Effects of Solar UV Radiation on Morphology and Photosynthesis of Filamentous Cyanobacterium Arthrospira platensis

Abstract

To study the impact of solar UV radiation (UVR) (280 to 400 nm) on the filamentous cyanobacterium Arthrospira (Spirulina) platensis, we examined the morphological changes and photosynthetic performance using an indoor-grown strain (which had not been exposed to sunlight for decades) and an outdoor-grown strain (which had been grown under sunlight for decades) while they were cultured with three solar radiation treatments: PAB (photosynthetically active radiation [PAR] plus UVR; 280 to 700 nm), PA (PAR plus UV-A; 320 to 700 nm), and P (PAR only; 400 to 700 nm). Solar UVR broke the spiral filaments of A. platensis exposed to full solar radiation in short-term low-cell-density cultures. This breakage was observed after 2 h for the indoor strain but after 4 to 6 h for the outdoor strain. Filament breakage also occurred in the cultures exposed to PAR alone; however, the extent of breakage was less than that observed for filaments exposed to full solar radiation. The spiral filaments broke and compressed when high-cell-density cultures were exposed to full solar radiation during long-term experiments. When UV-B was screened off, the filaments initially broke, but they elongated and became loosely arranged later (i.e., there were fewer spirals per unit of filament length). When UVR was filtered out, the spiral structure hardly broke or became looser. Photosynthetic O 2 evolution in the presence of UVR was significantly suppressed in the indoor strain compared to the outdoor strain. UVRinduced inhibition increased with exposure time, and it was significantly lower in the outdoor strain. The concentration of UV-absorbing compounds was low in both strains, and there was no significant change in the amount regardless of the radiation treatment, suggesting that these compounds were not effectively used as protection against solar UVR. Self-shading, on the other hand, produced by compression of the spirals over adaptive time scales, seems to play an important role in protecting this species against deleterious UVR. Our findings suggest that the increase in UV-B irradiance due to ozone depletion not only might affect photosynthesis but also might alter the morphological development of filamentous cyanobacteria during acclimation or over adaptive time scales.

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

“…Previously, solar UVR was found to reduce photosynthetic O 2 evolution of A. platensis D-0083 (Wu et al, 2005b) under similar conditions as in the present study. Here, we demonstrated that presence of UV-A and UV-B resulted in significant inhibition of the effective quantum yield and growth of this species, though high levels of PAR caused most of the inhibition.…”

“…UVR inhibits photosynthesis of cyanobacteria (Sinha et al, 1995;Kumar et al, 1996;Han et al, 2003;Wu et al, 2005b) and other photosynthetic organisms (Häder et al, 2001;Gao et al, 2007b). High levels of UV-B may reduce phycobilisomesrelated fluorescence and PSII activity in Arthrospira (Rajagopal et al, 2000(Rajagopal et al, , 2005.…”

“…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.…”

“…are notorious for reducing efficiency of harvest and production (Vonshak and Richmond, 1988;Belay, 1997). The spiral structure of A. platensis has been previously found to be broken and altered by solar UVR (Wu et al, 2005b). However, how sensitive the minor variant is to solar UVR and whether its occurrence relates to the solar exposures and the consequent spiral breakage are not known.…”

“…Solar UV radiation (UVR, 280-400 nm) is an important environmental factor that affects Arthrospira in natural environment as well as in outdoor commercial production ponds. It was found recently that the spiral structure of A. platensis filament was broken under solar radiation (PAR + UVR) on sunny days (Wu et al, 2005b), and its photosynthetic performance was negatively affected by UVR (Rajagopal et al, 2000;Wu et al, 2005b).…”