ORIGINAL ARTICLE Siderophore-assisted cadmium hyperaccumulation in Bacillus subtilis Azmi Khan 1 & Adity Gupta 1 & Pratika Singh 1 & Arun Kumar Mishra 2 & Rajesh Kumar Ranjan 3 & Amrita Srivastava 1 Received: 19 June 2019 /Revised: 12 September 2019 /Accepted: 16 September 2019 # Springer Nature Switzerland AG 2019 Abstract Siderophores (Gk iron carriers) are low molecular weight secondary metabolites produced by bacteria, fungi, and plants that have strong binding affinity for iron. Owing to their iron-chelating ability, they are produced mainly when the organism faces iron scarcity. The present study empirically investigated the importance of applying hydroxamate siderophore extracted from Aspergillus nidulans to the cells of Bacillus subtilis for bioremediation of cadmium salt. This investigation deals with siderophore-mediated intracellular Cd accumulation by bacterial cells, growth estimation, biochemical assays like lipid perox- idation, total protein content, carbohydrate content, and iron content estimation. In silico docking and STRING analyses revealed specific interaction between Aspergillus siderophore and receptors present on B. subtilis. Estimation of intracellular Cd by atomic absorption spectroscopy showed more accumulation of Cd ions by B. subtilis in the presence of hydroxamate siderophore. This suggests a possibility of confiscating environmental Cd 2+ by utilizing metal chelation property of siderophores and hence can lead to emerging bioremediation mechanisms for heavy metals. In silico studies support experimental investigation and suggest higher affinity of siderophore for Cd ions as compared with ferric ions. Keywords Siderophore . Cadmium bioremediation . Bacillus subtilis . Altruism . Aspergillus nidulans Introduction Metals are extremely essential for different biological func- tions but their accumulation in undesirable amount via in- creasing industrial influence has created immense metal pol- lution. The agricultural sector has also added to this atrocity through excess usage of chemical fertilizers, mainly phospho- rus fertilizers which add several metals including cadmium to the soils (Wuana and Okieimen 2011; Savci 2012; AlKhader 2015; Bitew and Alemayehu 2017). Cd has been ranked among the top 20 carcinogens, being a widespread pollutant and non-redox metal that leads to the formation of reactive oxygen species (ATSDR 2015). Adverse cellular conditions such as membrane disruption, protein degradation, loss of enzymatic activity, and damage to nucleic acids occur due to oxidative stress leading to growth limitation as well as cell death in several organisms (Biryukova et al. 2011). Thus, reducing its amount is an urgent need. Microbes are known to transform elements like arsenic and selenium via methyla- tion, oxidation, and reduction (Stolz et al. 2002). Containment of unwanted heavy metals through biological means projects an economic and ecologically stable alternative to more haz- ardous chemical-dependent methods. Biological means of heavy metal chelation provide a safe and cost-effective meth- od over traditional chemical-based methods. Siderophore- mediated bioremediation is one such alternative. Siderophores are essentially produced during iron scarcity by a number of microorganisms. Based on their structures, they are categorized as carboxylate, hydroxamate, catecholate, and mixed types (Khan et al. 2018). Apart from iron scaveng- ing, they are known to chelate and mobilize several other metals including Cu, Zn, Ni, Cd, Pb, U(IV), Th(IV), and Pu(IV) and are therefore also termed metallophore (Schalk et al. 2011). Siderophore-mediated detoxification of heavy * Amrita Srivastava amritasrivastava@cub.ac.in 1 Department of Life Science, School of Earth, Biological and Environmental Sciences, Central University of South Bihar, Gaya, Bihar 824236, India 2 Centre for Advanced Study in Botany, Banaras Hindu University, Varanasi, Uttar Pradesh, India 3 Department of Environmental Science, School of Earth, Biological and Environmental Sciences, Central University of South Bihar, Gaya, Bihar, India International Microbiology https://doi.org/10.1007/s10123-019-00101-4