Gymnodinium breve: population growth and development of toxicity in cultures

The population growth and toxicity of eight 12-liter cultures of the dinoflagellate, Gymnodinium breve, were followed for five months. Although each culture was inoculated with a volume of G. breve culture calculated to provide between 50,000 to 60,000 organisms per liter, counts made shortly after inoculation varied from 3,000 to 9,000 per liter. During the first five weeks, 200ml samples were removed at 2 to 4-day intervals for population estimates and bacterial enumeration. After the fifth week, 350 ml samples were taken at 7-day intervals to perform toxicity assays with the mosquito fish (Gambusia affinis). All experimental populations of G. breve increased uniformly by about three orders of magnitude and reached levels of five to nine million cells per liter (an average growth rate (K) of approximately 0.3/day) during the first five weeks. Afterwards, population levels differed considerably between cultures. At five months, culture population levels were from one-half to 24 million per liter. The eight cultures could be roughly classified as three types according to timing of peak growths: (1) early peak; (2) late peak; and (3) multiple small peaks. The causes for these differences between apparently replicate cultures are not known. The toxicity of the cultures and bacterial population levels varied directly with G. breve population levels. Electronic sizing of G. breve cells from populations of different ages and growth phases were performed with a Coulter Counter. Although there were shifts in the size distribution, these were not large and could not be related to age but the ratio of large:small cell volumes was higher in cultures with increasing population levels than in those with decreasing levels.