| The buoyancy and buoyancy regulation are very important to the dominance of blue-green algae.Considering the characteristics of different morphological cyanobacteria, two methods includingfiltration and centrifugation were applied here to concentrate algal suspensions. Using the method ofcapillary compression tube and the concentrated algae suspensions, critical pressure distribution of gasvesicles in six cyanobacteria that isolated from three shallow lakes in different area were alsoinvestigated. To investigate the interactions of gas vesicles and light; absorption spectra and turbidity ofcyanobacteria suspension with intact and collapsed gas vesicles was analyzed and absolute gas vesiclevolume was determined. Under the conditions of nitrogen-limit, phosphorus-limit and differenttemperature, the response of buoyancy regulation to nitrogen, phosphorus and temperature incyanobacteria were also investigated.The results showed that:1. The apparatus' lower limit of detection (LLD) reached to 0.0018μL·ml-1 cells suspension, andrelative standard deviation of measurement under 1％. Both filtration and centrifugation couldn'tconcentrate cells suspension effectively for unicellular M. aeruginosa, centrifugation and filtration wereeffective ways to concentrate colonies Microcystis wesenbegii and filiform Planktothrix mougeotiirespectively. Two preserved methods were also carried out to study the effects of cold storage andpreservation by adding Lugol's reagent on amount and gas vesicles content of cells, the amount of threealgae cells didn't lose obviously after cold storage and preservation by adding Lugol's reagent for oneweek. During the processes of concentration and preservation, losses of gas vesicle contents of cellswere negligible.2. Gas vesicles interact with light of different wavelength light in different ways. Red light with longwavelength penetrated cyanobacteria suspension more easily than short-wave light did, and the effectswere more significant in colonial and filamentous cyanobacteria. In colonial and filamentouscyanobacteria, measurement of relative gas vesicles volume by optical methods may resulted in greaterrors due to light multi-ply scattering in colony an filaments, however, capillary pressure tube methodis convenient to measure absolute volume of gas vesicles because colonial and filamentouscyanobacteria can be easily concentrated.3. The mean critical pressure values of five strains Microcystis were very close, which ranged from0.63～0.65 MPa, and the mean critical pressure of P. mougeotii was 0.715MPa; At the same condition of illumination density and temperature, turgor pressures of two unicellular Microcystis were higher thanthose of colonial Microcystis.4.When temperature decrease form 28℃to 13℃, the buoyancy of both algae decrease, moreover,the content of cellular carbohydrates of M. flos-aquae and P. mougeotii became 2.2 times and 2.6 timesof the initial content within 72 h cultivation while gas vesicle and protein content changedinsignificantly. As a result, cells density increased and the buoyancy lost. Therefore, M. flos-aquae mayaccess dormancy phase when temperature were lower than 13℃while P. mougeotii became benthic algaand continue to growth.5. Buoyancy regulation of Microcystis during nitrogen-limited growth and phosphorus-limitedgrowth was complex and different. Short-term buoyancy regulation was achieved by the accumulationand consumption of carbohydrates responding to light and long-term buoyancy regulation was achievedby the change of gas vesicle volume, which controlled by cell nutrient status. Nitrogen limitation led togreat decrease in gas vesicle volume and cell buoyancy compared with phosphorus limitation, and asbuoyancy is one of the most important factor lead to water blooms, it implies that water bloomsdisappeared soon in nitrogen limited water body while it can persist a long time in phosphorus limitedwater body.
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