Growth rates and nitrate uptake of co-occurring red-tide dinoflagellates Alexandrium affine and A. fraterculus as a function of nitrate concentration under light-dark and continuous light conditions

Abstract

The dinoflagellate genus Alexandrium is known to often form harmful algal blooms causing human illness and large-scale mortality of marine organisms. Therefore, the population dynamics of Alexandrium species are of primary concern to scientists and aquaculture farmers. The growth rate of the Alexandrium species is the most important parameter in prediction models and nutrient conditions are critical parameters affecting the growth of phototrophic species. In Korean coastal waters, Alexandrium affine and Alexandrium fraterculus, of similar sizes, often form red-tide patches together. Thus, to understand bloom dynamics of A. affine and A. fraterculus, growth rates and nitrate uptake of each species as a function of nitrate (NO3) concentration at 100 mu mol photons m(-2) s(-1) under 14-h light : 10-h dark and continuous light conditions were determined using a nutrient repletion method. With increasing NO3 concentration, growth rates and NO3 uptake of A. affine or A. fraterculus increased, but became saturated. Under light : dark conditions, the maximum growth rates of A. affine and A. fraterculus were 0.45 and 0.42 d(-1), respectively. However, under continuous light conditions, the maximum growth rate of A. affine slightly increased to 0.46 d(-1), but that of A. fraterculus largely decreased. Furthermore, the maximum nitrate uptake of A. affine and A. fraterculus under light : dark conditions were 12.9 and 30.1 pM cell(-1) d(-1), respectively. The maximum nitrate uptake of A. affine under continuous light conditions was 16.4 pM cell(-1) d(-1). Thus, A. affine and A. fraterculus have similar maximum growth rates at the given NO3 concentration ranges, but they have different maximum nitrate uptake rates. A. affine may have a higher conversion rate of NO3 to body nitrogen than A. fraterculus. Moreover, a longer exposure time to the light may confer an advantage to A. affine over A. fraterculus.