Nitrogen utilization by the raphidophyte Heterosigma akashiwo: Growth and uptake kinetics in laboratory cultures

Abstract The nitrogen uptake and growth capabilities of the potentially harmful, raphidophycean flagellate Heterosigma akashiwo (Hada) Sournia were examined in unialgal batch cultures (strain CCMP 1912). Growth rates as a function of three nitrogen substrates (ammonium, nitrate and urea) were determined at saturating and sub-saturating photosynthetic photon flux densities (PPFDs). At saturating PPFD (110 μE m −2  s −1 ), the growth rate of H. akashiwo was slightly greater for cells grown on NH 4 + (0.89 d −1 ) compared to cells grown on NO 3 − or urea, which had identical growth rates (0.82 d −1 ). At sub-saturating PPFD (40 μE m −2  s −1 ), both urea- and NH 4 + -grown cells grew faster than NO 3 − -grown cells (0.61, 0.57 and 0.46 d −1 , respectively). The N uptake kinetic parameters were investigated using exponentially growing batch cultures of H. akashiwo and the 15 N-tracer technique. Maximum specific uptake rates ( V max ) for unialgal cultures grown at 15 °C and saturating PPFD (110 μE m −2  s −1 ) were 28.0, 18.0 and 2.89 × 10 −3  h −1 for NH 4 + , NO 3 − and urea, respectively. The traditional measure of nutrient affinity—the half saturation constants ( K s ) were similar for NH 4 + and NO 3 − (1.44 and 1.47 μg-at N L −1 ), but substantially lower for urea (0.42 μg-at N L −1 ). Whereas the α parameter ( α  =  V max / K s ), which is considered a more robust indicator for substrate affinity when substrate concentrations are low ( K s ), were 19.4, 12.2 and 6.88 × 10 −3  h −1 /(μg-at N L −1 ) for NH 4 + , NO 3 − and urea, respectively. These laboratory results demonstrate that at both saturating and sub-saturating N concentrations, N uptake preference follows the order: NH 4 +  > NO 3 −  > urea, and suggests that natural blooms of H. akashiwo may be initiated or maintained by any of the three nitrogen substrates examined.