors of three factors,and to reduce the complexity of the modeling and establish a recruitment model.These experimental results show that prediction error for the model proposed are less than 5% of the total recruited amount by this study,when the fluctuation of surface sediment temperature was less than 5°C.When the temperature fluctuation reaches 10-15°C degrees,the prediction error is too large to reach the predictive purpose.