Inducing phytoplankton iron limitation in iron-replete coastal waters with a strong chelating ligand

Dissolved iron (Fe) concentrations in the California coastal upwelling regime vary over two orders of magnitude (from <0.05 to >5 nM), which leads to a wide range in Fe effects on phytoplankton growth. Fe-addition experiments are appropriate to use to assess the biological role of Fe in low-Fe areas, but other methods are needed in Fe-replete regions. We present experiments that use additions of the exogenous siderophore desferrioxamine B (DFOB, obtained from a terrestrial actinomycete fungus) to sequester ambient Fe and to markedly decrease its availability to the biota. DFOB additions resulted in artificial Fe limitation of the phytoplankton community in high-Fe areas of the upwelling region. Results of these "Fe-removal" experiments mirror those of Fe-addition experiments in low-Fe, high-nutrient, low-chlorophyll (HNLC) waters. When DFOB is added to Fe-replete waters, changes in nutrient concentrations, biomass, and other biological parameters closely resemble those seen in Fe-limited controls in HNLC areas, while the controls without DFOB behave much like HNLC Fe-addition bottles. DFOB additions in high-Fe waters greatly reduced biological Fe uptake and, consequently, nitrate, silicic acid, and carbon-uptake rates as well as particulate production. Diatoms and other phytoplankton bloomed profusely in unamended controls but not in Fe-Limited +DFOB bottles. Bacterial numbers and zooplankton grazing activity were also severely reduced in DFOB-addition bottles. These experiments demonstrate that artificially lowering Fe availability can induce limitation of autotrophic and heterotrophic plankton and can prevent utilization of the high ambient levels of upwelled nutrients along the California coast. Our results suggest that DFOB-bound Fe is highly unavailable to the plankton community, a result that offers researchers an important tool to use to probe the influence of Fe on biological community development in high-Fe regimes.