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

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Abstract

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/108 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 35S labeled sulfate. IC50 values of chromate for 35sulfate 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.

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Abbreviations

EC50 of chromate:

Concentration of potassium dichromate at which number of cells ml−1 was 50 % as compared to control

IC50 of chromate:

Concentration of chromate required for 50 % reduction in uptake of sulfate as compared to control

OD:

Optical density

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Acknowledgments

The authors thank Dr. A. Ballal and Dr. R. Agarwal MBD, BARC, Mumbai for critical reading of the manuscript. We also thank Drs. S.F. D’Souza and J.S. Melo for useful suggestions. We thank Ms. N. Jadhav for sample preparation for Electron Microscopy. JKS acknowledges Department of Atomic Energy, India for Raja Ramanna Fellowship.

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Authors and Affiliations

  1. Molecular Biology Division, Bhabha Atomic Research Centre, Trombay, Mumbai, 400085, India

    Alka Gupta & Jayashree K. Sainis

  2. Nuclear Agriculture and Biotechnology Division, Bhabha Atomic Research Centre, Trombay, Mumbai, 400085, India

    Suresh G. Bhagwat

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  1. Alka Gupta
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  2. Suresh G. Bhagwat
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  3. Jayashree K. Sainis
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Correspondence to Jayashree K. Sainis.

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Gupta, A., Bhagwat, S.G. & Sainis, J.K. Synechococcus elongatus PCC 7942 is more tolerant to chromate as compared to Synechocystis sp. PCC 6803. Biometals 26, 309–319 (2013). https://doi.org/10.1007/s10534-013-9614-6

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