Bacterial Community Response to Iron Availability in the California Current Ecosystem

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Here we describe the results of incubation experiments conducted during the California Current Ecosystem (CCE) LTER process cruises of 2012 and 2014 that were aimed at understanding the role of iron availability and iron-binding ligands in the remineralization of organic matter by heterotrophic marine bacteria. Dissolved iron is an essential trace element for phytoplankton and bacteria growth in the world’s oceans. However, it is scarce in seawater and almost always associated with a heterogeneous pool of organic ligands. The identity and behavior of organic ligands is therefore important for understanding the mechanisms of iron acquisition in the ocean. However, little is known about the production of iron-binding ligands during organic matter remineralization by heterotrophic bacteria, a key linkage between iron and carbon biogeochemical cycles. Surface seawater was incubated on-deck over the course of six days under low and high iron conditions and analyzed for iron-binding ligands via competitive ligand exchange-adsorptive cathodic stripping voltammetry and the composition of the microbial community via 16S tag sequencing. We found a significant correlation between strong L₁ ligands and copiotroph-type bacterial populations, possibly indicating a direct link between specific taxa and biological ligand production during remineralization processes.

In addition, while it is now widely accepted that iron concentrations can limit the growth of phytoplankton in some regions of the world’s oceans, including the CCE, less is understood about how it may directly limit heterotrophic bacteria growth and activity. Heterotrophic bacteria have a significant iron requirement – a majority of which is involved in the enzymatic steps facilitating the breakdown and remineralization of organic matter. Thus, understanding these iron requirements is also important for understanding the fate of organic carbon fixed during primary production. To address this, seawater from the deep chlorophyll maximum was incubated in the dark under low and high iron conditions over the course of five days and analyzed for total organic carbon concentration, bacterial carbon production, and bacterial biomass.  We found that under high iron conditions, a greater amount of organic carbon was processed by the bacterial community but did not significantly increase bacterial biomass; possibly indicating decreased bacterial growth efficiency and a larger proportion of organic carbon lost to respiration with greater iron availability.