Abstract
The distribution of dissolved iron (Fe), total organic Fe-binding ligands, and siderophores were measured between the surface and 400 m at Station ALOHA, a long term ecological study site in the North Pacific Subtropical Gyre. Dissolved Fe concentrations were low throughout the water column and strong organic Fe-binding ligands exceeded dissolved Fe at all depths; varying from 0.9 nmol L-1 in the surface to 1.6 nmol L-1 below 150 m. Although Fe does not appear to limit microbial production, we nevertheless found siderophores at nearly all depths, indicating some populations of microbes were responding to Fe stress. Ferrioxamine siderophores were most abundant in the upper water column, with concentrations between 0.1-2 pmol L-1, while a suite of amphibactins were found below 200 m with concentrations between 0.8-11 pmol L-1. The distinct vertical distribution of ferrioxamines and amphibactins may indicate disparate strategies for acquiring Fe from dust in the upper water column and recycled organic matter in the lower water column. Amphibactins were found to have conditional stability constants (log ) ranging from 12.0-12.5, while ferrioxamines had much stronger conditional stability constants ranging from 14.0-14.4, within the range of observed L1 ligands by voltammetry. We used our data to calculate equilibrium Fe speciation at Station ALOHA to compare the relative concentration of inorganic and siderophore complexed Fe. The results indicate that the concentration of Fe bound to siderophores was up to two orders of magnitude higher than inorganic Fe, suggesting that even if less bioavailable, siderophores were nevertheless a viable pathway for Fe acquisition by microbes at our study site. Finally, we observed rapid production of ferrioxamine E by particle-associated bacteria during incubation of freshly collected sinking organic matter. Fe-limitation may therefore be a factor in regulating carbon metabolism and nutrient regeneration in the mesopelagic.
Highlights
Iron (Fe) is an essential trace metal for many cellular processes in marine phytoplankton and bacteria, including nitrogen fixation and photosynthesis (Morel and Price, 2003)
Concentrations of dissolved Fe remained relatively constant between 0.15 nmol L−1 (15 m) and 0.12 nmol L−1 (400 m), with potentially a slight decrease immediately below the deep chlorophyll maximum (DCM; 125 m), where values ranged from 0.06 nmol L−1 (150 m) to 0.08 nmol L−1 (200 m) before recovering to 0.10 nmol L−1 at 300 m. (Table 1)
At the time of our sampling, dissolved Fe concentrations were >0.06 nmol L−1, but siderophores were present at several depths between 5 and 400 m, with strikingly different distributions in the upper and lower portions of our profile
Summary
Iron (Fe) is an essential trace metal for many cellular processes in marine phytoplankton and bacteria, including nitrogen fixation and photosynthesis (Morel and Price, 2003). The Hawaii Ocean Time-series (HOT) at Station ALOHA has been a major study site to evaluate the annual and interannual variability in Fe chemistry and its influence on oligotrophic marine ecosystems These timeseries measurements have revealed a dynamic Fe cycle, with atmospheric dust deposition and mixing of deeper waters into the photic zone acting as a fluctuating source of bioavailable Fe (Boyle et al, 2005; Fitzsimmons et al, 2015). Larger, more transiently abundant taxa such as diatoms and nitrogen fixing photoautotrophs such as Trichodesmium spp. often have high cellular Fe quotas (Berman-Frank et al, 2001; Kustka et al, 2003; Roe et al, 2012) These microbes could face Fe stress at times when the atmospheric delivery of Fe is low. While oligotrophic ecosystems such as the one at Station ALOHA may not show a chlorophyll a or biomass response to Fe amendments, the traditional way by which Fe limitation is defined, the cycling of Fe within the community may be important in determining the success of specific groups of microbes and for the ecosystem as a whole
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