Cyanide utilization and degradation by microorganisms

Batch and semi-continuous treatment of cassava wastewater using microbial fuel cells and metataxonomic analysis

The treatment of agroindustrial wastewater using microbial fuel cells (MFCs) is a technological strategy to harness its chemical energy while simultaneously purifying the water. This manuscript investigates the organic load effect as chemical oxygen demand (COD) on the production of electricity during the treatment of cassava wastewater by means of a dual-chamber microbial fuel cell in batch mode. Additionally, specific conditions were selected to evaluate the semi-continuous operational mode. The dynamics of microbial communities on the graphite anode were also investigated. The maximum power density delivered by the batch MFC (656.4 μW m- 2 ) was achieved at the highest evaluated organic load (6.8 g COD L- 1 ). Similarly, the largest COD removal efficiency (61.9%) was reached at the lowest organic load (1.17 g COD L- 1 ). Cyanide degradation percentages (50-70%) were achieved across treatments. The semi-continuous operation of the MFC for 2 months revealed that the voltage across the cell is dependent on the supply or suspension of the organic load feed. The electrode polarization resistance was observed to decreases over time, possibly due to the enrichment of the anode with electrogenic microbial communities. A metataxonomic analysis revealed a significant increase in bacteria from the phylum Firmicutes, primarily of the genus Enterococcus.

The Autotrophic Core: An Ancient Network of 404 Reactions Converts H2, CO2, and NH3 into Amino Acids, Bases, and Cofactors

The metabolism of cells contains evidence reflecting the process by which they arose. Here, we have identified the ancient core of autotrophic metabolism encompassing 404 reactions that comprise the reaction network from H₂, CO₂, and ammonia (NH₃) to amino acids, nucleic acid monomers, and the 19 cofactors required for their synthesis. Water is the most common reactant in the autotrophic core, indicating that the core arose in an aqueous environment. Seventy-seven core reactions involve the hydrolysis of high-energy phosphate bonds, furthermore suggesting the presence of a non-enzymatic and highly exergonic chemical reaction capable of continuously synthesizing activated phosphate bonds. CO₂ is the most common carbon-containing compound in the core. An abundance of NADH and NADPH-dependent redox reactions in the autotrophic core, the central role of CO₂, and the circumstance that the core’s main products are far more reduced than CO₂ indicate that the core arose in a highly reducing environment. The chemical reactions of the autotrophic core suggest that it arose from H₂, inorganic carbon, and NH₃ in an aqueous environment marked by highly reducing and continuously far from equilibrium conditions. Such conditions are very similar to those found in serpentinizing hydrothermal systems.

Effects of physicochemical properties of Au cyanidation tailings on cyanide microbial degradation

The initial cyanide (CN^-) concentration and amount of co-contaminants in GCTs can inhibit bacterial growth and reduce the CN^--degrading ability of bacteria. Several microorganisms can biotransform a wide range of organic and inorganic industrial contaminants into nontoxic compounds. However, active enzymatic CN^- metabolism processes are mostly constrained by the physical and chemical characteristics of GCTs. High concentrations of toxic metal co-contaminants, such as, Pb, and Cr, and factors, such as pH, temperature, and oxygen concentration create oxidative stress and limit the CN^--degrading potential of cyanotrophic strains. The effects of such external and internal factors on the CN^--degrading ability of bacteria hinder the selection of suitable microorganisms for CN^- biodegradation. Therefore, understanding the effects of the physicochemical properties of GCTs on cyanobacteria strains can help identify suitable microbes and favorable environmental conditions to promote microbial growth and can also help design efficient CN^- biodegradation processes. In this review, we present a detailed analysis of the physicochemical properties of GCTs and their effects on microbial CN^- degradation.

Optimization of nitric oxide donors for investigating biofilm dispersal response in Pseudomonas aeruginosa clinical isolates

Pseudomonas aeruginosa biofilms contribute heavily to chronic lung infection in cystic fibrosis patients, leading to morbidity and mortality. Nitric oxide (NO) has been shown to disperse P. aeruginosa biofilms in vitro, ex vivo and in clinical trials as a promising anti-biofilm agent. Traditional NO donors such as sodium nitroprusside (SNP) have been extensively employed in different studies. However, the dosage of SNP in different studies was not consistent, ranging from 500 nM to 500 μM. SNP is light sensitive and produces cyanide, which may lead to data misinterpretation and inaccurate predictions of dispersal responses in clinical settings. New NO donors and NO delivery methods have therefore been explored. Here we assessed 7 NO donors using P. aeruginosa PAO1 and determined that SNP and Spermine NONOate (S150) successfully reduced > 60% biomass within 24 and 2 h, respectively. While neither dosage posed toxicity towards bacterial cells, chemiluminescence assays showed that SNP only released NO upon light exposure in M9 media and S150 delivered much higher performance spontaneously. S150 was then tested on 13 different cystic fibrosis P. aeruginosa (CF-PA) isolates; most CF-PA biofilms were significantly dispersed by 250 μM S150. Our work therefore discovered a commercially available NO donor S150, which disperses CF-PA biofilms efficiently within a short period of time and without releasing cyanide, as an alternative of SNP in clinical trials in the future. KEY POINTS: • S150 performs the best in dispersing P. aeruginosa biofilms among 7 NO donors. • SNP only releases NO in the presence of light, while S150 releases NO spontaneously. • S150 successfully disperses biofilms formed by P. aeruginosa cystic fibrosis clinical isolates.

Potential Mechanism of Detoxification of Cyanide Compounds by Gut Microbiomes of Bamboo-Eating Pandas

The giant panda (Ailuropoda melanoleuca) and red panda (Ailurus fulgens), two obligate bamboo feeders, have distinct phylogenetic positions in the order Carnivora. Bamboo is extraordinarily rich in plant secondary metabolites, such as allied phenolic and polyphenolic compounds and even toxic cyanide compounds. Here, the enrichment of putative cyanide-digesting gut microbes, in combination with adaptations related to morphology (e.g., pseudothumbs) and genomic signatures, show that the giant panda and red panda have evolved some common traits to adapt to their bamboo diet. Thus, here is another story of diet-driven gut microbiota in nature.

Gut microbes can enhance the ability of hosts to consume secondary plant compounds and, therefore, expand the dietary niche breadth of mammalian herbivores. The giant and red pandas are bamboo-eating specialists within the mammalian order Carnivora. Bamboo contains abundant plant secondary metabolites (e.g., cyanide-containing compounds). However, Carnivora species, including the giant panda, have deficient levels of rhodanese (one of the essential cyanide detoxification enzymes) in their tissues compared with the same tissues of herbivores. Here, we make a comparative analysis of 94 gut metagenomes, including 25 from bamboo-eating pandas (19 from giant pandas and 6 from red pandas), 30 from Père David’s deer, and 39 from published data for other mammals. The bamboo-eating pandas’ gut microbiomes had some common features, such as high proportions of Pseudomonas bacteria. The results revealed that bamboo-eating pandas’ gut microbiomes were significantly enriched in putative genes coding for enzymes related to cyanide degradation (e.g., rhodanese) compared with the gut microbiomes of typical herbivorous mammals, which might have coevolved with their special bamboo diets. The enrichment of putative cyanide-digesting gut microbes, in combination with adaptations related to morphology (e.g., pseudothumbs) and genomic signatures, show that the giant panda and red panda have evolved some common traits to adapt to their bamboo diet.

IMPORTANCE The giant panda (Ailuropoda melanoleuca) and red panda (Ailurus fulgens), two obligate bamboo feeders, have distinct phylogenetic positions in the order Carnivora. Bamboo is extraordinarily rich in plant secondary metabolites, such as allied phenolic and polyphenolic compounds and even toxic cyanide compounds. Here, the enrichment of putative cyanide-digesting gut microbes, in combination with adaptations related to morphology (e.g., pseudothumbs) and genomic signatures, show that the giant panda and red panda have evolved some common traits to adapt to their bamboo diet. Thus, here is another story of diet-driven gut microbiota in nature.

Enzymatic assimilation of cyanide via pterin-dependent oxygenolytic cleavage to ammonia and formate in Pseudomonas fluorescens NCIMB 11764

Utilization of cyanide as a nitrogen source by Pseudomonas fluorescens NCIMB 11764 occurs via oxidative conversion to carbon dioxide and ammonia, with the latter compound satisfying the nitrogen requirement. Substrate attack is initiated by cyanide oxygenase (CNO), which has been shown previously to have properties of a pterin-dependent hydroxylase. CNO was purified 71-fold and catalyzed the quantitative conversion of cyanide supplied at micromolar concentrations (10 to 50 micro M) to formate and ammonia. The specific activity of the partially purified enzyme was approximately 500 mU/mg of protein. The pterin requirement for activity could be satisfied by supplying either the fully (tetrahydro) or partially (dihydro) reduced forms of various pterin compounds at catalytic concentrations (0.5 micro M). These compounds included, for example, biopterin, monapterin, and neopterin, all of which were also identified in cell extracts. Substrate conversion was accompanied by the consumption of 1 and 2 molar equivalents of molecular oxygen and NADH, respectively. When coupled with formate dehydrogenase, the complete enzymatic system for cyanide oxidation to carbon dioxide and ammonia was reconstituted and displayed an overall reaction stoichiometry of 1:1:1 for cyanide, O(2), and NADH consumed. Cyanide was also attacked by CNO at a higher concentration (1 mM), but in this case formamide accumulated as the major reaction product (formamide/formate ratio, 0.6:0.3) and was not further degraded. A complex reaction mechanism involving the production of isocyanate as a potential CNO monooxygenation product is proposed. Subsequent reduction of isocyanate to formamide, whose hydrolysis occurs as a CNO-bound intermediate, is further envisioned. To our knowledge, this is the first report of enzymatic conversion of cyanide to formate and ammonia by a pterin-dependent oxygenative mechanism.

CynD, the cyanide dihydratase from Bacillus pumilus: gene cloning and structural studies

The cyanide dihydratase in Bacillus pumilus was shown to be an 18-subunit spiral structure by three-dimensional reconstruction of electron micrographs of negatively stained material at its optimum pH, 8.0. At pH 5.4, the subunits rearrange to form an extended left-handed helix. Gel electrophoresis of glutaraldehyde cross-linked enzyme suggests that the fundamental component of the spiral is a dimer of the 37-kDa subunit. The gene was cloned, and the recombinant enzyme was readily expressed at high levels in Escherichia coli. Purification of the recombinant enzyme was facilitated by the addition of a C-terminal six-histidine affinity purification tag. The tagged recombinant enzyme has K(m) and V(max) values similar to those published for the native enzyme. This is the first cyanide dihydratase from a gram-positive bacterium to be sequenced, and it is the first description of the structure of any member of this enzyme class. The putative amino acid sequence shares over 80% identity to the only other sequenced cyanide dihydratase, that of the gram-negative Pseudomonas stutzeri strain AK61, and is similar to a number of other bacterial and fungal nitrilases. This sequence similarity suggests that the novel short spiral structure may be typical of these enzymes. In addition, an active cyanide dihydratase from a non-cyanide-degrading isolate of B. pumilus (strain 8A3) was cloned and expressed. This suggests that cynD, the gene coding for the cyanide dihydratase, is not unique to the C1 strain of B. pumilus and is not a reflection of its origin at a mining waste site.

Accumulation of alpha-keto acids as essential components in cyanide assimilation by Pseudomonas fluorescens NCIMB 11764

Pyruvate (Pyr) and alpha-ketoglutarate (alphaKg) accumulated when cells of Pseudomonas fluorescens NCIMB 11764 were cultivated on growth-limiting amounts of ammonia or cyanide and were shown to be responsible for the nonenzymatic removal of cyanide from culture fluids as previously reported (J.-L. Chen and D. A. Kunz, FEMS Microbiol. Lett. 156:61-67, 1997). The accumulation of keto acids in the medium paralleled the increase in cyanide-removing activity, with maximal activity (760 micromol of cyanide removed min-1 ml of culture fluid-1) being recovered after 72 h of cultivation, at which time the keto acid concentration was 23 mM. The reaction products that formed between the biologically formed keto acids and cyanide were unambiguously identified as the corresponding cyanohydrins by 13C nuclear magnetic resonance spectroscopy. Both the Pyr and alpha-Kg cyanohydrins were further metabolized by cell extracts and served also as nitrogenous growth substrates. Radiotracer experiments showed that CO2 (and NH3) were formed as enzymatic conversion products, with the keto acid being regenerated as a coproduct. Evidence that the enzyme responsible for cyanohydrin conversion is cyanide oxygenase, which was shown previously to be required for cyanide utilization, is based on results showing that (i) conversion occurred only when extracts were induced for the enzyme, (ii) conversion was oxygen and reduced-pyridine nucleotide dependent, and (iii) a mutant strain defective in the enzyme was unable to grow when it was provided with the cyanohydrins as a growth substrate. Pyr and alphaKg were further shown to protect cells from cyanide poisoning, and excretion of the two was directly linked to utilization of cyanide as a growth substrate. The results provide the basis for a new mechanism of cyanide detoxification and assimilation in which keto acids play an essential role.

Utilization of cyanide as nitrogenous substrate by Pseudomonas fluorescens NCIMB 11764: evidence for multiple pathways of metabolic conversion

The growth of Pseudomonas fluorescens NCIMB 11764 on cyanide as the sole nitrogen source was accomplished by use of a modified fed-batch cultivation procedure. Previous studies showing that cyanide metabolism in this organism is both an oxygen-dependent and an inducible process, with CO2 and ammonia representing conversion products, were confirmed. However, washed cells (40 mg ml-1 [dry weight]) metabolized cyanide at concentrations far exceeding those previously described; 85% of 50 mM KCN was degraded in 6 h. In addition, two other C1 metabolites were detected in incubation mixtures; their identities were confirmed as formamide and formate by 13C nuclear magnetic resonance spectrocopy, high-pressure liquid chromatography, radioisotopic trapping experiments, and other analytical means. The relative yields of all four metabolites (CO2, formamide, formate, and ammonia) were shown to be dependent on the KCN concentration and availability of oxygen; at 0.5 to 10 mM substrate, CO2 was the major C1 product, whereas at 20 and 50 mM substrate, formamide and formate were principally formed. The latter two metabolites also accumulated during prolonged anaerobic incubation, suggesting that P. fluorescens NCIMB 11764 can elaborate several pathways of cyanide conversion. One is formally similar to that proposed previously (R. E. Harris and C. J. Knowles, FEMS Microbiol. Lett. 20:337-341, 1983), involving the oxygen-dependent conversion of cyanide to CO2 and ammonia. The other two, occurring in the presence or absence of oxygen, involve separate reactions to yield, respectively, formate plus ammonia or formamide.(ABSTRACT TRUNCATED AT 250 WORDS)