Effects of iron and nitrogen source on the sinking rate, physiology and metal composition of an oceanic diatom from the subarctic Pacific

The effect of iron (Fe) on the sinking rate of an oceanic diatom Actinocyclus sp, and an oceanic coccolithophore Emiliania huxleyi, both isolated from the subarctic Pacific, was examined in natural oceanic seawater. 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-replete to Fe-stressed conditions. In contrast, Fe had no effect on the sinking rate of the oceanic coccolithophore, which maintained its sinking rate at 0.12 m d(-1) The cell volume of the diatom decreased slightly under Fe-stressed conditions, but the cell volume of the coccolithophore decreased substantially (46 %) under Fe-stressed conditions. The effect of nitrogen source (nitrate vs ammonium) on the chlorophyll a (chl a), carbon (C), and nitrogen (N) quotas of the oceanic diatom Actinocyclus sp, was also examined. Under Fe-stressed conditions when the energy-stress on the cells is the greatest, ammonium-grown cells appeared to have a physiological advantage over nitrate-grown cells in this oceanic diatom. Ammonium-grown cells were able to maintain normal N and C quotas under Fe-stress, whereas nitrate-grown cells were not, resulting in an 80 % reduction in N cell(-1) for nitrate grown cells under Fe-stress. Also, in vivo fluorescence:chl a increased and chi a:C decreased more drastically for nitrate-grown cells under Fe-stress than for ammonium-grown cells, indicating that nitrate-grown cells under Fe-stress are less capable of trapping and utilizing light energy. These findings support theoretical predictions based on Fe and energy requirements for nitrate versus ammonium utilization. Metal quotas (Fe, Mn, Zn) were measured simultaneously using cold-metal techniques to determine the metal content of the cells. There were no significant differences in metal to carbon ratios between nitrate and ammonium-grown cells 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 difference in Fe quotas. In this study we observed that 2 different species of phytoplankton from the subarctic Pacific responded physiologically differently to similar Fe conditions. Our results suggest that the solitary, 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 meet its nitrogen requirements in situ under the low Fe conditions found in the northeast subarctic Pacific. Actinocyclus sp. appears incapable of effectively changing its cell volume to help alleviate Fe- (and other nutrient) stress, whereas the coccolithophore can reduce its cell volume substantially, allowing it to reduce its requirements for N, C, and Fe. These physiological results help to explain phytoplankton composition dynamics in the subarctic Pacific.