Turbidity changes caused by collapse of cyanobacterial gas vesicles in water pumped from lake Kinneret into the Israeli national water carrier

The cyanobacterium Aphanizomenon ovalisporum Forti appeared in Lake Kinneret for the first time in the summer and autumn of 1994 and reappeared in 1995–1997. In 1994 it formed a large bloom (6000 filaments ml−1) that caused high turbidity (up to 7 NTU), which is attributed mainly to light scattering by gas vesicles. When lake water was pumped into the high-pressure pipe (2.5 MPa) of the Israeli National Water Carrier (INWC) the turbidity of the water decreased by 65%, the Aphanizomenon filaments lost their buoyancy and some filaments fragmented. Application of 0.6 MPa pressure to cultures of the Lake Kinneret strain of A. ovalisporum in a pressure nephelometer caused collapse of all gas vesicles; the turbidity decreased by 80% and all of the filaments sedimented. The mean critical collapse pressure of the gas vesicles (pc) was 0.58±0.02 MPa (with cells suspended in hypertonic sucrose solution), the apparent mean critical collapse pressure (pa) was 0.27±0.02 MPa (with cells suspended in culture medium) and the mean turgor pressure (pt) was 0.33±0.02 MPa. The value of pa is lower than the pressures in any of the five high-pressure pipes (0.6–2.5 MPa) along the INWC. It is concluded that gas vesicle collapse caused the observed decrease in turbidity in the system. The combined physical, chemical and biological treatments given in the INWC system resulted in a 99.9% decrease in the A. ovalisporum biomass.