Sulfur is one of the most important and versatile elements in biology and forms the basis of energy metabolism in many prokaryotes. Its extensive large-scale redox transformations drive the global biogeochemical sulfur cycle and profoundly influence environmental chemistry. In dissimilatory sulfur oxidizers, the oxidation of reduced sulfur compounds is directly coupled to energy conservation via photosynthetic or respiratory electron transport chains. Sulfur-oxidizing bacteria and archaea are remarkably diverse, reflecting both the wide range of habitats they inhabit and the variety of metabolic pathways through which sulfur compounds are exploited as electron donors. Over the past decades, these pathways have been examined in depth using a spectrum of molecular genetic, biochemical, and omics-based approaches, primarily in model organisms amenable to genetic manipulation. Particular emphasis has been placed on thiosulfate oxidation mediated by the Sox multienzyme system and thiosulfate dehydrogenases, as well as on cytoplasmic sulfane sulfur oxidation involving dissimilatory sulfite reductase and the sulfur-oxidizing heterodisulfide reductase-like sHdr complex. Recent advances have highlighted the central role of specific lipoate-binding proteins that are essential for efficient sHdr-dependent sulfur oxidation. This review provides an overview of current knowledge on prokaryotic sulfur oxidation pathways. In addition, emerging insights into the complex regulatory networks operating in facultative sulfur oxidizers are provided, with particular attention to how these organisms coordinate sulfur metabolism with changing environmental and energetic conditions.
Bacteria obtain nutrients and other essential factors from their immediate surrounding. For bacteria that reside in a specialized or limited niche range, their metabolism reflects adaptation to that environment(s). Bordetella pertussis and B. parapertussis have evolved from a B. bronchiseptica-like ancestor that survives within hosts and in the environment to become restricted to a single niche, the human respiratory tract. This restriction has shaped their metabolism. Here, the multifaceted role of nicotinic acid in Bordetella biology is reviewed. It serves as a precursor for the synthesis of NAD, for which these Bordetella are auxotrophic, but also induces profound changes in gene expression profiles in a concentration-dependent manner. While these Bordetella display very high levels of conservation in many aspects, they differ in their utilization of nicotinic acid. Understanding the consequences of this difference may shed light on host-pathogen interactions of these niche specialized species.
Glycyl radical enzymes (GRE) use radical-based biochemistry to catalyze the cleavage or formation of the carbon-carbon and carbon-heteroatom bonds found in organic molecules. These evolutionarily ancient enzymes are highly oxygen-sensitive but widespread in obligate as well as facultative anaerobes. When synthesized, GRE lack activity. The radical must be installed through the action of an activating enzyme (AE) that belongs to the radical S-adenosylmethionine superfamily. Typically, each GRE has a dedicated GRE-AE. Hydrogen atom abstraction from a highly conserved glycine residue located within the GRE's C-terminal domain introduces the radical into its storage location. Upon substrate binding, the radical is transferred to a conserved cysteine residue within the active site, transiently generating a thiyl radical species and initiating catalysis. There are currently five biochemical classes of GRE. Nevertheless, these GRE classes share a common mechanism of glycyl-radical storage and thiyl radical-mediated generation of a substrate radical. GRE exhibit similar tertiary structures and conserved active sites, but otherwise show limited primary structural conservation. These features are consistent with them diverging from an ancient common ancestor. Recent genome mining analyses, combined with structural and biochemical studies, are revealing the ubiquity of GRE, especially in uncultured gut microbiota and in association with disease states. Moreover, GRE are predominant in environmental microbiota associated with anaerobic degradation of hydrocarbons. GRE now have mainly catabolic roles in fermentative heterotrophs, but their universal involvement in anaerobic DNA synthesis and in other chemically challenging reactions suggests earlier in evolution biosynthesis of organic molecules might have been their main function.
Iron-sulfur (Fe-S) clusters are essential cofactors that support a wide range of metabolic and regulatory processes across all domains of life. The assembly and distribution of these chemically labile cofactors require tightly coordinated biosynthetic and trafficking systems that respond dynamically to cellular iron availability and redox conditions. Monothiol CGFS-type glutaredoxins (Grxs) have emerged as central components of these networks. Once considered primarily thiol-disulfide oxidoreductases, these proteins are now recognized as versatile Fe-S cluster carriers that coordinate and exchange [2Fe-2S] clusters through glutathione (GSH)-dependent mechanisms. This review synthesizes current understanding of monothiol Grxs across bacteria, fungi, and protists, highlighting both conserved biochemical functions and lineage-specific adaptations. In all systems, CGFS Grxs function as intermediates in Fe-S cluster trafficking pathways, facilitating the transfer of clusters from assembly machineries to downstream targets. In fungi, these proteins have been further co-opted into regulatory circuits, where Grxs alone or in partnership with BolA proteins directly couple mitochondrial Fe-S cluster biogenesis to nuclear transcriptional control of iron homeostasis. In contrast, bacterial and protist systems exhibit more indirect or emerging regulatory roles, often integrating Fe-S metabolism with broader redox and stress-response networks. We propose a unifying model in which monothiol Grxs act as dynamic Fe-S "rheostats" that sense and redistribute labile clusters in response to cellular conditions, thereby linking iron metabolism to physiological adaptation. Understanding how this conserved molecular framework is differentially deployed across organisms provides new insight into microbial iron homeostasis and reveals potential targets for therapeutic intervention in pathogenic systems.
Electronic waste (E-waste) is one of the fastest growing waste streams worldwide, posing serious challenges for environmental and human health, while representing a rich secondary source of valuable metals. Among these technology-critical metals (TCMs) are those, which are essential for renewable energy systems, electronics and low carbon technologies, yet face increasingly disrupted supply chains. This chapter explores the role of E-waste as an urban mine within a circular economy framework and highlights the emerging bio-based strategies for sustainable metal recovery, with particular emphasis on bacterial systems. Bacteria employ versatile mechanisms such as biosorption, bioaccumulation, bioleaching and biomineralization to capture, dissolve and transform metals, offering a green alternative to conventional physical and chemical methods for metal recovery. Beyond recovery, bacteria can upcycle the recovered metals into value added nanoparticles with diverse biomedical, environmental and catalytic applications. Advanced omics approaches including transcriptomics, metabolomics and proteomics provide key insights for the rational design of synthetic biology strategies aimed to enhance metal recovery and enable controlled, efficient synthesis of metal nanoparticles with tailored properties. Overall, the chapter combines E-waste management with microbial biotechnology and nanotechnology presenting a sustainable way to secure technology-critical metals, reducing environmental impact and promoting the transition to a green circular economy.
