Screening of buoyant marine cyanobacteria using sucrose gradient centrifugation

A centrifugation method using a sucrose density gradient was established to distinguish buoyant cyanobacterial cells based on their cell density. The marine strain Nostoc NKBG 500017 was selected, because it showed the lowest density among 71 strains examined from our culture collection and also exhibited stable buoyancy. The cell suspension was homogeneous in a 100 mL graduated cylinder in the dark for at least 12 h. Stability was also confirmed using an artificial water column consisting of polyethylene pipe 2 m in height and 11 cm in diameter with a halogen light source at the top. Cells were suspended well throughout the water column and no precipitation was observed at the bottom of the column after 24 h incubation. Most cells were retained in the upper part of the water column from 10 cm to 30 cm depth. Growth was inhibited by the addition of tributyltin (TBT), an endocrine disruptor. The autofluorescence intensity of the cells decreased with increasing TBT concentration. The stable buoyancy and TBT sensitivity of Nostoc NKBG 500017 indicate that the strain is a possible candidate for monitoring contamination by toxic chemicals in the marine environment.