Carbon versus Iron Limitation of Bacterial Growth in the California Upwelling Regime

The importance of iron versus dissolved organic carbon (DOC) in limiting the growth of heterotrophic bacteria is unresolved, even though iron, DOC, and heterotrophic bacteria are recognized to be critical components of oceanic biogeochemical cycles and food web dynamics. We used enrichment experiments to examine the roles of organic carbon and iron in limiting bacterial growth in the California upwelling regime, where phytoplankton can be iron limited depending on the time and location of the upwelling-induced phytoplankton bloom. In no-addition controls and in incubations with added Fe, bacterial production and abundance did not change substantially over time. In contrast, in all eight experiments, addition of glucose alone stimulated bacterial production and growth rates as much as tenfold. In Fe-replete areas ($>$1 nM), bacterial production and growth rates in glucose plus Fe treatments were similar to those incubations receiving only glucose. However, in low-Fe regions, addition of glucose plus Fe enhanced bacterial production and growth rates significantly more than glucose alone. We were unable to detect any impact of iron on glucose and amino acid catabolism, and the data offer no support for the hypothesis that bacteria are colimited by Fe and DOC. Rather, these results suggest that growth of heterotrophic bacteria is limited primarily by organic carbon even in Fe-poor waters, but when DOC limitation is relieved Fe may rapidly become a limiting factor.