In this laboratory, Jose R. Carraquero demonstrated that toxicity could be expressed to salmonids only if the Heterosigma were cultured under longer light period than 12 hours light: 12 hours dark, and only if cultured in an enriched sea water medium (HESNW), but not if cultured in O-3, a highly enriched chemically defined medium that we had used for most of our studies.  When grown with continuous light and in HESNW medium, Heterosigma grown with bacteria (an algal-bacteria-consortium, ABC, obtained from Drs. Albright and Yang of the Department of Biological Sciences, Simon Frazer University, Burnaby, BC, Canada) was toxic to salmonids (chinook, Oncorhynchus tschawytscha, coho, O. kisutch, and rainbow trout, O. mykiss).  Neither supernatant nor the bacteria were themselves toxic.  Axenic cultures of Man-1 became toxic when grown with bacteria from ABC or when frown with either of two t pes of non-toxic bacteria. Deaths occurred more rapidly when cultures were grown and fish exposed at 20 C than at 12 C. The toxic cultures produced superoxide radicals, as measured by a luminol reaction, a measure of superoxide radicals.  Addition of Vitamin C, eliminated the luminol reaction, but did not prevent mortality. The addition of a catalase in addition to Vitamin C did eliminate mortality. This suggests that hydroxyl radicals may have been responsible for the mortalities, but the role of superoxide, in the absence of Vitamin C addition, cannot be eliminated. These data lend strong support to the hypothesis that free radicals or Reactive Oxygen Species (ROS) are involved in the mortality, but cannot exclude the potential of other toxic factors being present. Salmonids exposed to ABC or Man-1 with bacteria behaved as if anesthetized. Histological examination of the gills of exposed fish showed edema and a considerable degree of epithelial separation from the capillary core of the second lamellae. This could have caused death by hypoxia by decreasing oxygen and carbon dioxide exchange.  Exposure of Man-1 Heterosigma grown in HESNW to 4 hours of light intensity including UV-B did not result in toxicity to salmonids.  The algal cultures were not exposed to UV during the fish exposures. During the exposure period, the light intensities were (in W/m(squared)) in the treatment (PAR=8.3, UV-A=2.8, and 0.1) as compared to the controls (PAR=8.2, UV-A=2.5, and 0.002).  These values are only about 2-3% of full sunlight. Luminol reactions showed no superoxide radicals.