Divoka, P. | bioRxiv Science

bioRxiv2026

Utilization of cyanobacterial siderophore cyanochelin B by phylogenetically distant heterotrophs suggest its role in mediating microbial interactions

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.

microbiology↗

bioRxiv2023

More than just an Eagle Killer: The freshwater cyanobacterium Aetokthonos hydrillicola produces highly toxic dolastatin derivatives

Cyanobacteria are infamous producers of toxins. While the toxic potential of planktic cyanobacterial blooms is well documented, the ecosystem level effects of toxigenic benthic and epiphytic cyanobacteria are an understudied threat. The freshwater epiphytic cyanobacterium Aetokthonos hydrillicola has recently been shown to produce the "eagle killer" neurotoxin aetokthonotoxin causing the fatal neurological disease Vacuolar Myelinopathy. The disease affects a wide array of wildlife in the southeastern United States, most notably waterfowl and birds of prey, including the bald eagle. In an assay for cytotoxicity, we found the crude extract of the cyanobacterium to be much more potent than pure aetokthonotoxin, prompting further investigation. Here, we describe the isolation and structure elucidation of the aetokthonostatins, linear peptides belonging to the dolastatin compound family, featuring a unique modification of the C-terminal phenylalanine derived moiety. Using immunofluorescence microscopy and molecular modeling, we confirmed that aetokthonostatin acts as a potent tubulin binder. We also show that aetokthonostatin inhibits reproduction of the nematode C. elegans, resulting in increased population lethality of the combined action of the two toxins produced by A. hydrillicola. Bioinformatic analysis revealed the aetokthonostatin biosynthetic gene cluster encoding a non-ribosomal peptide synthe-tase/polyketide synthase accompanied by a unique tailoring machinery. The biosynthetic activity of a specific N-terminal methyltransferase was confirmed by in vitro biochemical studies, establishing a mechanistic link between the gene cluster and its product. Significance StatementCyanotoxins have adverse effects on ecosystems. Our understanding of their potential risk has recently been expanded by the discovery of aetokthonotoxin, produced by the cyanobacterium Aetokthonos hydrillicola growing on invasive plants. Via trophic transfer, it acts as a neurotoxin causing mortality in animals including top predators like Bald Eagles. Closer examination of A. hydrillicola revealed that it also produces highly toxic dolastatin derivatives. A. hydrillicola is the first cultured cyanobacterium producing dolastatin derivatives, allowing us to uncover biosynthetic gene clusters of this compound family. In contrast to all other known dolastatin-producers, which are marine cyanobacteria, A. hydrillicola thrives in freshwater reservoirs, making it a potential threat also for human health. Monitoring of the cyanobacterium and its toxins is strongly recommended.

microbiology↗