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 Kinnere t for the first time in the summer and autumn of 1994 and reappeared in 199 5-1997. 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 scatteri ng 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 an d some filaments fragmented. Application of 0.6 MPa pressure to cultures of the Lake Kinneret strain of A. ovalisporum in a pressure nephelometer caus ed collapse of all gas vesicles; the turbidity decreased by 80% and all of the filaments sedimented. The mean critical collapse pressure of the gas ve sicles (p(c)) was 0.58 +/- 0.02 MPa (with cells suspended in hypertonic suc rose solution), the apparent mean critical collapse pressure (p(a)) was 0.2 7 +/- 0.02 MPa (with cells suspended in culture medium) and the mean turgor pressure (p(t)) was 0.33 +/- 0.02 MPa, The value of p(a) is lower than the pressures in any of the five high-pressure pipes (0.6-2.5 MPa) along the I NWC. 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. (C) 1999 Elsevier Science Ltd. All rights reserved.