Differential effects of UV-B radiation fluence rates on growth, photosynthesis, and phosphate metabolism in two cyanobacteria under copper toxicity

Abstract

This work was undertaken to ascertain the impact of different fluence rates of ultraviolet-B (UV-B) radiation on two cyanobacterial biofertilizers, Phormidium foveolarum and Nostoc muscorum, growing under copper toxicity. Copper (2 and 5 µmol L⁻¹) and high UV-B fluence rate (UV-B_H; 1.0 µmol m⁻²s⁻¹) decreased the growth, pigment content, photosynthetic oxygen yield, phosphate uptake, and acid phosphatase activity in both the strains analyzed after 24 and 72 h of experiments, and combined exposure further enhanced the toxic effects. Respiration and alkaline phosphatase activities were stimulated appreciably. The damaging effect was shown on the order on pigments: phycocyanin > chlorophyll a > carotenoids, and on photosystems: whole chain photosynthetic reaction > photosystem II > photosystem I. Partial recovery in the photosystem II activity in the presence of artificial electron donors; diphenyl carbazide (DPC), hydroxylamine (NH₂OH), and manganese chloride (MnCl₂) pointed out the interruption of electron flow on the oxidation side of photosystem II. Unlike UV-B_H, low UV-B fluence rate (UV-B_L; 0.1 µmol m⁻²s⁻¹), rather than causing damaging effect partially, alleviated the toxic effects of Cu. This study suggests that the cyanobacterium P. foveolarum is less sensitive against UV-B_H and excess Cu (2 and 5 µmol L⁻¹), thus P. foveolarum may be used as a biofertilizer for sustainable agriculture.

Keywords:

• copper
• cyanobacteria
• phosphate uptake
• photosynthesis
• UV-B radiation

Acknowledgements

Authors are thankful to The Head, Department of Botany, University of Allahabad, Allahabad for providing necessary laboratory facilities. VPS and PKS are thankful to University Grants Commission, New Delhi, for financial support in the forms of JRF and SRF.