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Aspects of iron and nitrogen nutrition in two red tide dinoflagellates, Gymnodinium sanguineum Hirasaka and Protogonyaulax tamarensis (Lebour) Taylor Doucette, Gregory John


Iron stress-mediated effects on growth, biochemical composition, iron and nitrogen uptake, and ultrastructure have been examined in the red tide dinoflagellates Gymnodinium sanguineum Hirasaka and Protogonyaulax tamarensis (Lebour) Taylor. The influence of nitrogen source (i.e. NO₃ or NH₄) on certain iron stress-mediated effects was studied, and some comparisons were made with nitrogen stress-mediated changes in biochemical composition The half-saturation constant for iron-limited growth (K[sub μ] = 1.7∙10⁻²⁰ M) of G. sanguineum was estimated to be 10-1000 times greater than for other neritic species investigated previously. Also, the iron requirement of this dinoflagellate, in terms of Fe/C ratios, exceeded those of certain coastal diatoms by one to two orders of magnitude. Fe/N ratios demonstrated a larger (1.5-fold) minimum iron requirement for NO₃- than NH₄-grown cells, likely reflectin the iron content of NO₃ assimilatory enzymes. Acquisition of nitrogen by Fe-deplete, NO₃-grown cells was sufficiently inhibited to yield symptoms of N deficiency, revealed by decreased (ca. 1.4-fold) N quotas and free amino acid/protein ratios compared to Fe-deplete, NH₄-grown cells Reductions in chlorophyll a (chl a) quotas (Q[sub chl]) and photosynthetic electron transport (PET) efficiency (as measured by in vivo fluorescence indices) occurred under Fe depletion, and are consistent with the essential role of iron in chl a and PET component (i.e. cytochromes and Fe-S proteins) biosynthesis. Nitrogen depletion affected Q[sub chl] similarly, but altered PET efficiency to a markedly lesser extent than did Fe depletion. Iron-deplete G. sanguineum exhibited an enhanced iron transport capacity, which failed to be manifested following a transition from NH₄ to NO₃ nutrition. This suppression may result from concurrent iron and nitrogen stress, due to the inability of Fe-deplete, NH₄-grown cells to rapidly assimilate NO₃. The complete initial inhibition of NO₃ uptake when Fe-deplete, NH₄-grown cells were given saturating iron additions supports this idea. Iron stress caused reductions in chloroplast number and some degeneration of lamellar organization in this species. For P. tamarensis, iron limitation induced the formation of temporary (= pellicle) and resting (= hypnozygotes) cysts. Degenerative changes in organelle (i.e. chloroplasts, mitochondria and chromosomes) ultrastructure were largely restricted to pellicular cysts, consistent with their hypothesized role of maintaining viability over brief, rather than extended (cf. hypnozygotes) exposure to adverse conditions.

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