A Comparative Study of Iron Uptake Rates and Mechanisms amongst Marine and Fresh Water Cyanobacteria: Prevalence of Reductive Iron Uptake.

Hagar Lis,Chana Kranzler,Yeala Shaked,Nir Keren

doi:10.3390/life5010841

Hagar Lis, Chana Kranzler + Show 2 more

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https://doi.org/10.3390/life5010841

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Abstract

In this contribution, we address the question of iron bioavailability to cyanobacteria by measuring Fe uptake rates and probing for a reductive uptake pathway in diverse cyanobacterial species. We examined three Fe-substrates: dissolved inorganic iron (Fe') and the Fe-siderophores Ferrioxamine B (FOB) and FeAerobactin (FeAB). In order to compare across substrates and strains, we extracted uptake rate constants (kin = uptake rate/[Fe-substrate]). Fe' was the most bioavailable Fe form to cyanobacteria, with kin values higher than those of other substrates. When accounting for surface area (SA), all strains acquired Fe' at similar rates, as their kin/SA were similar. We also observed homogeneity in the uptake of FOB among strains, but with 10,000 times lower kin/SA values than Fe'. Uniformity in kin/SA suggests similarity in the mechanism of uptake and indeed, all strains were found to employ a reductive step in the uptake of Fe' and FOB. In contrast, different uptake pathways were found for FeAB along with variations in kin/SA. Our data supports the existence of a common reductive Fe uptake pathway amongst cyanobacteria, functioning alone or in addition to siderophore-mediated uptake. Cyanobacteria combining both uptake strategies benefit from increased flexibility in accessing different Fe-substrates.

Highlights

Cyanobacteria are a diverse and widespread group of prokaryotes found in a range of marine, fresh, and brackish water environments
We studied the uptake rates and mechanisms of dissolved inorganic iron (Fe') by seven strains of cyanobacteria
free inorganic iron (Fe)-limitation (Figure 1a,c) and those grown under Fe-sufficient conditions (Figure 1b,d), with uptake rate constants of all strains increasing by an order of magnitude under iron limitation, with the exception of Anabaena UTEX2576

Summary

Introduction

Cyanobacteria are a diverse and widespread group of prokaryotes found in a range of marine, fresh, and brackish water environments. Apart from contributing significantly to global primary production [1,2] and nitrogen fixation [3], cyanobacteria influence chemical cycling, ecological structure, and water quality on a regional scale. Due to the high iron content of their photosynthetic apparatus and the role of ferric enzymes in nitrogen fixation, cyanobacteria have high Fe demands relative to heterotrophic bacteria and eukaryotic phytoplankton [4,5,6,7]. Iron often limits cyanobacterial carbon e.g., [8,9] and nitrogen fixation rates [5] and was shown to influence species composition and cyanobacterial abundance e.g., [10]. Iron is characterized by its exceedingly low solubility in oxic, circum-neutral pH waters, and, as such, Fe rapidly precipitates out of solution as ferric oxyhydroxide species [11]. The more accessible soluble iron pool (

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