Carbon versus iron limitation of bacterial growth in the California upwelling regime

The importance of iron versus dissolved organic carbon (DOC) in limiting th e growth of heterotrophic bacteria is unresolved, even though iron, DOG, an d heterotrophic bacteria are recognized to be critical components of oceani c biogeochemical cycles and food web dynamics. We used enrichment experimen ts to examine the roles of organic carbon and iron in limiting bacterial gr owth in the California upwelling regime, where phytoplankton can be iron li mited depending on the time and location of the upwelling-induced phytoplan kton bloom. In no-addition controls and in incubations with added Fe, bacte rial production and abundance did not change substantially over time. In co ntrast, in all eight experiments, addition of glucose alone stimulated bact erial 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 g rowth rates significantly more than glucose alone. We were unable to detect any impact of iron on glucose and amino acid catabolism, and the data offe r no support for the hypothesis that bacteria are colimited by Fe and DOG. Rather, these results suggest that growth of heterotrophic bacteria is Limi ted primarily by organic carbon even in Fe-poor waters, but when DOC limita tion is relieved Fe may rapidly become a limiting factor.