Differential responses of Nostoc sphaeroides and Arthrospira platensis to solar ultraviolet radiation exposure

Abstract

During October to December 2003 we carried out experiments to assess the impact of high solar radiation levels (as normally occurring in a tropical region of Southern China) on the cyanobacteria Nostoc sphaeroides and Arthrospira (Spirulina) platensis. Two types of experiments were done: a) Short-term (i.e., 20 min) oxygen production of samples exposed to two radiation treatments (i.e., PAR+UVR-280-700 nm, and PAR only-400-700 nm, PAB and P treatments, respectively), and b) Long-term (i.e., 12 days) evaluation of photosynthetic quantum yield (Y) of samples exposed to three radiation treatments (i.e., PAB; PA (PAR+UV-A, 320-700 nm) and P treatments, respectively). N. sphaeroides was resistant to UVR, with no significant differences (P > 0.05) in oxygen production within 20 min of exposure, but with a slight inhibition of Y within hours. A fast recovery of Y was observed after one day even in samples exposed to full solar radiation. A. platensis, on the other hand, was very sensitive to solar radiation (mainly to UV-B), as determined by oxygen production and Y measurements. A. platensis had a circadian rhythm of photosynthetic inhibition, and during the first six days of exposure to solar radiation, it varied between 80 and 100% at local noon, but cells recovered significantly during afternoon hours. There was a significant decrease in photosynthetic inhibition after the first week of exposure with values less than 50% at local noon in samples receiving full solar radiation. Samples exposed to PA and P treatments recovered much faster (within 2-3 days), and there were no significant differences in Y between the three radiation treatments when irradiance was low (late afternoon to early morning). Long-term acclimation seems to be important in A. platensis to cope with high UVR levels however, it is not attained through the synthesis of UV-absorbing compounds but it seems to be mostly related to adaptive morphological changes.

Isolate-specific effects of ultraviolet radiation on photosynthesis, growth and mycosporine-like amino acids in the microbial mat-forming cyanobacterium Microcoleus chthonoplastes

“…On the other hand, photosynthesis of WW6 was able to partially acclimate to UVR but without signiWcant diVerence in MAAs contents between PAR and PAR + UVR treated cells. The same phenomenon was observed in A. platensis which required a long-term acclimation process to cope with high UVR levels, which seem not to be related to the accumulation of UV-absorbing compounds rather than to adaptive morphological changes (Helbling et al 2006). Conversely, photosynthesis of STO under UVR slightly deteriorated over time but a signiWcant diVerence in MAAs content was observed between treatments.…”

“…Conversely, no signiWcant changes were observed in the morphology of N. sphaeroides under diVerent radiation treatments (Helbling et al 2006). The terrestrial cyanobacteria Nostoc microscopicum and Tolypothrix arenophila possessing scytonemin as UV-A absorbing pigment in the extracellular sheath layer were able to tolerate 48 h continuous UV-B radiation.…”

“…Under this condition, cell survival would entail higher metabolic cost for maintenance and therefore no net growth was observed after longer exposure time while only a minimal growth was measured in WW6 within the Wrst 6 days of exposure. However, under much higher UVR (53.1 W m ¡2 UV-A; 1.3 W m ¡2 UV-B), A. platensis exhibited detectable but reduced growth rates (Helbling et al 2006). Acclimation and maintenance of eYcient photosynthetic performance under UVR in WW6 and BRE were most probably measured from surviving cells.…”

“…In a marine planktonic cyanobacterium Oscillatoria sp., pre-culture of cells with UV-A led to a higher tolerance of cells against UV-A-induced inhibition of photosynthesis (Wachi et al 1995). While Nostoc sphaeroides is slightly photoinhibited under ambient solar radiation and capable of fast recovery which is typical for dynamic photoinhibition (Helbling et al 2006), Arthrospira platensis is much more sensitive to UVR but photoinhibition decreased within 1 week of repeated exposure to solar radiation exemplifying partial acclimation of photosynthesis. In M. chthonoplastes photosynthetic performance was observed to be isolate-speciWc where acclimation and deterioration in F v /F m was observed in WW6 and STO, respectively.…”

“…Impact of UVR on the morphology of A. platensis indicated Wlament breakage or formation of tight helical structures as some kind of photoprotective mechanism (Wu et al 2005;Helbling et al 2006). Conversely, no signiWcant changes were observed in the morphology of N. sphaeroides under diVerent radiation treatments (Helbling et al 2006).…”