Response Of Microcystis Associated Bacterial Community To Nitrogen And Phosphorus Limitation

In recent years,cyanobacterial blooms caused by eutrophication have frequently erupted in freshwater lakes such as Taihu Lake and Dianchi Lake in China,which have become a serious environmental problem.Discharge of excess nitrogen and phosphorus into the freshwater system can trigger eutrophication of water bodies.In eutrophic water bodies,bacteria are closely associated with Microcystis,and they can affect the formation and decline of Microcystis blooms.In order to understand how Microcystis-associated bacteria respond to the different cultivation conditions and Microcystis strains,we monitored the bacterial community structure in different Microcystis cultures,at different cultivation stages,and under the nitrogen-or phosphoruslimiting condition.and its possible impact on Microcystis.Three strains of Microcystis aeruginosa(PCC7806,FACHB912,and FACHB930)which represent 3 different phylotypes were used in this study.The main results are summarized as follows:1.The bacterial community structure of three different strains of Microcystis FACHB912,FACHB930 and PCC7806,was similar in the same culture system.The nitrogen and phosphorus nutrient factors of Microcystis culture system were the important factors affecting the difference of community structure.2.At the initial stage,bacterial species in the cultures of Microcystis FACHB912,FACHB930 and PCC7806 were different at the order level.In the PCC7806 system,Pseudomonadales,Rhizobiales and Cytophagales were the main taxa;Pseudomonadales dominated the FACHB912 system;In the FACHB930 system,Pseudomonadales was also dominant3.When Microcystis strains were cultivated in the nitrogen-limited condition for more than 90 days,Pseudomonadales,Rhodobacterales,Caulobacterales and Cytophagales were the main bacteria in the PCC7806 system.In the FACHB912 system,Sphingomonadales,Caulobacterales,Rhodobacterales and Xanthomonadales were dominant.The FACHB930 system was mainly composed of Pseudomonadales,Sphingomonadales and Cytophagales.In the phosphorus limited system,Burkholderialesand Saprospirales were dominant in the PCC7806 system.The FACHB912 system was mainly composed of Pseudomonadales,Rhodobacterales,and Caulobacterales.Rhizobiales and Saprospirales made up the majority of bacterial community in the FACHB930 culture.4.The associated bacteria community structures of the three Microcystis under different conditions are different.The bacterial community associated with each Microcystis strain changed with culture time.When PCC7806 was grown in the nitrogen-limited condition.Alphaproteobacteria became less abundant,while the relative abundance of Gammaproteobacteria gradually increased.In the phosphorus-limited PCC7806culture,the change of Alphaproteobacteria and Gammaproteobacteria followed the opposite trend as that in the nitrogen-limited condition.At the order level,Pseudomonadales was the dominant group in the nitrogen--limited PCC7806 culture,and Rhizobiales was the dominant group in the phosphorus-limited PCC7803 culture.In the nitrogen-limited FACHB912 system,as the cultivation time increases,the relative abundances of Alphaproteobacteria and Cytophagia in the nitrogen-limited system are significantly higher than the phosphorus-limited conditions,while Gammaproteobacteria and Saprospirae are significantly lower than the phosphorus-restricted conditions;at the order level.Xanthomonadales,Caulobacterales and the relative abundance of Sphingomonadales was significantly higher than that of normal culture system and phosphorus-restricted system;Pseudomonadales dominated the phosphorus system.In the FACHB930 system,the relative abundance of Alphaproteobacteria and Saprospirae in the nitrogen-restricted bacterial community at the end of the culture was significantly lower than the phosphorus-restricted conditions,while Cytophagia and Gammaproteobacteria were significantly higher than the phosphorus-restricted conditions;at the order level,Pseudomonadales and Pseudomonadales in the nitrogen-restricted system Cytophagales dominate,and the relative abundance of phosphorus-restricted systems Rhizobiales,Saprospirales.Burkholderiales and Rhodospirillales are significantly higher than those of normal culture systems and nitrogen-restricted systems.