PAR+UV - A Research Articles

Impacts of ultraviolet and photosynthetically active radiations on photosynthetic efficiency and antioxidant systems of the cyanobacterium Spirulina subsalsa HKAR-19.

This study summarizes the response of cyanobacterium Spirulina subsalsa HKAR-19 under simulated light conditions of photosynthetically active radiation (PAR), PAR+UV-A (PA), and PAR+UV-A+UV-B (PAB). Exposure to UV radiation caused a significant (P < 0.05) decrease in chlorophyll a, phycocyanin, and total protein. In contrast, total carotene content increased significantly (P < 0.05) under PA and PAB with increasing irradiation time. The photosynthetic efficiency of photosystem II also decreased significantly in PA and PAB radiation. We havealso recorded a decrease in the fluorescence emission intensity of phycocyanin under PA and PAB exposure. The phycocyanin fluorescence shifted towards shorter wavelengths (blue-shift) after 72h of PA and PAB exposure. Intracellular reactive oxygen species (ROS) levels increased significantly in PA and PAB. Fluorescence microscopic images showed an increase in green fluorescence, indicating ROS generation in UV radiation. We havealso quantified ROS generation using green and red fluorescence ratio represented as G/R ratio. A 2-6-foldincrease in antioxidative enzymes activity was observed to overcome the damaging effects caused by UV stress as compared to untreated control cultures. The lipid peroxidation was assessed in terms of malondialdehyde content which increases significantly (P < 0.05) as the duration of exposure increases. These results suggest that a combined effect of PAR, UV-A, and UV-B was more deleterious than an individual one.

Photosynthetic responses of a golden tide alga (Sargassum horneri) to ultraviolet radiation

In recent years, massive Sargassum drifting on the sea surface, known as “golden tides,” negatively impacted on the local marine ecology. However, the physiological mechanisms of its formation remain unclear. To investigate the photosynthetic responses of golden tide algae to UVR, one key factor for drifting Sargassum population, we cultivated thalli of S. horneri, a golden tide alga, under three light treatments: P (photosynthetically active radiation, PAR), PA (PAR+UVA) and PAB (PAR+UVA+UVB) for 120 mins, followed by low light recovery for 240 mins. The photosynthetic characteristics of alga were determined. The results showed that UVR exposure decreased photosynthetic activity, reflected by depressed maximum photochemical quantum yield (Fv/Fm) and contents of Chla and Chlc in the PA and PAB treatments. Higher content of malondialdehyde (MDA) was found in thalli exposed to UVR, which verified the damage of UVR. Electron transfer rate (ETR) was slowed down by UVR, accompanied by the increments of net closing rate of the reaction center and the energy absorbed and dissipated by unit reaction center in PSII. In these effects on photosynthesis of UVR, the PAB treatment expressed more significant inhibition, indicating a remarkable role of UVB. However, based on our results, S. horneri also took some strategies to protect itself from photodamage of UVR. UVR exposure enhanced the contents of UV-absorbing compounds (UVACs) and carotenoid, and simultaneously expedited heat consumption of excess light energy, indicated by the increased non-photochemical quenching coefficient (NPQ) in the PA and PAB treatments. Increased activities of superoxide dismutase (SOD) and peroxidase (POD), and higher content of PsbA (D1) protein were found in the treatments with UVR, which suggested that antioxidant system and the turnover of D1 protein played important roles in protection from UV-induced damages. Due to the above protection pathways, Fv/Fm and ETR gradually recovered when thalli were transferred to low light recovery. Therefore, we suggest that various protection and restoration pathways in S. horneri work together to effectively protect against UVR damage, which may be the reason why drifting populations can adapt to UVR on the seawater surface and form golden tide in case of suitable temperature and nutrients.

Ecophysiological implications of UV radiation in the interspecific interaction of Pyropia acanthophora and Grateloupia turuturu (Rhodophyta)

Radiation, both photosynthetic active radiation (PAR, l = 400–700 nm) and Ultraviolet (UVR, l = 280–400 nm) is one of the key factors regulating algal distribution in aquatic environments. Pyropia acanthophora and Grateloupia turuturu have been found over upper rocky shore areas in Southern Brazil, occupying the same niche space. The first species is native and the second one is exotic and considered a potential invader of South Atlantic. The aim of the present study was to evaluate the effects of radiation on physiological responses of both species and infer mechanisms that allow their niche competition in the environment. Samples were cultured in the following conditions: associated or separated, and with an addition of PAR, PAR + UVA (PA) and PAR + UVA + UVB (PAB), totalizing six factorial treatments during 5 days of exposure. Photosynthetic responses of Fv/Fm and ETR were daily evaluated. At the beginning and at the end of the experiment, samples were analyzed for pigment content (chlorophyll a and phycobiliproteins), and mycosporine-like amino acids (MAAs), while oxygen evolution was evaluated at the end of the experiment. As the main results, G. turuturu died when cultivated in PAB conditions. P. acanthophora presented higher amounts of chlorophyll a than G. turuturu during the whole experiment. Phycoerythrin and Fv/Fm remained constant in P. acanthophora but diminished for G. turuturu in UV treatments. ETR was higher for samples that were cultivated in associative treatment. The presence of G. turuturu in the same flask enhanced MAA synthesis in P. acanthophora, regardless of radiation condition. In addition, UV radiation can be a factor controlling species distribution and could counteract the spreading of invasive species, like G. turuturu, allowing P. acanthophora survival in upper rocky shore zones of the natural ecological distribution area.

Effects of solar UV radiation and temperature on morphology and photosynthetic performance of Chaetoceros curvisetus

This study investigated the effect of solar ultraviolet radiation (UVR) and temperature on a chain length and photosynthetic performance of diatom Chaetorceros curvisetus. The cells were cultured in large quartz tubes and exposed to PAR, PAR + UV-A (PA), or PAR + UV-A + UV-B (PAB) radiation at 20°C and 28°C for six days, respectively. After recovery for 1 h, the cells were exposed again to three different radiations for 1 h. Then, a change in the photochemical efficiency (FPSII) was examined and UVR-induced photoinhibition was calculated. The percentage of long chains (more than five single cells per chain) in C. curvisetus significantly increased from 8.2% (PAR) to 38.9% (PAB) at 20°C; while it was not notably affected at 28°C. Mycosporine-like amino acids (MAAs) concentration obviously increased by irradiance increment from PAR to PAB at 20°C. Chlorophyll (Chl) a concentration significantly declined with increasing irradiance at 20°C. Both MAAs and Chl a concentrations were not obviously changed by irradiance at 28°C. Before and after reexposure, FPSII was significantly reduced both at 20°C and 28°C. UVR-induced photoinhibition at 20°C (39%) was higher than that at 28°C (30.9%). Solar UV radiation, especially UV-B, could significantly influence the percentage of long chains of C. curvisetus, especially at low temperature. UVR-induced photoinhibition can be alleviated by higher temperatures.

Influence of light history on the photosynthetic and motility responses of Gymnodinium chlorophorum exposed to UVR and different temperatures

In the wake of global climate change, phytoplankton productivity and species composition is expected to change due to altered external conditions such as temperature, nutrient accessibility, pH and exposure to solar visible (PAR) and ultraviolet radiation (UVR). The previous light history is also of importance for the performance of phytoplankton cells. In order to assess the combined impacts of UVR and temperature on the dinoflagellate Gymnodinium chlorophorum we analyzed the effective photochemical quantum yield (Y), relative electron transport rate vs. irradiance curves (rETR vs. I), percentage of motile cells and swimming velocity. Cells were grown at three different temperatures (15, 20 and 25°C) and two PAR intensities: low light (LL, 100μmol photons m−2s−1) and high light (HL, 250μmol photons m−2s−1). Pre-acclimated cells were then exposed to either PAR only (P), PAR+UV-A (PA) or PAR+UV-A+UV-B (PAB) radiation at two different irradiances, followed by a recovery period in darkness. The Y decreased during exposure, being least inhibited in P and most in PAB treatments. Inhibition was higher and recovery slower in LL-grown cells than in HL-grown cells at 15° and 20°C, but the opposite occurred at 25°C, when exposed to high irradiances. Maximal values of rETR were determined at t0 as compared to the different (before and after exposure) radiation treatments. The effects of temperature and UVR on rETR were antagonistic in LL-grown cells (i.e., less UVR inhibition at higher temperature), while it was synergistic in HL cells. Swimming velocity and percentage of motile cells were not affected at all tested temperatures and exposure regimes, independent of the light history. Our results indicate that, depending on the previous light history, increased temperature and UVR as predicted under climate change conditions, can have different interactions thus conditioning the photosynthetic response of G. chlorophorum.

Short-term effects of solar UV radiation and NO3 - supply on the accumulation of mycosporine-like amino acids in Pyropia columbina (Bangiales, Rhodophyta) under spring ozone depletion in the sub-Antarctic region, Chile

Abstract Short-term variations of mycosporine-like amino acids (MAAs) in Pyropia columbina (Montage) W.A. Nelson exposed to nitrate (NO3 -) enrichment under different outdoor light treatments during the spring ozone depletion of 2008 in Punta Arenas (Chile) were investigated. Segments of P. columbina thalli were cultivated under three treatments of solar radiation without or with NO3 - supply (0.38 mmol l-1): PAR (P), PAR+UVA (PA), and PAR+UVA+UVB (PAB). Samples were taken at 8:00 h (initial value), 9:30, 12:30, 15:30, and 18:00 h on November 8 and at 9:00 h on November 9 (recovery period). A complex dynamic of MAAs affected by light quality and NO3 - supply was observed. During the light period, the highest content of MAAs was reached under PAB and NO3 - enrichment, whereas MAAs increased during the recovery period in P and PAB with no NO3 - supply. Five MAAs were identified: porphyra-334, shinorine, asterina-330, palythine, and mycosporine-glycine. The hourly accumulation of each MAA varied mainly according to the time of exposure and NO3 - supply. In general, the percentage of porphrya-334 increased, whereas the other MAAs decreased during the exposure period. These results suggest that MAA content in P. columbina varied in the short-term (hours) and the changes were related to the solar irradiance received and NO3 - availability.

Ultraviolet radiation-induced generation of reactive oxygen species, DNA damage and induction of UV-absorbing compounds in the cyanobacterium Rivularia sp. HKAR-4

The effects of ultraviolet radiation (UVR) on certain biochemical processes were studied in the cyanobacterium Rivularia sp. HKAR-4. The generation of reactive oxygen species (ROS) was investigated under 295, 320 and 395nm cut-off filters using the ROS sensing probe 2′,7′-dichlorodihydrofluorescein diacetate (DCFH-DA). Contrary to photosynthetically active radiation (PAR) and PAR+UV-A (PA) radiation, ROS signals were more prominent under PAR+UV-A+UV-B (PAB) radiation. The low levels of ROS were also detected in the cells growing under dark as well as normal light conditions. The integrity of genomic DNA, the amplification of 16S rDNA and random amplified polymorphic DNA (RAPD) profile were considerably affected by PAB radiation. The formation of cyclobutane thymine dimers (T<>T) was observed under PAR as well as PA and PAB radiations but the intensity was found to be the highest under UV-B radiation. The formation of T<>T under PAR as well as PA radiation has not previously been reported in cyanobacterial systems. Furthermore, the synthesis of UVR-absorbing/screening compounds such as mycosporine-like amino acids (MAAs) was investigated under PAR and PAB. Dose-dependent induction of MAAs, mycosporine-glycine (MG; λmax310nm) as well as an unknown UV-absorbing compound (λmax 335nm) was observed under both PAR as well as UVR. The concentration of unknown MAA (hereafter, M-335) was higher than MG. Overall, in response to harmful effects of solar UVR, cyanobacteria have developed some photoprotective machinery to overcome its impact to grow in the adverse natural environments.

UVR defense mechanisms in eurytopic and invasive Gracilaria vermiculophylla (Gracilariales, Rhodophyta)

The invasive success of Gracilaria vermiculophylla has been attributed to its wide tolerance range to different abiotic factors, but its response to ultraviolet radiation (UVR) is yet to be investigated. In the laboratory, carpospores and vegetative thalli of an Atlantic population were exposed to different radiation treatments consisting of high PAR (photosynthetically active radiation) only (P), PAR+UV-A (PA) and PAR+UV-A+UV-B (PAB). Photosynthesis of carpospores was photoinhibited under different radiation treatments but photosystem II (PSII) function was restored after 12 h under dim white light. Growth of vegetative thalli was significantly higher under radiation supplemented with UVR. Decrease in chlorophyll a (Chl a) under daily continuous 16-h exposure to 300 µmol photons m(-2) s(-1) of PAR suggests preventive accumulation of excited chlorophyll molecules within the antennae to minimize the generation of dangerous reactive oxygen species. Moreover, an increase in total carotenoids and xanthophyll cycle pigments (i.e. violaxanthin, antheraxanthin and zeaxanthin) further suggests effective photoprotection under UVR. The presence of the ketocarotenoid β-cryptoxanthin also indicates protection against UVR and oxidative stress. The initial concentration of total mycosporine-like amino acids (MAAs) in freshly-released spores increased approximately four times after 8-h laboratory radiation treatments. On the other hand, initial specific MAAs in vegetative thalli changed in composition after 7-day exposure to laboratory radiation conditions without affecting the total concentration. The above responses suggest that G. vermiculophylla have multiple UVR defense mechanisms to cope with the dynamic variation in light quantity and quality encountered in its habitat. Beside being eurytopic, the UVR photoprotective mechanisms likely contribute to the current invasive success of the species in shallow lagoons and estuaries exposed to high solar radiation.

Dynamic response of UV-absorbing compounds, quantum yield and the xanthophyll cycle to diel changes in UV-B and photosynthetic radiations in an aquatic liverwort

We studied the diel responses of the liverwort Jungermannia exsertifolia subsp. cordifolia to radiation changes under laboratory conditions. The samples were exposed to three radiation regimes: P (only PAR), PA (PAR + UV-A), and PAB (PAR + UV-A + UV-B). The day was divided in four periods: darkness, a first low-PAR period, the high-PAR plus UV period, and a second low-PAR period. After 15 days of culture, we measured photosynthetic pigments, chlorophyll fluorescence and UV-absorbing compounds in the four periods of the day on two consecutive days. With respect to UV-absorbing compounds, we analyzed their global amount (as the bulk UV absorbance of methanolic extracts) and the concentration of seven hydroxycinnamic acid derivatives, both in the soluble (mainly vacuolar) and insoluble (cell wall-bound) fractions of the plant extracts. PAB samples increased the bulk UV absorbance of the soluble and insoluble fractions, and the concentrations of p-coumaroylmalic acid in the soluble fraction and p-coumaric acid in the cell wall. Most of these variables showed significant diel changes and responded within a few hours to radiation changes (more strongly to UV-B), increasing at the end of the period of high-PAR plus UV. F v/ F m, Φ PSII, NPQ and the components of the xanthophyll cycle showed significant and quick diel changes in response to high PAR, UV-A and UV-B radiation, indicating dynamic photoinhibition and protection of PSII from excess radiation through the xanthophyll cycle. Thus, the liverwort showed a dynamic protection and acclimation capacity to the irradiance level and spectral characteristics of the radiation received.

Photosynthetic response of Arctic kelp zoospores exposed to radiation and thermal stress

Zoospores of Arctic kelp species, Alaria esculenta, Laminaria digitata and Saccharina latissima were exposed to different temperature (2 degrees C to 19 degrees C) and radiation (photosynthetically active radiation (PAR=P), PAR+UV-A (PA), and PAR+UV-A+UV-B (PAB)) conditions in the laboratory. Species-specific responses to the combined effect of light and temperature stress showed sensitivity in the order S. latissima>L. digitata>A. esculenta. The optimum temperature range for photosynthesis in different Arctic kelp species' zoospores was between 7-13 degrees C, temperatures higher than in the natural environment. Short-term response to increasing temperature was non-lethal while moderate temperature increase had an ameliorating effect on the overall biological effect of UVR; where the lowest photoinhibition was observed at 13 degrees C under PAB and higher photosynthetic recovery was observed in UVR-pre-exposed zoospores at 7-13 degrees C compared to 2 degrees C. Above the temperature optima, continued cultivation under high temperature had a negative impact on the recovery of photoinhibition. The higher capacity for non-photochemical quenching (NPQ) in A. esculenta and L. digitata helped to regulate and protect photosynthesis under light and temperature stress compared to S. latissima. The investigated Arctic kelp species may be able to locally survive under the influence of UVR at a certain range of temperature increase but the southernmost distribution range of the species may shift to higher latitudes; although natural selection may result in genotypes adapted to stressful environment.

The effects of UV radiation on photosynthesis estimated as chlorophyll fluorescence in Zygnemopsis decussata (Chlorophyta) growing in a high mountain lake (Sierra Nevada, Southern Spain)

The effect of increased UV radiation on photosynthesis estimated as in vivo chlorophyll fluorescence i.e. optimal quantum yield (Fv/Fm) and electron transport rate (ETR) in the green filamentous alga Zygnemopsis decussata (Streptophyta, Zygnematales) growing in the high mountain lake La Caldera (Sierra Nevada, Spain) at 3050 m altitude was evaluated. Two sets of in situ experiments were conducted: (1) On July 2006, Fv/Fm was measured throughout the day at different depths (0.1, 0.25, 0.5 and 1 m) and in the afternoon, ETR and phenolic compounds were determined. In addition, in order to analyze the effect of UV radiation, Fv/Fm was determined in algae incubated for 3 days at 0.5m under three different light treatments: PAR+UVA+UVB (PAB), PAR+UVA (PA) and PAR (P). (2) On August 2007, Fv/Fm was determined under PAB, PA and P treatments and desiccation/rehydration conditions. Fv/Fm decreased in algae growing in surface waters (0.1 m) but also at 1 m depth compared to that at 0.5 m depth. The decrease of Fv/Fm at noon due to photoinhibition was small (less than 10%) except in algae growing at 1 m depth (44%). The maximal electron transport rate was 3.5-5 times higher in algae growing at 0.25-0.5 m respectively than that at 0.1 and 1 m depth. These results are related to the accumulation of phenolic compounds: i.e. the algae at 0.25-0.5 m presented respectively about a 3-5 times higher concentration of phenolic compounds than that of algae at 0.1-1 m depth. The protection mechanisms seem to be stimulated by UVB radiation, since Fv/Fm was higher in the presence of UVB (PAB treatment) compared to PA or P treatments. UVA exerts the main photoinhibitory effect, not only at midday, but also in the afternoon. UVB radiation also had a protective effect in algae grown under desiccation conditions for three days. During re-hydration, the rapid increase of Fv/Fm (after 1 h) was higher in the UVB-grown algae than in algae grown under UVA radiation. After 5 h, Fv/Fm values were similar in algae submitted to desiccation/rehydration under PAB and P treatments as they were in the control (submerged algae). The combined effect of desiccation and UVA produced the greatest decrease of photosynthesis in Z. decussata. Thus UVB, in contrast to other species, may support the recovery process. Z. decussata can acclimate to severe stress conditions in this high mountain lake by the photoprotection mechanism induced by UVB radiation through dynamic photoinhibition and the accumulation of phenolic compounds (UV screen and antioxidant substances).

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

The impact of ultraviolet radiation (UVR) on a semi-natural, soft-bottom diatom commu- nity 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 treat- ments: PAR + UV-A + UV-B (PAB), PAR + UV-A (PA) and PAR (P). The most frequently observed spe- cies were Pleurosigma obscurum and Gyrosigma fasciola. Within the 0.7 mm substrate, UV radiation significantly reduced the Photosystem II (PS II) maximum efficiency (Fv/Fm). 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 mecha- nism. 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.

UVB effects on early developmental stages of commercially important macroalgae in southern Chile

High levels of ultraviolet-B radiation (UVB) could represent a danger to seaweeds by affecting their physiological processes and development. The aim of this work was to study the effects of UVB radiation on early developmental stages of commercially and ecologically important marine algal species in southern Chile, considering spores survival and embryos growth. Spores of Mazzaella laminarioides, Gigartina skottsbergii, Sarcothalia crispata and embryos of G. skottsbergii and Macrocystis pyrifera were submitted to treatments of a) photosynthetically active radiation (PAR: Control), b) PAR+UVA (PA) and c) PAR+UVA+UVB (PAB). UV radiation did not affect spore survival of M. laminarioides S. crispata and G. skottsbergii (P = 0.55, P = 0.6 & P = 0.25 respectively), but did provoke differences in the growth rate of G. skottsbergii embryos (P = 0.00). Differences in survival and growth of M. pyrifera embryos were also observed (P = 0.001 & P = 0.007, respectively). Differences in growth of M. pyrifera embryos were observed only in the first five days, whereas changes in survival persisted until the end of the experiment. Additionally, UVB provoked morphological alteration in M. pyrifera embryos, as evidenced by progressive curling. These results suggest that the initial stages of the subtidal algae species G. skottsbergii and M. pyrifera cultivated in laboratory conditions were sensitive to UVA and UVB radiation, and their recruitment and development could be affected as well in natural conditions found in southern South America, where the ozone layer has thinned more than in other parts of the planet.

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

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.

Different physiological responses of two aquatic bryophytes to enhanced ultraviolet-B radiation

Two aquatic bryophytes, the moss Fontinalis antipyretica Hedw. and the foliose liverwort Jungermannia exsertifolia Steph. subsp. cordifolia (Dumort.) Váña were analysed with regard to their ultraviolet-B (UV-B) tolerance. The plants were cultivated in the laboratory for 36 days under three different radiation regimes which were set up using appropriate lamps and filters: P (PAR alone, control), PA (PAR + UV-A) and PAB (PAR + UV-A + UV-B). The two bryophytes responded differently to the enhancement in UV-B radiation, while UV-A radiation had only a slight biological effect. The samples of the moss which were irradiated with UV-B showed, with respect to the control, decreases in the chlorophyll and carotenoids concentration, the chlorophyll a/b quotient, the chlorophylls/phaeopigments ratios, the net photosynthesis rates, the light saturation point, the maximum quantum yield of photosystem II (Fv/Fm) and the apparent electron transport rate (ETR). They also showed increases in the sclerophylly index and the dark respiration rates. The majority of these changes are indicative of plant stress and some of them had been found previously in bryophytes exposed to enhanced UV-B radiation. However, the UV-B-irradiated samples of the liverwort only showed a decrease in Fv/Fm, which might be the most sensitive physiological variable to UV-B, together with an increase in the concentration of UV-absorbing compounds. This defence mechanism, rarely described in bryophytes, would enable the liverwort to have a higher tolerance than the moss against UV-B radiation, at least under the specific experimental conditions used in this study. Also, the increment of UV-absorbing compounds in the liverwort, which seems to be a specific response to the enhancement in UV-B radiation, might be a useful ecophysiological tool in the bioindication of this phenomenon; long-term field research is needed to confirm this usefulness. The different UV-B tolerances of the species studied may be related, to a certain extent, with their different distribution and ecology. Given that UV-B tolerance depends on the species, bryophytes should not be grouped as a single functional type in the prediction of the consequences of a hypothetic enhancement in UV-B radiation.