Cyanobacteria are key prokaryotic primary producers in diverse ecosystems, yet the role of cyanobacterial siderophores in shaping their associated microbiomes remains unexplored. Our study demonstrates the benefits provided to the heterotrophic co-habitants of filamentous cyanobacteria in terrestrial microbial biofilms, focusing on the recently discovered widespread siderophores cyanochelins. To address the acceptance of cyanochelin B (CychB) across multiple bacterial classes, we first investigated its role in providing iron to a model siderophore producer P. aeruginosa PAO1 and selected Pseudomonas natural isolates, which were found to utilize CychB under iron limiting conditions while downregulating endogenous siderophore production. In response to CychB, PAO1 expresses a siderophore internalization cluster, which is localized in multiple Pseudomonas natural isolates. Using metagenome analysis, we characterized the bacterial community recruited along with CychB producing Phormidesmis cyanobacteria under long-term iron starvation. Potential CychB acceptor bacteria associated with the CychB producer were predominantly lacking endogenous siderophore machineries. Using siderophore selective pressure, we isolated a genuine CychB acceptor, gram-negative bacterium Methyloversatilis sp. S146 and demonstrated that its genome hosts an iron processing cluster overexpressed after CychB feeding, recognizing Methyloversatilis as a candidate for further mechanistic investigation of iron acquisition–driven microbial interactions. Our results indicate that CychB supports a specific subset of co-habiting heterotrophic bacteria during iron starvation, further emphasizing the role of cyanobacteria as key drivers of nutrient flows within globally important microbial soil crust ecosystems, supporting microbial life in nutrient-limited environments. These findings provide a mechanistic foundation to elucidate the role of cyanochelins as a public good in these communities.
B. P. Falcao, Martinez Yerena Jose Alberto, T. Galica et al.· bioRxiv· 0 citations
Siderophore transport is central to microbial competition, because it determines access to iron, frequently a limiting nutrient. While siderophore-mediated iron uptake via TonB-dependent transporters (TBDTs) has been extensively studied in heterotrophic bacteria, little is known about the functionality and specificity of TBDTs in cyanobacteria. In the present study we functionally characterise the import system of cyanochelin B, a photolytic β-hydroxy aspartate siderophore produced by several filamentous cyanobacteria, including Leptolyngbya sp. NIES-3755. We have identified a cyanochelin B putative transport cassete localized in the vicinity of the cyanochelin biosynthetic gene cluster in Leptolyngbya genome. By expressing the import genes heterologously in a model unicellular cyanobacterium Synechocystis sp. PCC 6803, we established that the transport cassette reconstitutes cyanochelin B-dependent growth, consistent with cyanochelin-mediated iron acquisition. Systematic gene dissection showed that the TBDT (CctA) and the substrate-binding protein (CctB), responsible for binding the siderophore in the periplasm, are alone sufficient for cyanochelin import into Synechocystis cells, with the permeases, ATPase and a cassette-associated ferredoxin supplied in trans by the host. CctA carries an N-terminal AMIN domain, a fusion found only in cyanobacterial TBDTs. The cassette accepts the structurally similar cyanochelin A but not cyanochelin C, enterobactin or pyoverdine, indicating limited promiscuity. The phylogenetic placement of cyanochelin receptors within a broader clade containing citrate-hydroxamate-type siderophore receptors suggests an evolutionary link between transport systems for chemically distinct cyanobacterial siderophores. Our study reports the first functional heterologous expression of a cyanobacterial TonB-dependent transporter and establishes Synechocystis as a promising platform for cyanobacterial xenosiderophore-uptake studies.
Jan Mašek, B. P. Falcao, Lucie Kajan Grodecká et al.· bioRxiv· 0 citations