3c Bacteria apparently do not produce or directly contribute to 

 the production of the toxic substance present in G. brevis cultures. 

 Bacteria-free cultures were just as toxic to fish as the unialgal ones. 



4. The toxicity of G. brevis does not depen4 on the presence of 

 living organisms. Removing the organisms from cultures by millipore 

 filtration or killing them with gentle heat did not appear to alter 

 the toxicity. The high dissolved oxygen content of aerated G. brevis 

 cultiires eliminates oxygen deficiency as the cause of toxicity. 



5. The toxic substance produced by G. brevis readily passes through 

 a millipore membrane, but for the most part is retained by filter 



paper . 



6. Studies with bacteria-free and inialgal cultures indicate 

 that the six species of test fish are differentially sensitive 



to G. brevis cultures. The test fish, listed in order of decreasing 

 sensitivity, were: Membras vagrans, Mugil cephalus . Fundulus grandis , 

 Mollienisia latipinna , Fundulus similis, and CT/prinodon variegatus . 



7. Some chromogenlc bacteria isolated from the Gulf of Mexico 

 or adjoining bays are toxic to fish in laboratory tests. However, 

 an association of such bacteria with mass fish kills in the Gulf of 

 Mexico has not been established. Flavobacter ium piscicida , previously 

 found to be toxic to several species of fish in undetermined concen- 

 trations, was not toxic to Mugil cephalus at an initial concentration 

 of about 2 million per ml, A "red bacterium" isolated from G. brevis - 

 infested waters appears to be toxic to fish at concentrations in the 

 order of 0.5 to 2 million per ml. This bacterium was uncommon during 

 our survey since only two colonies were obtained from 15 samples of 

 1,0 ml each. 



43 



