The ocean has a huge effect on global climate, in part due to the influence of marine phytoplankton on the global carbon cycle. Iron (Fe) availability can play an important role in the structure and function of marine ecosystems, though effects on phytoplankton species composition and on nutrient cycling. It has been speculated that zinc (Zn) may also play an important role in these processes, although only a few studies have looked at the effect of Zn addition on natural marine phytoplankton. My dissertation investigates the effects of Fe and Zn addition on diatom productivity and nutrient cycling in three high-nutrient, low-chlorophyll (HNLC) regions with low Fe and/or Zn availability. Nutrient uptake studies focused on silicon and nitrate, two nutrients known to regulate new production and carbon export in marine systems. The effects of Zn and combined Fe/Zn additions on diatom productivity and nutrient uptake were complex and so only the effects of Fe addition will be summarized here.

Studies were conducted in 10-20 liter deck-board enrichment experiments in regions off central California (1997 and 1999), the Pacific sector of the Southern Ocean (1997-1998) and the eastern tropical Pacific (2000). In all three areas, Fe additions to low-Fe waters (<0.2 nM) resulted in higher diatom biomass as well as greater Si, NO3 and inorganic C uptake in natural phytoplankton assemblages. Fe addition also resulted in greater maximum potential uptake rates (Vmax) of Si and NO3, by a factor of 2-7. In Southern Ocean waters, Si Vmax increased with increasing dissolved Fe concentration, approaching a maximum value at ~1.0 nM Fe. In some instances, Fe addition also affected Si uptake affinity, significantly altering the half-saturation constant (Ks). Fe addition also greatly reduced Si:NO3 uptake ratios. Results from studies on Si and NO3 uptake kinetics suggest that higher Si:NO3 uptake ratios in Fe-limited phytoplankton may be due in part to the direct effect of Fe on Si and NO3 Vmax (i.e., transporter synthesis). In addition, these studies suggest a physiological basis for the reduced nutrient drawdown observed in HNLC regions, together with the more general effects of slower phytoplankton growth rates.

At the eastern tropical Pacific locations, the effect of Fe on size-fractionated Si and NO3 uptake was measured in order to tease apart the response of different sub-components of the microbial community to Fe addition. These studies revealed that increases in Si and NO3 uptake after Fe addition were restricted to phytoplankton in the larger (>5 micron) size fractions, and that uptake rates in the larger size fractions can outstrip those in the small (0.6-5 micron) size fraction at high Fe concentrations. Microautoradiographic experiments measuring cell-specific Si uptake were also performed in order to separate the response of the dominant diatom species in these waters to Fe addition. Results showed that silica production by Rhizosolenia spp. diatoms, which represented a significant proportion of total silica production in both controls and Fe additions, was not affected by Fe addition. In contrast, silica production by Pseudo-nitzschia spp. diatoms increased more than two-fold, increasing the contribution of this group to total silica production.

These studies explored potential mechanisms for higher Si:NO3 uptake ratios observed in low-iron waters and showed that Fe can play a direct role in Si and N biogeochemistry as well as phytoplankton productivity and species composition.