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

摘要

Uptake rates of three potential N-sources (ammonium, nitrate and N-urea) and two potential C-sources (HCO_3~ 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 Curea. 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 NO_3, NH_4~+ 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 Cstorage strategies correspond to a benefit in terms of competitiveness for A catenella, optimized by the migrating abilities of this species.