Growth and biomass stimulation of the toxic dinoflagellate Gymnodinium catenatum (Graham) by dissolved organic substances

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Journal Article
Journal of Experimental Marine Biology and Ecology, 1999, 236 (1), pp. 33 - 47
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Blooms of the toxic dinoflagellate Gymnodinium catenatum are annually recurrent events in south-east Tasmanian waters. Extensive blooms are preceded by a rainfall 'trigger' and the associated influx of dissolved organic matter (DOM; otherwise known as humic substances) from land runoff. Culture studies were conducted to investigate the potential nutritive influence of DOM on G. catenatum. Using a seawater-based growth medium (28‰ salinity), enriched with nutrients but no DOM, the trace element selenium (as 10-9 M selenite) limited growth rate and biomass (cell density and chlorophyll a). Addition of standard humic acid (aquatic Suwannee River Humic Acid available from the International Humic Substances Society; SHA) caused a 5-25 and 30-90% decrease in G. catenatum growth and biomass, respectively. In contrast, growth and biomass of G. catenatum were increased by approximately 15 and 30-60%, respectively, when Suwannee River SHA and natural DOM (isolated from Huon River water, south-east Tasmania, by ultrafiltration) were added to cultures in the absence of any other chelators or associated trace metals. Under these conditions, biomass was limited by the macronutrients nitrate and phosphate suggesting that dissolved organic substances can change overall nutrient availability and/or uptake by G. catenatum, either by addition of nutrients (e.g. nitrogen and phosphorus) to the system, or by interaction with other nutrients (e.g. selenium). The importance of selenium for G. catenatum growth, its introduction into coastal systems in land runoff, and the interaction of selenium with DOM supports the widely accepted hypothesis that dissolved organic substances alter the supply of nutrients available for algal growth. From these observations it is clear that the outcome of DOM-nutrient interactions on algal growth is variable, and depends on the composition and amount of DOM, the concentration of nutrients in solution and the species- or strain-specific cellular nutrient requirements.
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