Modulation of photosynthesis in Synechocystis and Synechococcus grown with chromium (VI).

Synechocystis sp. PCC 6803 and Synechococcus elongatus PCC 7942 exhibit dissimilar tolerance to Cr(VI) with a tenfold difference in their EC50 value for Cr(VI). This contrasting tolerance was attributed to the difference in the ability to transport Cr(VI) and to detoxify ROS. The present study used biochemical assays and chlorophyll fluorescence to investigate the effect of growth with Cr(VI) on photosynthesis in the two cyanobacteria. In absence of Cr(VI), all the measured parameters viz., rates of CO2 fixation, PSII and PSI activities were higher in Synechocystis in comparison to Synechococcus, suggesting intrinsic differences in their photosynthesis. Growth in the presence of Cr(VI) reduced the pigment content and photosystems' activities in both cyanobacteria. It was further observed that photosynthetic functions were more adversely affected in Synechocystis in comparison to Synechococcus, in spite of exposure to tenfold lower Cr(VI) concentration. The effective quantumyield of PSII and PSI obtained by chlorophyll fluorescence measurements increased in the presence of Cr(VI) in Synechococcus whereas it decreased in Synechocystis. However, the overall CO2 fixation remained unchanged. These results indicated that, in addition to the intrinsic difference in photosynthetic rates, the two cyanobacteria exhibit differential modulation of photosynthetic machinery upon Cr(VI) exposure and Synechococcus could adapt better it's photosystems to counter the oxidative stress.

Synechococcus elongatus PCC 7942 is more tolerant to chromate as compared to Synechocystis sp. PCC 6803.

Two unicellular cyanobacteria Synechocystis sp. PCC 6803 and Synechococcus elongatus PCC 7942 showed contrasting responses to chromate stress with EC50 of 12 ± 2 and 150 ± 15 µM potassium dichromate respectively. There was no depletion of chromate in growth medium in both the cases. Using labeled chromate, very low accumulation (<1 nmol/10(8) cells) was observed in Synechocystis after incubation for 24 h in light. No accumulation of chromate could be observed in Synechococcus under these conditions. Chromate oxyanion is known to enter the cells using sulfate uptake channels. Therefore, inhibition of sulfate uptake caused by chromate was monitored using (35)S labeled sulfate. IC50 values of chromate for (35)sulfate uptake were higher in Synechococcus as compared to Synechocystis. The results suggested that the sulfate transporters in Synechococcus have lower affinity to chromate than those from Synechocystis possibly due to differences in affinity of sulfate receptors for chromate. Bioinformatic analyses revealed presence of sulfate and chromate transporters with considerable similarity; however, minor differences in these may play a role in their differential response to chromate. In both cases the IC50 values decreased when sulfate concentration was reduced in the medium indicating competitive inhibition of sulfate uptake by chromate. Interestingly, Synechococcus showed stimulation of growth at concentrations of chromate less than 100 µM, which affected its cell size without disturbing the ultrastructure and thylakoid organization. In Synechocystis, growth with 12 µM potassium dichromate damaged the ultrastructure and thylakoid organization with slight elongation of the cells. The results suggested that Synechococcus possesses efficient strategies to prevent entry and to remove chromate from the cell as compared to Synechocystis. This is the first time a differential response of Synechococcus 7942 and Synechocystis 6803 to chromate is reported. The contrasting characteristics observed in the two cyanobacteria will be useful in understanding the basis of resistance or susceptibility to chromate.

Enhancement of soluble dietary fibre, polyphenols and antioxidant properties of chapatis prepared from whole wheat flour dough treated with amylases and xylanase.

BACKGROUND: Chapati preparation involves various processing steps such as mixing the flour into dough, sheeting and baking. During these processing steps, flour components are likely to undergo changes in their nutrient and polyphenol composition and their antioxidant properties due to phenol-mediated crosslinking of proteins and carbohydrates. Therefore, in the present study, changes in nutritional, nutraceutical and antioxidant properties of chapatis prepared from doughs treated with amylases and xylanase were determined. RESULTS: An increase in insoluble dietary fibre content and a decrease in soluble polyphenol content were observed during preparation of control chapatis from whole wheat flours. However, significant increases in soluble dietary fibre and soluble polyphenol contents were observed in chapatis prepared from amylase-treated doughs compared with control chapatis. Extracts of chapatis prepared from amylase- and xylanase-treated doughs showed better antioxidant properties than extracts of control chapatis. Among these enzyme treatments, chapatis prepared from amylase-treated doughs showed better antioxidant properties than chapatis prepared from xylanase-treated doughs. High-performance liquid chromatography analysis of extracts of chapatis prepared from doughs treated with amylases showed the presence of potential antioxidant phenolic acids such as caffeic, gentisic and syringic acids in addition to the phenolic acids present in control chapatis. CONCLUSION: Treatment of doughs with amylases increased the contents of soluble dietary fibre and soluble polyphenols as well as improving the antioxidant properties of chapatis.