Differential effects of iron additions on organic and inorganic carbon production by phytoplankton

Bottle and mesoscale experiments have demonstrated that iron additions enhance phytoplankton growth and reduce surface pCO(2) in high-nutrient, low-chlorophyll (HNLC) regions of the world oceans. Here we show that iron additions specifically stimulate organic but not inorganic carbon production in the HNLC Subarctic Pacific. Five-hour C-14 labeling experiments performed during incubation of surface water samples demonstrated a large increase in the rate of organic carbon produced but no change in the rate of inorganic carbon production. The same result was obtained on two different dates: one when coccolithophores formed a relatively large proportion of total autotrophic biomass; the other when coccolithophores were less abundant. Together with previous taxonomic observations, our results imply that iron fertilization may be particularly effective in drawing down CO2 in surface waters by stimulating primary production but not calcium carbonate precipitation, which augments CO2.