Impacts of metal stress on extracellular microbial products, and potential for selective metal recovery.

Harnessing microbial capabilities for metal recovery from secondary waste sources is an eco-friendly and sustainable approach for the management of metal-containing wastes. Soluble microbial products (SMP) and extracellular polymeric substances (EPS) are the two main groups of extracellular compounds produced by microorganisms in response to metal stress that are of great importance for remediation and recovery of metals. These include various high-, and low, molecular weight components, which serve various functional and structural roles. These compounds often contain functional groups with metal binding potential that can attenuate metal stress by sequestering metal ions, making them less bioavailable. Microorganisms can regulate the content and composition of EPS and SMP in response to metal stress in order to increase the compounds specificity and capacity for metal binding. Thus, EPS and SMP represent ideal candidates for developing technologies for selective metal recovery from complex wastes. To discover highly metal-sorptive compounds with specific metal binding affinity for metal recovery applications, it is necessary to investigate the metal binding affinity of these compounds, especially under metal stressed conditions. In this review we critically reviewed microbial EPS and SMP production as a response to metal stress with a particular emphasis on the metal binding properties of these compounds and their role in altering metal bioavailability. Furthermore, for the first time, we compiled the available data on potential application of these compounds for selective metal recovery from waste streams.

Interrelating EPS, soluble microbial products and metal solubility in a methanogenic consortium stressed by nickel and cobalt.

The relationships between extracellular polymeric substances (EPS), soluble microbial product production, metal solubility, and methanogenic activity were investigated. The individual, and joint, toxic effects of nickel and cobalt on methanogenic consortia fed with glucose as model substrate were studied using biomethane potential assays. Cobalt was found to be less toxic to methanogens than nickel at each concentration tested, and the combined effects of Ni and Co on methane production in the bimetal experiment was higher than the sum of the effects of each metal alone. The protein content of EPS, and extracellular soluble protein fractions, decreased with increasing concentrations of total metals. Meanwhile, no significant change in response to metal stress was apparent for carbohydrate content of EPS or extracellular soluble carbohydrate. Decreasing protein content of EPS was accompanied by reduced methanogenic activity and an increase in the soluble metal fraction. The strong associations observed between these variables could be due to the critical role of EPS in protecting microbial cells against nickel and cobalt stress, possibly by capturing metal cations through their functional groups, thus reducing metal availability to the microbial cells in the methanogenic consortia underpinning the anaerobic digestion process.

Biosurfactant production by Mucor circinelloides: Environmental applications and surface-active properties.

Biosurfactants are structurally a diverse group of surface-active molecules widely used for various purposes in industry. In this study, among 120 fungal isolates, M-06 was selected as a superior biosurfactant producer, based on different standard methods, and was identified as Mucor circinelloides on the basis of its nucleotide sequence of the internal transcribed spacer (ITS) gene. M. circinelloides reduced the surface tension to 26 mN/m and its EI24 index was determined to be 66.6%. The produced biosurfactant exhibited a high degree of stability at a high temperature (121°C), salinity (40 g/L), and acidic pH (2-8). The fermentation broth's ability to recover oil from contaminated sand was 2 and 1.8 times higher than those of water and Tween 80, respectively. The ability of biosurfactant to emulsify crude oil in the sea and fresh water was 64.9 and 48% respectively. This strain could remove 87.6% of crude oil in the Minimal Salt Medium (MSM) crude oil as the sole carbon source. The results from a primary chemical characterization of crude biosurfactant suggest that it is of a glycolipid nature. The strain and its biosurfactant could be used as a potent candidate in bioremediation of oil-contaminated water, soil, and for oil recovery processes.