Protective effect of supplemental low intensity white light on ultraviolet-B exposure-induced impairment in cyanobacterium Spirulina platensis: formation of air vacuoles as a possible protective measure

Abstract

Intact trichomes of Spirulina platensis were exposed to 1-5 h of low (0.2 mW cm(-2)) or high (0.6 mW cm(-2)) intensity UV-B (280-320 nm) radiation, alone or with photosynthetically active radiation (PAR) of supplemental 50 muE m(-2) s(-1) white light (WL). The mitigating effect of supplemental WL on UV-B induced alterations in Spirulina were investigated by monitoring time-dependent change in photosystem (PS) II mediated O(2) evolution, absorption, circular dichroism (CD) spectra, and ultrastructure. At low intensity, UV-B induced loss in PS II-catalyzed O(2) evolution, but caused no change in the absorption spectrum. At high intensity, UV-B caused a decrease in absorption by phycobilisomes (PBsomes), which was only partly prevented by the presence of low-intensity supplemental WL. The CD spectral analysis revealed that UV-B exposure caused time-dependent enhancement of the negative psi-type bands at 452 and 689 nm, reflecting alterations in the macroaggregation of chlorophyll-protein complexes. This enhancement of negative PS II-type bands was substantially arrested by the presence of supplemental WL exposure, even when UV-B exposure was continued for 5 h. These changes in UV-B-induced CD spectrum suggest alterations in the antenna structure of Spirulina involving both PBsomes and Chlorophyll a. Thus, supplemental low intensity WL arrests, to large extent, the macroaggregation of pigment-protein complexes. Furthermore, the electron micrographs of Spirulina revealed that UV-B exposure caused disorganization of the cellular ultrastructure, while the inclusion of supplemental WL enhanced the formation of air vacuoles in Spirulina. We suggest that the formation of vacuoles by supplemental WL is a protective feature against UV-B.

Developmental phase-dependent photosynthetic responses to ultraviolet-B radiation: damage, defence, and adaptation of primary leaves of wheat seedlings

“…Similar protective role of low irradiance by "white light" against UV-B induced impairment in photosynthetic apparatus of cyanobacterium Spirulina platensis was also found (Rajagopal et al 2005). The redox status of PS2 controls the degree of UV-B-induced degradation of D1 and D2 proteins under the combined irradiation by PAR and UV-B (Babu et al 1999).…”

Photosynthesis and growth of Arthrospira (Spirulina) platensis (Cyanophyta) in response to solar UV radiation, with special reference to its minor variant

“…High levels of UV-B may reduce phycobilisomesrelated fluorescence and PSII activity in Arthrospira (Rajagopal et al, 2000(Rajagopal et al, , 2005. Damage to D1 protein is known to be responsible for the inhibited activity of PSII (Sass et al, 1997).…”

“…The amount of UVAC in D-0083 and its minor variant was negligible and not induced by exposures to UVR, suggesting that synthesis of UVAC was not a functional mechanism for A. platensis to protect cells from UV radiation damage. Gas vacuoles in the cells (Rajagopal et al, 2005) and tightened spiral structure (Wu et al, 2005b) were suggested to play protective roles against UVR in Arthrospira spp. via scattering or shading effects.…”

Using a two-layered sphere model to investigate the impact of gas vacuoles on the inherent optical properties of <i>Microcystis aeruginosa</i>

“…The peripheral arrangement of vacuoles has caused speculation of light shielding of the photosynthetic lamella (e.g. van baalen and Brown, 1969;Rajagopal et al, 2005); however this is somewhat disputed (Ogawa et al, 1979). Polar arrangement is observed in Pseudanabaena, while both central and polar arrangements exist in Oscillatoria species (Meffert et al, 1981).…”