New insights into iron acquisition by cyanobacteria: an essential role for ExbB-ExbD complex in inorganic iron uptake

Abstract

Cyanobacteria are globally important primary producers that have an exceptionally large iron requirement for photosynthesis. In many aquatic ecosystems, the levels of dissolved iron are so low and some of the chemical species so unreactive that growth of cyanobacteria is impaired. Pathways of iron uptake through cyanobacterial membranes are now being elucidated, but the molecular details are still largely unknown. Here we report that the non-siderophore-producing cyanobacterium Synechocystis sp. PCC 6803 contains three exbB-exbD gene clusters that are obligatorily required for growth and are involved in iron acquisition. The three exbB-exbDs are redundant, but single and double mutants have reduced rates of iron uptake compared with wild-type cells, and the triple mutant appeared to be lethal. Short-term measurements in chemically well-defined medium show that iron uptake by Synechocystis depends on inorganic iron (Fe 0 ) concentration and ExbB-ExbD complexes are essentially required for the Fe 0 transport process. Although transport of iron bound to a model siderophore, ferrioxamine B, is also reduced in the exbB-exbD mutants, the rate of uptake at similar total [Fe] is about 800-fold slower than Fe 0 , suggesting that hydroxamate siderophore iron uptake may be less ecologically relevant than free iron. These results provide the first evidence that ExbB-ExbD is involved in inorganic iron uptake and is an essential part of the iron acquisition pathway in cyanobacteria. The involvement of an ExbB-ExbD system for inorganic iron uptake may allow cyanobacteria to more tightly maintain iron homeostasis, particularly in variable environments where iron concentrations range from limiting to sufficient.

Long-Term Acclimation to Iron Limitation Reveals New Insights in Metabolism Regulation of Synechococcus sp. PCC7002

“…Acclimation to different Fe' concentrations is essential to directly investigate the physiological and transcriptomic responses resulting from Fe homeostasis (Mock and Hoch, 2005). Typically, transcriptomic studies, report short-term responses to Fe stress induced by starvation through omission of Fe in the culture medium (Nodop et al, 2008;Jiang et al, 2015) or addition of a strong Fe binding ligand such as desferrioxamine B (Shcolnick et al, 2009;Ludwig and Bryant, 2012). However, with exception of final phases of a phytoplankton bloom, abrupt increase in Fe limitation is not common in marine environment.…”

“…However, we observed that also genes encoding for siderophore receptors were expressed already under mild-Felim. Thus, together with the decrease of cell size (Figure 1B), that increases the cellular specific surface, the upregulation of high-affinity uptake systems (Kranzler et al, 2014;Jiang et al, 2015) enhances Fe bioavailability to the cell already at Fe' concentration typical of oceanic Fe rich systems.…”

Physiological responses of the freshwater N2‐fixing cyanobacterium Raphidiopsis raciborskii to Fe and N availabilities

“…Previous laboratory-based studies have revealed that Fe bioavailability is principally determined by the concentration of dissolved unchelated Fe (rather than total Fe) buffered by organic ligands for both eukaryotic and prokaryotic cells (Morel et al, 2008;Fujii et al, 2011b). The prevalence of unchelated Fe uptake by non-diazotrophic freshwater cyanobacteria including Microcystis (Fujii et al, 2014b) and Synechocystis (Jiang et al, 2015) as well as the lack of siderophore-associated genes in a number of prokaryotic phytoplankton (Hopkinson and Barbeau, 2012) is indicative of the importance of unchelated Fe uptake by phytoplankton. The prevalence of unchelated Fe uptake by non-diazotrophic freshwater cyanobacteria including Microcystis (Fujii et al, 2014b) and Synechocystis (Jiang et al, 2015) as well as the lack of siderophore-associated genes in a number of prokaryotic phytoplankton (Hopkinson and Barbeau, 2012) is indicative of the importance of unchelated Fe uptake by phytoplankton.…”

Identification of an iron permease, cFTR1, in cyanobacteria involved in the iron reduction/re‐oxidation uptake pathway

“…Cyanobacteria, like other gram‐negative bacteria, possess an outer membrane (OM) as well as a plasma membrane that poses an additional barrier to the transport of iron and other essential solutes. Unchelated inorganic iron is thought to be the predominant form of iron for uptake by most freshwater and coastal cyanobacteria (Fujii et al ., ; Kranzler et al ., ) and its uptake is mediated by a TonB‐ExbB‐ExbD‐dependent OM transport pathway (Jiang et al ., ). Once in the periplasmic space, unchelated Fe 3+ is likely complexed by the abundant FutA2 protein (Badarau et al ., ) and then transported across the plasma membrane by the FutABC complex (Kranzler et al ., ).…”