Filtrates of G,<. brevis eultiires are toxic; however, the method 

 of filtration determines whether the more toxic portion of the 

 culture either passes through or is retained by the filter membrane. 

 In our studies conducted with both iinialgal and bacteria-free 

 cultijres, the filter paper residues eluted in either sea water or 

 cultiire medium were more toxic to the fish than the filtrates „ 

 The results were reversed when a culture was passed thz'ough a 

 millipore membrane under suction: the more toxic portion passed 

 through the membrane. The reasons for the different effects of 

 these two methods of filtration are not known. The filter materials 

 differ in composition and size; the millipore membrane (diameter 

 4.7 mm) is made of cellulose esters whereas the filter paper (diameter 

 180 5 cm) consists of cellulose fibers. Retention of the toxic 

 substance by filter paper may be due to greater adsorptive ax-ea and/or 

 differences in physical and chemical properties. Another possibility 

 is that filtration by gravity flow used with filter paper may 

 result in the retention of considerably more intact organisms than 

 in the case of millipore filtration under suction. Assuming that 

 greater niimbers of G, brevis were broken up by millipore filtrati'-n, 

 more toxin might be released in such case. However, our preliminar-y 

 studies showed no apparent increase in the toxicity of G, brevis 

 cultures in which the organisms were cytolysed by gentle heatings 



There are no indications that the fish kills caused by G, brevis 

 result from the depletion of oxygen in sea water by great masses of 

 this organism, Connell and Cross (1950) suggested that anaerobic 

 conditions created by the high biochemical oxygen demand of an 

 armored dinoflagellate, Gonyaulax . was the cause of mass mortality 

 of fish associated with discolored water in Offatts Bayou (Galveston 

 Bay) during the summer of 19/49. Gunter et al, (1948) concluded that 

 the 194-6-194-7 incidence of mass mortality of marine animals on the 

 west coast of Florida was not associated with low oxygen. Oxygen 

 deficiency can be excluded as a factor in the death of the fish in 

 the G, brevis cultures which were aerated. In experiment 8 (Tables 

 6 and 7) the dissolved oxygen content of all aerated test materials 

 exceeded 75^ saturation and some were as high as 90^, With 

 continuous gentle aeration one of the G. brevis raltures was about 

 90^ saturated although the bacterial count was 24 million per ml 

 at the time the dissolved oxygen was determined. 



The results of an attempt to determine the effects of aeration 

 on the toxicity of bacteria-free G, brevis cult^ores were contra- 

 dictory. In experiment 8 the fish, especially M. cephalus , shewed 

 distress and died more rapidly in the aliquot which was aerated 

 (dissolved oxygen — 90^ saturation) than in the nonaerated one 

 (dissolved oxygen — 45^ satioration) . The fish in another non- 

 aerated aliquot (dissolved oxygen — 55^ saturation) in this experiment 

 showed distress much sooner than those in the aerated culture 



44 



