Impact of ionic and nanoparticle speciation states of silver on light harnessing photosynthetic events in Spirodela polyrhiza.

Owing to wide range of applications, nanotechnology is growing expeditiously. Likely negative impact of nanoparticles (NPs), which are inevitably released into our surroundings, on living organisms is of growing concern. Findings presented here are outcome of investigations carried out to evaluate the impact of ionic and NP speciation states of silver on light harnessing photosynthetic events in Spirodela polyrhiza fronds. Fronds exposed to ionic speciation state showed significant decline in PS (photosystem) II efficiency (Fv/Fm; variable fluorescence/maximal fluorescence), while those exposed to silver nanoparticles (Ag-NPs) showed marginal decline. Accordingly, decline in amplitude of Chl a fluorescence transients was sharper in fronds treated with Ag(+) than those treated with Ag-NPs. Of the various phases Chl a fluorescence transient, J-I phase [which reflects reduction of plastoquinone (PQ) pool] was most sensitive to both Ag(+) and Ag-NPs. Phenomenological yield models, built using Biolyzer software, revealed that fronds exposed to Ag(+) possessed significantly lower potential to trap and harness absorbed light energy for photochemical reactions than those exposed to Ag-NPs. Accordingly, dissipation of absorbed light energy as heat was significantly higher in fronds exposed to Ag(+) than those exposed to Ag-NPs. These findings revealed that NP speciation state of silver is significantly less toxic to light harnessing photosynthetic machinery of S. polyrhiza, compared to ionic speciation state.