Dark metabolism and carbon–nitrogen uncoupling in the toxic dinoflagellate Alexandrium catenella (Dinophyceae)

Uptake rates of three potential N-sources (ammonium, nitrate and N-urea) and two potential C-sources (HCO3− and C-urea) were estimated during growth of Alexandrium catenella in both light and dark phases. According to the variations observed in 13C-isotopic ratio, A. catenella cells were not able to use C-urea. Furthermore, decreases in 13C cell content during darkness revealed a probably high involvement of C recently fixed in dark respiration. Dark N-uptake capacities of A. catenella were characterized by dark/light uptake ratio of 27%, 43% and 65% for NO3−, NH4+ and N-urea, respectively. An accumulation of C-rich compounds during the light period was highlighted through strong diel variations in C:N ratio and would provide C and energy for these dark uptake processes indicating an uncoupling between N and C metabolism. Total costs in terms of C associated with dark N-uptake and assimilation were estimated and revealed that the main part of those costs may be associated with maintenance metabolism in A. catenella cells. The relatively low C-costs of biosynthesis in darkness suggest that dark uptake and C-storage strategies correspond to a benefit in terms of competitiveness for A. catenella, optimized by the migrating abilities of this species.

► Dark nitrogen uptake and carbon storage of Alexandrium catenella are reported.
► Strong diel variations in C:N ratio show uncoupling between C and N metabolism.
► Dark/light uptake ratios are 27%, 43%, and 65% for nitrate, ammonium, and urea.
► Estimations of total C costs associated with dark N metabolism are 10–14 gC gN−1.
► Dark N uptake represents a low additional cost in terms of C for this species.