ASPECTS OF IRON AND NITROGEN NUTRITION IN THE RED TIDE DINOFLAGELLATE GYMNODINIUM-SANGUINEUM

Iron-stress-mediated effects on biochemical constituents of the red tide dinoflagellate Gymnodinium sanguineum Hirasaka were examined in 1988 by comparing Fe-replete and Fe-deplete batch cultures. The influence of nitrogen source (NO3 or NH4) on characteristics of Fe-deplete cells was also studied [i.e., Fe-deplete/NO3-grown (= -Fe/NO3) vs Fe-deplete/NH4-grown (= -Fe/NH4)]. Common to both N sources were reductions of chlorophyll a (chl a) and Fe quotas (per cell volume) by 75% and ca. 1.5 orders of magnitude, respectively, under Fe depletion. The Fe requirement of G. sanguineum exceeded those of certain neritic diatoms by one to two orders of magnitude. -Fe/NH4 cells exhibited 30 to 50% greater N quotas and free amino acid:protein ratios than did Fe-deplete cells grown on NO3. In vivo fluorescence:chl a increased with Fe deficiency particularly in -Fe/NO3 cultures, surpassing -Fe/NH4 values by ca. two-fold. Effects of Fe depletion were consistent with this element's essential role in the biosynthesis of chl a and components of the photosynthetic electron transport (PET) system, and also in NO3 utilization. Fe:N ratios were larger (1.5-fold) for iron-deficient NO3-grown than NH4-grown cells, likely reflecting the Fe content of NO3 assimilatory enzymes [nitrate (NR) and nitrite (NiR) reductase] and of electron transport components needed to provide reductant, coupled with a diminished capacity of -Fe/NO3 cells to acquire and assimilate nitrogen. Indicators of PET efficiency suggested that under iron stress, supply of Fe for NR and NiR is partly at the expense of iron-containing PET components. Utilization of nitrate by NO3-grown cells was inhibited sufficiently by Fe depletion to yield symptoms bordering on N deficiency. In an ecological context, the most important effect mediated by nitrogen source may be the determination of critical Q(Fe) (i.e., Fe required to just sustain maximal growth), thereby regulating the degree of growth limitation for a given subsaturating iron concentration.