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Boletín de la Sociedad Chilena de Química

versión impresa ISSN 0366-1644

Bol. Soc. Chil. Quím. v.47 n.1 Concepción mar. 2002

http://dx.doi.org/10.4067/S0366-16442002000100012 

EFFECTS OF SOLAR UV-B RADIATION IN THE ACCUMULATION
OF RHIZOCARPIC ACID IN A LICHEN SPECIES FROM ALPINE
ZONES OF CHILE

Cecilia Rubio*, Ernesto Fernández*, María Eliana Hidalgo**, and Wanda Quilhot**.

*Escuela de Química y Farmacia, Facultad de Medicina; Universidad de Valparaíso. Casilla 5001, Valparaíso Chile

**Instituto de Ciencias Biológicas y Químicas, Facultad de Ciencias, Universidad de Valparaíso, Valparaíso, Chile.

(Received: March 28, 2001 - Januar 11, 2002)

ABSTRACT

In the lichen Acarospora schleicheri collected in altitudinal gradients at 18°S, 19°S, and 33°S in alpine zones of Chile, the concentrations of rhizocarpic acid were determined and compared. Significant increments in the accumulation of rhizocarpic acid were observed at the highest altitudes and they were correlated with the increasing UV-B radiation levels in the selected gradients. The results obtained suggest that the synthesis of rhizocarpic acid is induced by UV-B radiation, and that this lichen compound represents a chemical adaptation to the high levels of UV-B radiation in lichen habitats.

KEY WORDS: lichens, shikimic acid derivatives, rhizocarpic acid, oxolane-carbonyl chromophore unit, UV radiation

RESUMEN

En la especie liquénica Acarospora schleicheri, recolectada en gradientes altitudinales a 18º, 19º y 33º Lat. S, en zonas alpinas de Chile, se determinaron y compararon las concentraciones de ácido rizocárpico. Se observaron incrementos significativos en la acumulación del metabolito con la altura s.n.m. y el aumento en los niveles de radiación UV-B en los gradientes seleccionados. Los resultados obtenidos sugieren que la síntesis de ácido rizocárpico es inducida por la radiación UV-B y que su presencia representa una adaptación química frente a los elevados niveles de radiación UV-B en los habitats de esta especie liquénica.

PALABRAS CLAVE: líquenes, derivados de ácido chiquímico, ácido rizocárpico, cromóforo oxolano-carbonilo, radiación UV-B.

INTRODUCTION

Stratospheric ozone is an effective filter of UV-B radiation (280-315 nm); as a consequence of the ozone decrease, this radiation has increased in the polar regions and mean latitudes, that for the Southern Hemisphere occurs between 15° and 53° Lat.S (1). In 1998-1999, the UV-B radiation is 15-20% higher than in the seventies (2). UV-A radiation (315-400 nm) which is unimpeded by ozone (3) remains relatively constant.

Inhabitants of terrestrial and aquatic ecosystems have developed strategies to cope with the potential stress induced by UV radiation; one of these strategies is the synthesis of photoprotector compounds that absorb UV radiation and the absorbed energy is released as fluorescence or heat (4, 5).

Lichens produce a variety of secondary products with filtering properties of UV radiation (6, 7, 8). It has conclusively been demonstrated that lichen chemicals produced by the acetate-polymalonate pathway and characterised by the orthohydroxycarboyl chromophore unit are efficient filters of both the UV-A and UV-B radiation emitting fluorescence in the blue region of the solar spectrum (9, 10, 11). The accumulation rates of these photoprotective compounds increase with the increasing levels of UV radiation in lichen habitats, as it occurs in lichens from the Antarctica (12) and from the Chile’s alpine zones (13, 14) where the concentrations of usnic acid matched the minimum ozone values or the increased UV radiation.

Shikimic acid derivatives are frequent in lichens from environments having high levels of UV radiation. Calycin co-exists with usnic acid in Lecanora somervellei from alpine zones of the Himalayas (15); thus suggesting that calycin represents a chemical acclimatization to high levels of irradiance. The accumulation of calycin in Pseudocyphellaria crocata significantly decreased when filters were used to eliminate radiation under 400 nm, if compared with individuals exposed to full solar radiation (16).

The study of the photophysical and photochemical behaviour of calycin and rhizocarpic acid, characterised by the oxolane-carbonyl chromophore unit, has revealed that both compounds absorb UV-A and UV-B radiation, and emit fluorescence at 440 nm and 460 nm, respectively. The compounds are photostable, given the low photoconsumption quantum yields (17), allowing the use of herbarium specimens with the certainty that the compounds assayed do not show significant photochemical changes (18).

Acarospora schleicheri, a lichen species from Chile’s alpine zones, accumulates rhizocarpic acid. In a previous work, it was observed that the accumulation of rhizocarpic acid increased with the altitude in selected sites of the Chilean Altiplano, where the UV radiation is also higher (13). In this work, the purpose was to compare the evolution of the concentrations of rhizocarpic acid in individuals collected in altitudinal gradients at different latitudes to know if they correlated with the levels of UV-B radiation in various lichen habitats.

METHODS

Lichen material: Acarospora schleicheri A. Massal was collected in altitudinal gradients in Chile’s alpine zones. The characteristics of the collection sites are included in Table 1. Voucher specimens are deposited in the Lichen Herbarium of the School of Chemistry and Pharmacy, University of Valparaíso.

Extraction and quantification of rhizocarpic acid. In each site, at least ten thalli were randomly collected from rock surfaces. Thalli were cleaned and washed with distilled water, and dried at 60°C. Rhizocarpic acid was extracted in acetone at room temperature (20°C ± 2°) for 48 and 24 h successively. The two combined extracts were filtered and concentrated in vacuum to 5 mL volume, and were spotted on Merck HPTLC Silica Gel plates, using a Camag Linomat applicator and were eluted in toluene-ethyl acetate-formic acid (35:1:0.5). The plates were scanned in a Camag HPTLC densitometer, equipped with a Chromatopac Shimadzu C-R6A Model data processor, at l = 313 nm.

The rhizocarpic acid concentrations were calculated using formulae from the standard curves with an authentic sample. Variability within thalli growing in different habitats was determined by ANOVA and the differences among concentration means were tested using the Student’s test. The UV-B solar irradiance values were calculated as per Cabrera et al. (19), considering an increase of 5.1% at 18°S and 19°S, and of 65% at 33° Lat. S per kilometre of altitude.

Table 1. Geographical characteristics of the A. schleicheri collection sites.

Transect

Locality

Geographical coordenates

Altitude

(m.a.s.l.)

Year

1

Zapahuira

18º20' S, 69º37' W

3150

1996

Zapahuira - Putre

18º17' S, 69º35' W

3350

1996

Chucuyo

18º13' S, 69º18' W

4500

1996

Chungará

18º16' S, 69º08' W

4691

1996

2

Enquelga-Isluga

19º14' S, 68º47' W

3690

1996

Chusmisa

19º44' S, 69º13' W

3750

1996

Enquelga

19º13' S, 68º45' W

3840

1996

Colchane

19º27' S, 68º44' W

3920

1996

3

San José de Maipo

33º36’ S, 70º21’ W

910

1998

Melocoton

33º46' S, 70º15' W

1200

1998

El Alfalfal

33º30' S, 70º12' W

1330

1998

Las Melosas

33º53' S, 70º12' W

1480

1998

cerca Lo Valdés

33º49' S, 70º06' W

1680

1998

Lo Valdés

33º49' S, 70º03' W

1850

1998

cerca Termas de Colina

33º48' S, 70º00' W

2370

1998

Termas de Colina

33º S *

2480

1998

* The geographical coordenates were not recorded because the GPS used did
not receive a sufficient amount of satellites.

RESULTS

At three different altitude gradients selected, it was observed the same trends in the accumulation of rhizocarpic acid. The largest accumulation occurs at the highest altitudes where the UV-B radiation levels are also higher.

At the 18°S gradient, significant differences were observed (p£ 0.05) in the accumulation of rhizocarpic acid between 4500 and 4691 m a.s.l. (Figure 1), the highest concentration, that is 4,3 g/%, was recorded in the site having the highest altitude.

The rhizocarpic acid concentration remains relatively constant between 3750 and 3920 m high, at 19°S (Figure 2); between 3690 and 3750, a significant increase was observed (p£ 0.05), with an average value of 2,4 g%. Between the 18°S and 19°S, there is a correspondence in the values of the rhizocarpic acid concentrations, and the height of A. schleicheri habitat.

In individuals collected between the 910 and 1850 m high, at the 33° S gradient, the concentrations of the metabolite are not significantly different (Figure 3). At 2480 m, the greatest increase was recorded (p£ 0.05), there reaching a value of 2.2 g/%.

Fig. 1: Variation in rhizocarpic acid concentration in A. scheleicheri and its relation with altitude and UV-B solar radiation, at 18°Lat. S

Fig. 2: Variation in rhizocarpic acid concentration in A. scheleicheri and its relation with altitude and UV-B solar radiation, at 19°Lat. S

Fig. 3: Variation in rhizocarpic acid concentration in A. scheleicheri and its relation with altitude and UV-B solar radiation, at 33°Lat. S

DISCUSSION

The photoacclimatization seems to be a common process in lower plants with photoprotector systems. Microalgae, and other planctonic organisms synthetize MAAs, which absorb UV radiation (20). In a photoacclimatization experiment of antarctic diatoms to UV radiation, the MAAs levels and those of the photosynthetic efficiency increased in a meaningful way with the acclimatization time (21). Besides, in diatoms from the Antarctica exposed to low dosis of UV-B radiation, the growth increased; while higher doses inhibited it (22). The UV-A radiation induces an increase in the photosynthetic efficiency in the seaweed Porphyra umbilicalis (23), while the UV-B produced a decrease in the photosynthesis and in the synthesis of chlorophyll. In vascular plants, there are photoreceptors for the UV-B radiation (24); in Antarctic mosses, a direct relation was observed among the UV-B radiation levels and the concentration of flavonoids (25)

The effect of UV radiation in the synthesis of photoprotector compounds in lichens has been demonstrated by means of studies on the spatial and temporal variation of lichen metabolites which are characterised by orthohydroxicarbonyl chromophore (13,26, 27) and through field and laboratory experiments. The total of phenols accumulated by Umbilicaria americana increase significantly under the exposure to both UV-A and UV-B radiation. The largest increases were induced by the UV-A radiation (28). Significant increases in the synthesis of usnic acid were observed in Xanthoparmelia microspora when treated with additional doses of UV-A and UV-B radiation, larger than normal ones, during 15 days. The greater effect was induced by UV-A. If the exposure to the UV radiation is lengthened, a photodecomposition of usnic acid is produced (29)

Solar UV radiation increases with altitude (19, 30). The significant increase in the accumulation of rhizocarpic acid in A. schleicheri individuals collected in the sites of the highest altitude would be demonstrating that the UV-B radiation induces the synthesis of shikimic acid derivatives which are characterised by the oxolane-carbonyl ring.

The photoprotector compounds dissipate the UV energy absorbed as fluorescence in one spectral zone in which chlorophyll is absorbed (11). Sonesson et al. (31) observed a significant increase in the fluorescence of chlorophyll under the effects of the UV-B radiation, in Cladonia arbuscula, Cetraria islandica and Stereocaulon paschale. The increase in the chlorophyll fluorescence quantum yields by the effects of the UV-B would be a consequence of the electron transfer from the lichen compounds to the chlorophyll, as is was proposed by Rao and Le Blanc (32). The low fluorescence quantum yields of the lichen compounds (13) make it improbable their participation as energy donors. This apparent inefficiency would be compensated by the high concentrations of lichen metabolites which in numerous species range from 1 and 5 % dry weight (33).

From the results of the research on lichen photoprotector compounds, it may be inferred that these organisms adapt themselves to variable levels of UV solar radiation. This adaptative capability is expressed by the increase of the synthesis of the photoprotector compounds in habitats having a high UV irradiance, thus projecting lichens as biological indicators of changes in the levels of UV radiation due to ozone thinning world-wide.

ACKNOWLEDMENTS

This research work was funded by Fondo Nacional de Ciencia y Tecnología (FONDECYT), Project N° 1960353.

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