The heterotrophic bacterial response during a mesoscale iron enrichment experiment (IronEx II) in the eastern equatorial Pacific Ocean

The response of the heterotrophic bacterial community to iron addition was determined during the mesoscale iron-enrichment experiment conducted in the eastern equatorial Pacific during May–June 1995 (IronEx II). Bacterial abundance and 3H-leucine incorporation rates were measured for samples collected from the middle of the mixed layer (15 m) over the course of the iron-induced phytoplankton bloom and its decline. Bacterial abundance and productivity increased 1.7and threefold, respectively, compared to un-enriched waters. Specific growth rates of heterotrophic bacteria increased threeto fourfold. These results demonstrate that iron addition to this high-nitrate, low-chlorophyll region affects both autotrophic and heterotrophic microorganisms and that bacterial carbon demand can be potentially met by the fivefold increase in photosynthetic productivity in the mixed layer. Over the last decade, considerable effort has been directed toward understanding the relationship between phytoplankton productivity and the availability of iron, particularly as the reason for the lack of significant autotrophic growth in high-nitrate, low-chlorophyll (HNLC) regions of the open ocean. The results of iron enrichment incubation experiments conducted in the subarctic North Pacific (e.g., Martin and Fitzwater 1988), equatorial Pacific Ocean (e.g., Martin et al. 1991) and the Southern Ocean (e.g., de Baar et al. 1990), as well as in situ Fe enrichment experiments conducted in the equatorial Pacific (e.g., Coale et al. 1996) and the Polar Front in the south of Australia (Boyd et al. 2000), strongly support the idea that phytoplankton growth in HNLC areas are limited, at least in part, by the availability of Fe. These studies have focused primarily on the phytoplankton response to an alleviation of Fe deficiency, presumably because of the crucial role of Fe in photosynthesis, chlorophyll synthesis, and nitrate assimilation. Heterotrophic bacteria have been less studied despite their numerical dominance in oligotrophic waters (e.g., Fukuda et al. 1998) and their importance in the cycling of carbon and nutrients, including iron (Tortell et al. 1996). The few studies examining iron limitation of heterotrophic bacterial communities have reached contradictory conclusions, and the relative importance of iron versus dissolved organic matter in controlling the rates of bacterial growth in HNLC regions is still unclear. Previous incubation experiments in Fe-depleted regions have suggested that Fe enrichment may affect heterotrophic bacteria, either directly by the alleviation of Fe limitation or indirectly through the effects of enhanced phytoplankton growth and the increased supply of dissolved organic matter suitable for bacterial utilization. During experiments conducted in the coastal Southern Ocean (Gerlache Strait), Pakulski et al. (1996) found that Fe enrichment increased both heterotrophic bacteria abundance and cell-specific growth rates. These experiments, conducted in the dark and in the absence of phytoplankton and bacterivores, suggest a direct stimulation of heterotrophic bacterial growth by Fe enrich-