UV radiation effects on pigments, photosynthetic efficiency and DNA of an Antarctic marine benthic diatom community

Abstract

The impact of ultraviolet radiation (UVR) on a semi-natural, soft-bottom diatom community from Potter Cove, King Georg Island, Antarctica, was investigated. The objective was to estimate the impact of UV-B (280 to 320 nm) and UV-A (320 to 400 nm) on photosynthetic efficiency, pigments, DNA damage and repair. The diatom community was exposed to different doses of radiation treatments: PAR + UV-A + UV-B (PAB), PAR + UV-A (PA) and PAR (P). The most frequently observed species were Pleurosigma obscurum and Gyrosigma fasciola. Within the 0.7 mm substrate, UV radiation significantly reduced the Photosystem II (PS II) maximum efficiency (F v /F m ). Complete recovery was observed after 6 h post-cultivation under dim white light. The accumulation of diatoxanthin increased with exposure time for the P and PA treatments, but not for the PAB treatment, indicating a UV-B-related blocking of the de-epoxidation process within the xanthophyll cycling process. The ratio of diatoxanthin:(diatoxanthin+diadinoxanthin) decreased again to initial values after 24 h of recovery. The amount of DNA damage, measured as accumulation of cyclobutane pyrimidine dimers (CPDs), was minimal and increased with increasing UV-B dose, but DNA lesions were completely repaired within 24 h under dim white light. Regardless of possible avoidance strategies, e.g. vertical migration, negative UV treatment effects were observed. However, these effects were transient, facilitated by the dynamic recovery of photoinhibition and an efficient DNA damage repair mechanism. Although results from laboratory experiments using artificial radiation can only be extrapolated to field conditions with great caution, we conclude that Antarctic marine benthic diatoms inoculated into a semi-natural habitat are resilient to unnaturally high UVR.

Efficiency of photoprotection in microphytobenthos: role of vertical migration and the xanthophyll cycle against photoinhibition

“…These results also are in agreement with previous observations on the seasonal variability of microphytobenthos photosynthetic performance, showing patterns of acclimation to higher light levels during spring/summer and to lower levels in autumn/winter (Blanchard et al 1997, Migné et al 2004, Serôdio et al 2006. In this context, seasonal variation of UV irradiance may have also played a role (Wulff et al 2008). The results were also consistent with the photoacclimation response of benthic diatoms grown in culture exposed to low-and high-light regimes , Schumann et al 2007.…”

A (too) bright future? Arctic diatoms under radiation stress

“…The HPLC was calibrated with pigment standards from DHI Water and Environment, Denmark. Peak identities were further confirmed by online recording of absorbance spectra (400-700 nm) (Wright and Jeffrey 1997). Chlorophyll a (chl a) is expressed as lg l -1 and to distinguish a possible physiological response to the treatments, pigment ratios (weight/weight) were determined; separate photosynthetic pigments to chl a, total carotenoids (fucoxanthin, diadinoxanthin, diatoxanthin and betacarotene) to chl a and xanthophyll-cycle pigments (diatoxanthin/(diadinoxanthin plus diatoxanthin).…”

“…The filtered cells were extracted in 1.5 ml 100 % methanol. The extract was sonicated for 45 s using a ultra-sonicator (Vibra-cell) equipped with a 3 mm diameter probe operating at 80 % in 5 s pulses (Wright and Jeffrey 1997). The extraction and HPLC analysis continued according to Wright and Jeffrey (1997) using an absorbance diode-array detector (SpectraphysicsUV6000LP).…”

“…These pigments are synthesized when algae experience stressful conditions and absorb partly the incoming excessive radiation energy, in order to protect the photosynthetic apparatus. In diatoms, excessive energy can be modulated through thermal dissipation by de-epoxidation of diadinoxanthin to diatoxanthin in the xanthophyll cycle (Olaizola and Yamamoto 1994;van de Poll et al 2006;Wulff et al 2008). Excessive irradiance also leads to a decrease in photosynthetic efficiency due to non-photochemical quenching of incoming radiation, and/or lasting photodamage (Hanelt 1998).…”

“…However, negative UVR effects on microalgae are not ubiquitous. For example, polar benthic pennate diatoms have been shown to be relatively tolerant to UVR (Zacher et al 2007;Wulff et al 2008), and species-specific or even strain-specific differences have to be expected. Concomitantly, irradiance (E) in the wavelength range of photosynthetically available radiation (PAR, 400-700 nm) increases in Arctic marine ecosystems as a consequence of decreasing ice cover.…”