EFFECTS OF IRON AND NITROGEN-SOURCE ON THE SINKING RATE, PHYSIOLOGY AND METAL COMPOSITION OF AN OCEANIC DIATOM FROM THE SUB-ARCTIC PACIFIC

The effect of iron (Fe) on the sinking rate of an oceanic diatom Actin ocyclus sp, and an oceanic coccolithophore Emiliania huxleyi, both iso lated from the subarctic Pacific, was examined in natural oceanic seaw ater. The Fe status of the diatom had a dramatic effect on its sinking rate, causing a 5 times increase from 0.17 to 0.93 m d(-1) from Fe-re plete to Fe-stressed conditions. In contrast, Fe had no effect on the sinking rate of the oceanic coccolithophore, which maintained its sink ing rate at 0.12 m d(-1) The cell volume of the diatom decreased sligh tly under Fe-stressed conditions, but the cell volume of the coccolith ophore decreased substantially (46 %) under Fe-stressed conditions. Th e effect of nitrogen source (nitrate vs ammonium) on the chlorophyll a (chl a), carbon (C), and nitrogen (N) quotas of the oceanic diatom Ac tinocyclus sp, was also examined. Under Fe-stressed conditions when th e energy-stress on the cells is the greatest, ammonium-grown cells app eared to have a physiological advantage over nitrate-grown cells in th is oceanic diatom. Ammonium-grown cells were able to maintain normal N and C quotas under Fe-stress, whereas nitrate-grown cells were not, r esulting in an 80 % reduction in N cell(-1) for nitrate grown cells un der Fe-stress. Also, in vivo fluorescence:chl a increased and chi a:C decreased more drastically for nitrate-grown cells under Fe-stress tha n for ammonium-grown cells, indicating that nitrate-grown cells under Fe-stress are less capable of trapping and utilizing light energy. The se findings support theoretical predictions based on Fe and energy req uirements for nitrate versus ammonium utilization. Metal quotas (Fe, M n, Zn) were measured simultaneously using cold-metal techniques to det ermine the metal content of the cells. There were no significant diffe rences in metal to carbon ratios between nitrate and ammonium-grown ce lls under Fe-replete conditions. Under Fe-stressed conditions, nitrate -grown cells had significantly higher Mn:C and significantly lower Zn: C ratios than ammonium-grown cells, but there was no observed differen ce in Fe quotas. In this study we observed that 2 different species of phytoplankton from the subarctic Pacific responded physiologically di fferently to similar Fe conditions. Our results suggest that the solit ary, centric, 20 to 60 pm diameter oceanic diatom would have a higher sinking rate than the oceanic coccolithophore in the subarctic Pacific , perhaps having implications for biogenic fluxes to depth. Moreover, our data indicate that this diatom is probably utilizing ammonium to m eet its nitrogen requirements in situ under the low Fe conditions foun d in the northeast subarctic Pacific. Actinocyclus sp. appears incapab le of effectively changing its cell volume to help alleviate Fe- (and other nutrient) stress, whereas the coccolithophore can reduce its cel l volume substantially, allowing it to reduce its requirements for N, C, and Fe. These physiological results help to explain phytoplankton c omposition dynamics in the subarctic Pacific.