All four test fish were alive after 24. hours; two of them were alive 

 after 5 days. The pH of the medium in container I4 was 6,9 at this 

 time. 



The effects of millipore and paper filtration on the toxicity 

 of bacteria-free G. brevis cultures were the same as observed when 

 unialgal cultures were so treated. The more toxic portion of the 

 culture passes through the millipore membrane whereas filter paper 

 retains the more toxic fraction. 



Bacteria-free G. brevis cultures proved just as toxic as the 

 unialgal ones in simultaneous tests with comparable concentrations 

 of this organism. For example, in experiment 8 (Table 6) the "death 

 times" for M. cephalus in initially bacteria-free cultures varied 

 from a minimum of 15 minutes to a maximum of 4- hours and 4-'4 minutes. 

 Seven of the 16 M. cephalus died in less time than was required 

 (1 hour and 13 minutes) to kill the two M. cephalus subjected to a 

 imialgal culture. The C, variegatus succumbed in the Ipacteria-free 

 cultures in periods varying from a minimum of 3 hours and I8 minutes 

 to a maximiffli of 32 hours; 13 of the 16 test fish died within 8 hours. 

 Nine of the 16 C. variegatus in bacteria-free cultures died in less 

 than the minimum time {6^ hours) required to kill the two C. variegatus 

 in a unialgal culture . Further data for comparison of the effects of 

 bacteria-free and unialgal cultures are available from experiment 9 

 (Table 8) . Two large M. cephalus subjected to unialgal cultures died 

 in 10 and L4 mihutes. The four large mullet in the bacteria-free 

 cultures died within 23 to 4I minutes. The "death times" for six 

 small M. cephalus in the unialgal cultures varied from 15 minutes to 

 2^ hours. The extremes of "death times" for the 12 small mullet in 

 bacteria-free cultures were quite similar — 25 minutes to 2 hours and 

 28 minutes . 



In spite of the evidence that bacteria are not directly 

 responsible for the toxic effects of G. brevis cultures, the possi- 

 bility that bacteria play a significant role in the development of 

 G. brevis "blooms" should not be overlooked. Such organisms may 

 contribute appreciably to the nutrition of G. brevis . For example, 

 some of the bacteria isolated from unialgal G. brevis cultures 

 produce vitamin B-|_2-active substances (Starr et al., in press). 

 Vitamin B-]_2 apparently stimulates the growth of G. brevis in 

 sea-water-base medium (Wilson and Collier, 1955). 



Once fish kills are Initiated by "blooms" of G. brevis, excessive 

 bacterial growth resulting from the increased availability of organic 

 matter may possibly cause the "blooms" to decline in isolated situ- 

 ations . Some of the ways in which bacteria could unfavorably affect 

 G. brevis "blooms" include: producing substances toxic to this 

 organism, competing for nutritive substances, and adversely altering 

 the pH. We have frequently observed the failure of unialgal G, brevis 

 to grow in tubes in which the medium became cloudy with bacterial growth. 



35 



