Effect of Time Interval on Arsenic Toxicity to Paddy Field Cyanobacteria as Evident by Nitrogen Metabolism, Biochemical Constituent, and Exopolysaccharide Content

Synergistic regulation of hydrogen sulfide and nitric oxide on biochemical components, exopolysaccharides, and nitrogen metabolism in nickel stressed rice field cyanobacteria

The present study examined the regulatory mechanism of hydrogen sulfide (H2S) and nitric oxide (NO) in nickel (Ni) stressed cyanobacteria viz., Nostoc muscorum and Anabaena sp. by analyzing growth, photosynthetic pigments, biochemical components (protein and carbohydrate), exopolysaccharides (EPS), inorganic nitrogen content, and activity of enzymes comprised in nitrogen metabolism and Ni accumulation. The 1 µM Ni substantially diminished growth by 18% and 22% in N. muscorum and Anabaena sp. respectively, along with declining the pigment contents (Chl a/Car ratio and phycobiliproteins), and biochemical components. It also exerted negative impacts on inorganic uptake of nitrate and nitrite contents; nitrate reductase and nitrite reductase; and ammonium assimilating enzymes (glutamine synthetase, glutamate synthase, and glutamate dehydrogenase exhibited a reverse trend) activities. Nonetheless, the adverse impact of Ni can be mitigated through the exogenous supplementation of NaHS [sodium hydrosulfide (8 µM); H2S donor] and SNP [sodium nitroprusside (10 µM); NO donor] which showed substantial improvement on growth, pigments, nitrogen metabolism, and EPS layer and noticeably occurred as a consequence of a substantial reduction in Ni accumulation content which minimized the toxicity effects. The accumulation of Ni on both the cyanobacterial cell surface (EPS layer) are confirmed by the SEM-EDX analysis. Further, the addition of NO scavenger (PTIO; 20 µM) and inhibitor of NO (L-NAME; 100 µM); and H2S scavenger (HT; 20 µM) and H2S inhibitor (PAG; 50 µM) reversed the positive responses of H2S and NO and damages were more prominent under Ni stress thereby, suggesting the downstream signaling of H2S on NO-mediated alleviation. Thus, this study concludes the crosstalk mechanism of H2S and NO in the mitigation of Ni-induced toxicity in rice field cyanobacteria.

Genome sequence and characterisation of a freshwater photoarsenotroph, Cereibacter azotoformans strain ORIO, isolated from sediments capable of cyclic light-dark arsenic oxidation and reduction

A freshwater photosynthetic arsenite-oxidizing bacterium, Cereibacter azotoformans strain ORIO, was isolated from Owens River, CA, USA. The waters from Owens River are elevated in arsenic and serve as the headwaters to the Los Angeles Aqueduct. The complete genome sequence of strain ORIO is 4.8 Mb genome (68% G + C content) and comprises two chromosomes and six plasmids. Taxonomic analysis placed ORIO within the Cereibacter genus (formerly Rhodobacter). The ORIO genome contains arxB2 AB1 CD (encoding an arsenite oxidase), arxXSR (regulators) and several ars arsenic resistance genes all co-localised on a 136 kb plasmid, named pORIO3. Phylogenetic analysis of ArxA, the molybdenum-containing arsenite oxidase catalytic subunit, demonstrated photoarsenotrophy is likely to occur within members of the Alphaproteobacteria. ORIO is a mixotroph, oxidises arsenite to arsenate (As(V)) photoheterotrophically, and expresses arxA in cultures grown with arsenite. Further ecophysiology studies with Owens River sediment demonstrated the interconversion of arsenite and As(V) was dependent on light-dark cycling. arxA and arrA (As(V) respiratory reductase) genes were detected in the light-dark cycled sediment metagenomes suggesting syntrophic interactions among arsenotrophs. This work establishes C. azotoformans str. ORIO as a new model organism for studying photoarsenotrophy and light-dark arsenic biogeochemical cycling.

The combined supplementation of melatonin and salicylic acid effectively detoxifies arsenic toxicity by modulating phytochelatins and nitrogen metabolism in pepper plants

The main objective of the study was to assess if joint application of melatonin (MT, 0.1 mM) and salicylic acid (SA 0.5 mM) could improve tolerance of pepper plants to arsenic (As) as sodium hydrogen arsenate heptahydrate (0.05 mM). The imposition of arsenic stress led to accumulation of As in roots and leaves, and increased contents of leaf proline, phytochelatins, malondialdehyde (MDA) and H₂O₂, but it reduced plant biomass, chlorophylls (Chl), PSII maximum efficiency (Fv/Fm) and leaf water potential. Melatonin and SA applied jointly or alone enhanced nitrogen metabolism by triggering the activities of glutamate synthase, glutamine synthetase, and nitrite reductases and nitrate. In comparison with a single treatment of MT or SA, the joint treatment of MT and SA had better impact on enhancing growth and key biological events and decreasing tissue As content. This clearly shows a cooperative function of both agents in enhancing tolerance to As-toxicity in pepper plants.

Genome description of Nostoc ellipsosporum strain NOK (Nostocales, Cyanobacteria) isolated from an arsenic contaminated paddy field of the Bengal Delta Plains

A diazotrophic cyanobacterium, Nostoc ellipsosporum strain NOK, was isolated from an arsenic contaminated paddy field of the Bengal Delta Plains (BDP). Located across India and Bangladesh, BDP, is one of the worst arsenic (As)-affected regions of the world. Previous studies have shown this isolate to be tolerant to high concentration of arsenic (≦400 μM). The genome of this isolate was sequenced to identify the genes involved in various metabolic pathways including arsenic resistance and biofilm formation. Whole genome analyses showed Nostoc ellipsosporum strain NOK to be closely related to N. punctiforme strain PCC73102. The genome is about 10.9 Mbp which assembled into 694 contigs. Genome annotation identified 10120 genes out of which 10000 were CDSs. There are a total of 9927 protein coding genes in addition to 120 RNA coding genes. The genome codes three 5S rRNA, four 16S rRNA and three 23S rRNA genes along with 103 tRNAs, 7 ncRNAs and 73 pseudo-genes. The G + C% of the genome is 54.28. The genome codes for crucial genes involved in biofilm formation in response to stress conditions including arsenic stress. The arsBHC operon is present within the genome which makes this tolerant to high concentration of arsenic which might lead to biofilm formation. A number of ABC transporters including cysUW and sbp (sulfate/thiosulfate), nrtABC (nitrate/nitrite/cyanate), cmpABCD (bicarbonate), ssuABC (alkane sulfonate), modABCF (molybdate; 2 copies), afuAC (iron), pstA (phosphate; 2 copies), pstBC, pstS (4 copies), cbiOQ (cobalt and nickel) and opuBC, opuBB and opuBA (osmoprotectants) were identified.

Regulation of redox homeostasis in cadmium stressed rice field cyanobacteria by exogenous hydrogen peroxide and nitric oxide

In the present study, defensive strategies of H2O2 mediated NO signaling were analyzed in Cd stressed Nostoc muscorum and Anabaena sp. Exogenously supplied SNP (10 µM) and H2O2 (1 µM) lessen the toxicity of Cd (6 µM) but without NO; H2O2 was unable to release the stress from cyanobacterial cells potentially. The reduced contents of exopolysaccharide, protein content, endogenous NO and enzymatic antioxidants (SOD, POD, CAT, and GST) due to Cd toxicity, were found increased significantly after exogenous application of H2O2 and SNP thereafter, cyanobacterial calls flourished much better after releasing toxic level of Cd. Moreover, increased level of ROS due to Cd stress also normalized under exogenous application of H2O2 and SNP. However, chelation of NO hindered the signaling mechanism of H2O2 that diminished its potential against Cd stress while signaling of NO has not been hindered by chelation of H2O2 and NO potentially released the Cd stress from cyanobacterial cells. In conclusion, current findings demonstrated the synergistic signaling between H2O2 and NO towards the improvement of cyanobacterial tolerance to Cd stress, thereby enhancing the growth and antioxidant defense system of test cyanobacteria that improved fertility and productivity of soil even under the situation of metal contamination.