Modeling cometabolism of hexavalent chromium by iron reducing bacteria in tertiary substrate system.

In this study, a bacterial strain Serratia sp. was employed for the reduction of synthetically prepared different concentration of Cr(VI) solution (10, 25, 40, 50 and 100 mg/L). Cometabolism study have been carried out in the binary substrate system as well as in the tertiary substrate system. The results revealed that when glucose was added as a co-substrate, at low Cr(VI) concentration, complete reduction was achieved followed by increased biomass growth, but when Cr(VI) concentration was increased to 100 mg/L, the reduction decline to 93%. But in presence of high carbon iron filings (HCIF) as co-substrate even at higher Cr(VI) concentration i.e. 100 mg/L, 100% reduction was achieved and the cell growth continued till 124 h. The study was illustrated via Monod growth kinetic model for tertiary substrate system and the kinetic parameters revealed that the HCIF and glucose combination showed least inhibition to hexavalent chromium reduction by Serratia sp.

Assessment of Serratia sp. isolated from iron ore mine in hexavalent chromium reduction: kinetics, fate and variation in cellular morphology.

Serratia sp. strain SU.ISM.1 was isolated from Noamundi iron ore mines for the first time and was observed for hexavalent chromium reduction, and growth kinetics modelling was applied for bacterial chromium reduction. For 4-8 ppm of hexavalent chromium concentration, complete reduction was observed within 36 h when the selected isolate was applied, and for 12-20 ppm chromium concentration, complete reduction was achieved within 48 h. The viable biomass concentration increased up to 36 h of treatment time, after which the biomass concentration gradually declined. The Aiba model of product inhibition growth kinetics best described the growth of biomass in the presence of hexavalent chromium. The total mass conversion of Cr(VI) to Cr(III) for 4, 8, 12, 16 and 20 ppm was found to be 94.9%, 88.5%, 74.66%, 70.75% and 78.8%, respectively. The AFM and FESEM studies showed that the roughness of the cell surface increased with increasing concentration of hexavalent chromium, probably due to adsorption of chromium.

High carbon iron filings (HCIF) and metal reducing bacteria (Serratia sp.) co-assisted Cr (VI) reduction: Kinetics, mechanism and longevity.

Permeable reactive barriers (PRBs) have been an area of interest for in-situ remediation of groundwater. However, the corrosion of iron used in PRBs has been an area of concern. This study was aimed to enhance the long term performance for reduction of Cr (VI) by high carbon iron filings (HCIF) co-assisted with Serratia sp. Cr (VI) reduction by HCIF alone followed pseudo-first order kinetics and the reaction rate was 0.382 h-1 for 50 mg/L of Cr (VI) which declined to 0.0017 mg-1 L h-1 in combined system. But in cyclic studies, the reduction of Cr (VI) with HCIF alone system declined to 70% after 2 cycles whereas more than 90% reduction was observed in combined system up to four cycles. The corrosion potential and XRD data supported that Serratia sp. have positive effect on longevity of HCIF for Cr (VI) reduction.

Enhanced production of steviol glycosides in mycorrhizal plants: a concerted effect of arbuscular mycorrhizal symbiosis on transcription of biosynthetic genes.

Stevia rebaudiana (Bertoni) produces steviol glycosides (SGs)--stevioside (stev) and rebaudioside-A (reb-A) that are valued as low calorie sweeteners. Inoculation with arbuscular mycorrhizal fungi (AMF) augments SGs production, though the effect of this interaction on SGs biosynthesis has not been studied at molecular level. In this study transcription profiles of eleven key genes grouped under three stages of the SGs biosynthesis pathway were compared. The transcript analysis showed upregulation of genes encoding 2-C-methyl-D-erythritol-4-phosphate (MEP) pathway enzymes viz.,1-deoxy-D-xylulose 5-phospate synthase (DXS), 1-deoxy-D-xylulose 5-phospate reductoisomerase (DXR) and 2-C-methyl-D-erytrithol 2,4-cyclodiphosphate synthase (MDS) in mycorrhizal (M) plants. Zn and Mn are imperative for the expression of MDS and their enhanced uptake in M plants could be responsible for the increased transcription of MDS. Furthermore, in the second stage of SGs biosynthesis pathway, mycorrhization enhanced the transcription of copalyl diphosphate synthase (CPPS) and kaurenoic acid hydroxylase (KAH). Their expression is decisive for SGs biosynthesis as CPPS regulates flow of metabolites towards synthesis of kaurenoid precursors and KAH directs these towards steviol synthesis instead of gibberellins. In the third stage glucosylation of steviol to reb-A by four specific uridine diphosphate (UDP)-dependent glycosyltransferases (UGTs) occurs. While higher transcription of all the three characterized UGTs in M plants explains augmented production of SGs; higher transcript levels of UGT76G1, specifically improved reb-A to stev ratio implying increased sweetness. The work signifies that AM symbiosis upregulates the transcription of all eleven SGs biosynthesis genes as a result of improved nutrition and enhanced sugar concentration due to increased photosynthesis in M plants.

Arbuscular mycorrhiza increase artemisinin accumulation in Artemisia annua by higher expression of key biosynthesis genes via enhanced jasmonic acid levels.

It is becoming increasingly evident that the formation of arbuscular mycorrhiza (AM) enhances secondary metabolite production in shoots. Despite mounting evidence, relatively little is known about the underlying mechanisms. This study suggests that increase in artemisinin concentration in Artemisia annua colonized by Rhizophagus intraradices is due to altered trichome density as well as transcriptional patterns that are mediated via enhanced jasmonic acid (JA) levels. Mycorrhizal (M) plants had higher JA levels in leaf tissue that may be due to induction of an allene oxidase synthase gene (AOS), encoding one of the key enzymes for JA production. Non-mycorrhizal (NM) plants were exogenously supplied with a range of methyl jasmonic acid concentrations. When leaves of NM and M plants with similar levels of endogenous JA were compared, these matched closely in terms of shoot trichome density, artemisinin concentration, and transcript profile of artemisinin biosynthesis genes. Mycorrhization increased artemisinin levels by increasing glandular trichome density and transcriptional activation of artemisinin biosynthesis genes. Transcriptional analysis of some rate-limiting enzymes of mevalonate and methyl erythritol phosphate (MEP) pathways revealed that AM increases isoprenoids by induction of the MEP pathway. A decline in artemisinin concentration in shoots of NM and M plants treated with ibuprofen (an inhibitor of JA biosynthesis) further confirmed the implication of JA in the mechanism of artemisinin production.