Introduction

Tierra del Fuego, at the southern tip of South America, has received increased UV-B radiation due to stratospheric ozone depletion for at least the last 20 years (Farman et al. 1985; Frederick et al. 1994). Whilst solar UV-B fluxes often remain low compared to those at low latitudes (about half of mid-summer values at 40° latitude), the relative increase in daily UV-B flux in Tierra del Fuego during the austral spring (October–November) can be up to twice that received prior to the occurrence of ozone depletion (Rousseaux et al. 1999; Searles et al. 1999). This substantial relative UV-B increase may influence community dynamics at different trophic levels in the forests and peatlands that dominate the region, e.g., slower decomposition rate (Pancotto et al. 2003), and changes in the extent of herbivory (Ballaré et al. 2001; Zaller et al. 2003).

Sphagnum mosses in peatlands create the hydrological conditions that allow carbon sequestration to occur through incomplete decomposition (van Breemen 1995; Clymo et al. 1998). Peatlands hold an estimated one-third of global soil organic carbon (Adams and Faure 1998; O’Neill 2000). The importance of peatlands in the global carbon cycle has spurred considerable research. In particular, the interaction of warming (Gerdol et al. 1998; Weltzin et al. 2001), increased N deposition (Berendse et al. 2001; Tomassen et al. 2003), increased atmospheric CO2 (Heijmans et al. 2001, 2002), and changing precipitation (Weltzin et al. 2003) on rates of plant growth and decomposition in peatlands (O’Neill 2000) have been investigated. However, few studies have involved the influence of UV-B radiation on processes and organisms associated with decomposition in heaths and peatlands (Johnson et al. 2002; Searles et al. 2001; Johnson 2003).

Water held between leaflets and in hyaline cells of the Sphagnum capitulum (dead water-filled cells of the uppermost Sphagnum leaflets) provides a niche for the peatland surface microfauna and fungi (Vitt 2000). Sphagnum-associated fungi in the acrotelm (Ingram 1978; Belyea 1996), the aerobic layer of the peatland, initiate most of the decomposition of Sphagnum (Thormann et al. 2002). Peatland microfungi are also important in the food web of the Sphagnum mat (Gilbert et al. 1998, 1999), and serve as a food source for some species of testate amoebae (Coûteaux and Dévaux 1983; Coûteaux and Pussard 1983).

Although the hydrology of the upper few centimeters of a peatland is largely dependent on environmental conditions (Bragazza et al. 2003), the volume of water held near the surface is influenced by Sphagnum capitulum size and density (Hayward and Clymo 1982). It is not known whether changes in Sphagnum capitulum growth and morphology under near-ambient UV-B (Robson et al. 2003) are expressed in the peatland microfungal community.

Microfungal communities are sensitive to changes in acidity and nutrient concentrations in Sphagnum capitulum water (Zabawski 1967; Thormann et al. 2003). Higher concentrations of solutes in the capitulum water, indicated by increased conductivity, under supplemental UV-B, were reported by Niemi et al. (2002a, b). As moss leaves are only one cell thick they are thought to be particularly susceptible to membrane damage, which leads to cytosolic leakage (Gerdol 1991). Nutrient-binding organochemicals, phenolics, and uronic acid, are excreted and leached by Sphagnum into the surrounding water (Rasmussen et al. 1995). These lower the pH of the peatland and are inhibitory to some microfungi (Heil et al. 2002), but are utilized by others (Flanagan and Scarborough 1974; Thormann et al. 2002).

From a preliminary microfungal sample in January of 1999, Mortierella, Penicillium, and Mucor were identified as the most abundant genera in the top 1 cm of the Sphagnum capitulum (Searles et al. 2001). Species of all three genera are known to respond to UV-B radiation. Supplemental UV-B treatments decreased the abundance of Mucor in decomposing leaf litter (Gehrke et al. 1995). Supplemental UV-B inhibited growth of Mortierella parvispora, compared to dark controls (Hughes et al. 2003). Penicillium frequentans was less abundant on senescent Gunnera magellanica leaves (Pancotto et al. 2003) under near-ambient compared to reduced UV-B.

We sampled the peatland microfungi during three field seasons of exposure to near-ambient and reduced UV-B (1999–2002), and concurrently measured plant growth and sampled water from the Sphagnum capitulum. We identified the different fungal species in the community where possible, and tracked changes in their abundance to assess the species specificity of response to UV-B. We examined the effect of solar UV-B on fungal diversity, as ubiquitous species were expected to have greater tolerance of UV-B than species native to Tierra del Fuego. We postulated that the combined effects of UV-B on fungal growth and the altered Sphagnum capitulum morphology with associated changes in capitulum water chemistry could act to alter the microfungal community. To test this hypothesis, we compared responses of cultured fungi in petri dishes with those of the peatland microfungal community under different UV-B treatments. We sampled Sphagnum capitula at depths of 0–5 and 5–10 mm from the peatland surface. Since solar radiation only penetrates a few millimeters into the Sphagnum mat, our goal was to contrast the microfungal community at the peatland surface where it is subject to direct solar radiation, with the community just below the surface where no solar radiation penetrates. We also observed whether the magnitude of response of the microfungal community to near-ambient UV-B followed a similar pattern during three field seasons.

Materials and methods

The study site is a Sphagnum magellanicum (Brid) dominated peatland in the Tierra del Fuego National Park (54°51′S 68°36′W).

A weather station and CS 21x datalogger at the site (Campbell Scientific, Logan, Utah) were used to record temperature, precipitation, wind speed, and UV-B radiation throughout each field season. Ambient UV-B (Table 1) and UV-B under the two filter types (data not shown) were measured using broadband global radiation sensors (Solar Light, Model PMA2102, Philadelphia, Penn.), calibrated to a double-monochromator spectroradiometer SUV-100 at the Ushuaia UV-monitoring station (Biospherical Instruments, San Diego, Calif.). Mean temperatures for the 5-month period, October–February, were 8.9°C (1999–2000), 7.9°C (2000–2001), and 9.7°C (2001–2002). Total precipitation for the same 5-month period was 260 mm (1999–2000), 312 mm (2000–2001), and 269 mm (2001–2002). Ground water was ca. 40 cm maximum depth below the surface. The Sphagnum mat remained frozen at 5-cm depth through the winter until at least the end of September (for more details of the climate and flora of the peatland see Robson et al. 2003).

Table 1 Monthly averages of daily integrated solar UV-B radiation over three 5-month field seasons recorded at our peatland field site in Tierra del Fuego National Park (1999–2000, 2001–2002). Data from a nearby fen field site are presented from 2000–2001 courtesy of J.G. Zaller, because of instrument failure at the peatland. The daily UV-B values (kJ m−2 day−1) are normalized to one at 300 nm and weighted with the generalized plant action spectrum (Caldwell 1971). Daily means for each month are shown ±1SD among days. Data from the NSF UV-monitoring station in Ushuaia (http://www.biospherical.com/nsf/login/update.asp) are given in parentheses for comparison
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Ten pairs of 1.4×2.0-m plots were established during October 1996. Plots were located within an area of relatively homogeneous vegetation without pronounced hummocks and hollows in the Sphagnum mat, between a small lake (Laguna Negra) and Nothofagus wood. These plots were maintained for the following six field seasons (September–March), and have been used in several long-term field experiments (Ballaré et al. 2001; Robson et al. 2003).

Plastic film filters were stretched (ca. 40-cm height) over the plots to create the near-ambient and reduced UV-B treatments (see photo Ballaré et al. 2001). A matrix of small louvred slits (ca. 25×2 mm) was melted into the filters prior to installation, to enable water from precipitation to penetrate evenly to the Sphagnum mat (Searles et al. 2001). Field trials showed that almost all the precipitation passed equally through the two filter types (Robson et al., unpublished data). Polyester filters (100 μm thick, optically equivalent to Mylar-D, Dupont, Wilmington, Del.), that attenuate ca. 83% of short wavelength UV-B radiation (300 nm) were perforated (Searles et al. 2002), and were used to achieve the reduced UV-B treatment. Polyfluorine filters (Aclar type 22A, 38-μm thick, Honeywell, Pottsville, Penn.) that block ca. 10% of the UV-B (Searles et al. 2002), were used to create our near-ambient UV-B treatment. Both films are equally transparent to longer wavelengths (UV-A and visible), each blocking ca. 10% of visible radiation. The UV-B treatments were randomly assigned within each pair of plots (“block”) and remained in place from late September–March each year. This period incorporates early spring when the ozone hole opens but the sun is low in the sky, through to autumn when UV-B again becomes very low (Díaz et al. 2001). During winter the peatland is usually covered by snow and ice.

The microfungal community, Sphagnum capitulum mass, and water held by the capitulum were sampled at four evenly spaced time intervals during the fourth (1999–2000) and sixth (2001–2002) field seasons, and three times during the fifth (2000–2001) field season of long-term UV-B treatments. To avoid edge effects, all samples were taken at least 20 cm from the edge of the area covered by the filters. To accommodate Sphagnum surface heterogeneity (Mitchell et al. 2000), a random sample of 28 Sphagnum capitula was removed from each plot using sterile forceps. Sphagnum capitula were maintained in isolated conditions and taken to the laboratory for immediate weighing and processing.

Under a laminar-flow hood, Sphagnum capitula from both near-ambient UV-B and reduced UV-B plots were cut into two 5-mm lengths, corresponding to the 0- to 5-mm and 5- to 10-mm depths beneath the surface of the Sphagnum mat. Previously, it was established that at least 99% of solar radiation is attenuated at 6-mm depth in the Sphagnum capitulum (Searles et al. 1999). The 28 capitula taken from each plot were pooled, weighed, and added to two tubes containing 6 ml of autoclaved deionized water. These tubes were agitated for 4 min, then 400-μl and 200-μl volumes were pipetted onto potato dextrose agar (PDA) under sterile conditions (following Dickinson 1982).

Agar plates were incubated in the dark at room temperature. The various fungal species grew at different rates, so it was necessary to monitor growth from 3 until 8 days after inoculation. Colony forming units (CFUs) of each species were counted to determine fungal abundance. Fungal species were identified using Domsch et al. (1980), and additional specific keys for Mortierella (Cabello 1997), and Biverticillium (Quintanilla 1985).

Ionic conductivity (DIST ATC dissolved solid tester, Cole Parmer, Chicago, Ill.) and pH (Corning pH-40 sensor, New York) of the 6 ml of water held by the Sphagnum capitulum were measured within 12 h after harvest. Samples were stored in the dark at 4°C until a second measurement of ionic conductivity was taken after 60 h. Ionic conductivity is considered a good relative indicator of ionic leaching from Sphagnum cells (Gerdol 1991). Ionic conductivity was adjusted for H+ ions following Sjörs (1950). Tests using alcohol to sterilize the sample confirmed that microbial activity did not affect ionic conductivity after 60 h. On termination of the experiment, Sphagnum capitula were oven dried at 65°C for 72 h and dry mass determined. This allowed the initial water content to be calculated gravimetrically by subtracting dry mass from the initial fresh mass measured directly after capitula were removed from the peatland.

In February 2003, the most prominent fungal species were isolated from Sphagnum capitula. Fungi from near-ambient UV-B and reduced UV-B plots exhibited the same inherent growth rates when inoculated onto petri dishes containing PDA. Cultures were left untreated for 2 days to confirm that inoculation was successful before UV-B treatments were administered in a growth chamber equipped with a 6,000-W xenon lamp. Daily growth, changes in sporulation, sclerotia, hyphal density, and pigmentation were recorded. The different UV-B treatments were created by replacing most of the petri-dish lid with a plastic filter. As with the solar UV-B treatments in the peatland, polyester film was used to block most of the UV-B radiation, and polyfluorine film allowed the lamp UV-B radiation to pass. A cellulose diacetate filter was wrapped around the lamp to remove radiation <290 nm. The fungi were exposed to 3.5–4 kJ m−2 day−1 UV-B weighted with the generalized plant action spectrum normalized at 300 nm (Caldwell 1971), measured with a modified double-monochromator (Optronic, Orlando, Fla.). In Ushuaia, Tierra del Fuego, the average November–February UV-B dose from 1996–2002 was ca. 4 kJ m−2 day−1 (Table 1; and Searles et al. 2002), rising to in excess of 8 kJ m−2 day−1 on the four or five occasions each austral spring when the “ozone hole” passed over the site. Fungal cultures also received ca. 46.8 mol m−2 day−1 (800 μmol m−2 s−1 for 16 h) of photosynthetically active radiation (total photon flux, 400–700 nm). These daily doses were calculated to approximately simulate our peatland radiation treatments in the field (Zaller et al. 2004).

Statistical analysis

Species occurrence was calculated as mean presence in the ten plots of each treatment type over the duration of the study (1999–2002). Unadjusted species richness was calculated at the 0- to 5-mm and 5- to 10-mm depths, and the Shannon-Weiner index was used to assess evenness of fungal distribution throughout the experiment. The effect of UV-B treatment on each fungal species occurrence, and on species richness was tested using a one-way factorial ANOVA.

To control for the influence of gradients in ground-water depth and pH, and floristic composition across the site, each pair of plots was considered as a block in the statistical models. The effect of UV-B treatment, depth, and month on Sphagnum dry mass, water, pH and conductivity, was assessed using an analysis of variance of a three-way factorial, blocked split-split-plot-in-time design. Ionic conductivity was only measured during the final field season of treatments. Hence, to allow for direct comparison between physical parameters, only results from the final field season are presented for Sphagnum capitulum mass, water, and pH.

The abundance of some fungi was low, and there were no distinct seasonal trends in abundance, so abundance data for each field season were pooled before analysis. The effect of UV-B treatment, depth and field season on abundance of each fungal species was assessed using an analysis of variance of a three-way factorial, blocked split-split-plot-in-time design. The UV-B, depth and field season (“year”) were the fixed-effects factors and block, block × UV-B, block × UV-B × depth were the random-effects factors in the design (Aldworth and Hoffman 2002). A first order autoregressive variance/covariance structure for repeated-measures-in-time was determined to be most appropriate for the data (Keselman et al. 2002). To assess the relationships amongst species within the microfungal community, and between fungal species, Sphagnum capitulum water and climatic variables, correlations across the ten blocks over the two UV-B treatments, at two depths were used.

The growth chamber experiment was performed twice. Each fungal species was inoculated into four pairs of petri dishes, which were rotated daily under the lamp. Daily measurements of the colony diameter of fungi cultured showed fungal growth to be constant. Growth rate of each colony was calculated by linear regression, and the mean calculated for each species under lamp-UV-B and lamp-UV-B controls. The effect of lamp UV-B on the growth of cultured fungi was tested using a one-way factorial ANOVA in a completely randomized design. All computations were performed in SAS, Version 8.2 (SAS Institute, Cary, N.C.).

Results

Nine species of hyphal fungi were isolated from the Sphagnum capitulum (Fig. 1), of which M. alpina and P. thomii were the most common. Additionally, several forms of Aureobasidium (including A. pullulans) a filamentous bacteria Actinomycetes and white and colored yeasts, were isolated from the Sphagnum (Fig. 1). There were no significant UV-B effects on occurrence of individual fungal species, except A. pullulans, which was present in fewer plots under near-ambient UV-B than under reduced UV-B at the 0- to 5-mm depth (Fig. 1).

Fig. 1
figure 1

Frequency of microfungal species occurrence in the Sphagnum capitulum, at 0- to 5-mm and 5- to 10-mm depths in the peatland. Mean (±1SE) number of plots with species present in each UV-B treatment of 11 samples over three field seasons. Species with less than 1-in-10 occurrence, on average, were not assessed. ** P<0.01

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Species richness of the microfungal community was slightly lower under near-ambient than under reduced UV-B (Fig. 2). When averaged over the entire sampling period, species richness was significantly lower at 0- to 5-mm depth from the Sphagnum surface under near-ambient UV-B (Fig. 2a; F 1,9=8.53, P=0.017), but not at the 5- to 10-mm depth (Fig. 2b; F 1,9=1.98, P=0.193). Shannon-Weiner evenness (J′) for the entire 0- to 10-mm depth was slightly less under near-ambient UV-B in mid-summer (January) during each of the three field seasons (Fig. 2c). This difference is statistically significant if the 11 samples over the 3-year period are considered (F 1,10=15.9, P=0.003).

Fig. 2a–c
figure 2

Fungal diversity: mean species richness at a 0- to 5-mm and b 5- to 10-mm depths (±1SE), and c Shannon-Weiner evenness (J′) at 0- to 10-mm depth, in the Sphagnum capitulum, of ten plots for each UV-B treatment. Diversity was calculated for each of 11 sample dates over three field seasons. In December 1999 and March 2000, only the 0- to 5-mm depth was assessed

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Effects of UV-B on the Sphagnum microenvironment

Solar UV-B affected Sphagnum morphology and properties of the capitulum water (Fig. 3). The Sphagnum capitula under near-ambient UV-B held more water than did those under reduced UV-B in our January and March samples (Fig. 3d). This effect was significantly more apparent at the 0- to 5-mm than at the 5- to 10-mm depth (Fig. 3d). Higher capitulum dry mass at the 0- to 5-mm depth under near-ambient UV-B was maintained throughout the growing season (Fig. 3a). Water associated with the Sphagnum capitulum was more acidic under near-ambient UV-B in the November and December samples (Fig. 3c). The initial ionic conductivity of Sphagnum capitulum water after harvest was greater under near-ambient UV-B (data not shown). The rate of increase in ionic conductivity between 24 and 74 h after harvest was also greater under near-ambient UV-B at three of the four sampling times (Fig. 3b).

Fig. 3a–d
figure 3

Physical properties of the Sphagnum capitulum. All four of the dependent variables tested demonstrated a significant effect of month (P<0.001) and depth (P<0.001). Other sources of variation had insignificant effects. Monthly mean, 2001–2002 field season ±1SE: a Sphagnum capitulum dry-mass. b Change in the ionic concentration of water from the Sphagnum capitulum between 24 and 72 h after harvest. c pH of water from the Sphagnum capitulum 24 h after harvest. d Water held by the Sphagnum capitulum

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Effects of UV-B on fungal species within the microfungal community

Despite a suggestion that abundance of hyphal fungi increased under near-ambient UV-B (Fig. 4, UV-B × Year), the abundance of hyphal fungi in the top 10 mm of the peatland was not significantly affected by solar UV-B when considered over the three field seasons (Fig. 4). However, the overall abundance of fungi did decrease over with time under both treatments (Fig. 4).

Fig. 4
figure 4

Abundance of all species of hyphal fungi (colony-forming units). Data represent the back-transformed means (±1SE) of three or four samples pooled over each field season

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The unpigmented species M. alpina was more abundant under near-ambient than reduced UV-B (Fig. 5a), but only at the depth of 5–10 mm. At the 0- to 5-mm depth, no effect of near-ambient UV-B on M. alpina was apparent (Fig. 5a). There was no overall treatment effect on a related pigmented species M. vinacea, but it was more abundant under near-ambient UV-B in the 2000–2001 field season (Fig. 5b), suggesting a UV-B × field season interaction. However, when pure cultures of M. alpina and M. vinacea were subjected to lamp UV-B approximating that of solar UV-B at the peatland surface, there was a reduction of more than 20% in colony growth rate of both species (Table 2).

Table 2 Growth rates of fungal species in pure culture under lamp UV-B radiation
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Fig. 5a–h
figure 5

Response of individual fungal species from the peatland microfungal community to near-ambient and reduced UV-B. a Mortierella alpina, b M. vinacea, c P. thomii, d P. frequentans, e Mucor hiemalis, f Aureobasidium sp, g White yeasts, and h Colored yeasts. The number of CFU per 200 ul from each 6-ml sample containing 14 Sphagnum capitula is shown. Back-transformed means (±1 SE) of multiple samples (3–4 occasions) for each field season. Three-way interaction terms were all non-significant, except for P. frequentans, F 2,72=3.60, P=0.032

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Two other related species, P. thomii and P. frequentans, were affected in different ways by near-ambient UV-B. Whilst P. frequentans growth and abundance were reduced by both near-ambient UV-B in the peatland and by lamp UV-B in pure culture, P. thomii was not affected by UV-B in either experiment (Fig. 5c, d; Table 2). However, an increase in pink sclerotia typical of P. thomii, and an apparent decrease in conidial frequency were observed under lamp UV-B, whilst no difference in the conidia of P. frequentans was evident.

Abundance in the peatland of the unpigmented species Mucor hiemalis under near-ambient UV-B was significantly lower in the final field season of treatments, but unaffected by UV-B overall (Fig. 5e). In pure culture under lamp UV-B, M. hiemalis growth was reduced (ca. 40%; Table 2), and hyphal density was observed to increase. Cladosporium herbarum abundance in the peatland was highly variable amongst years and did not differ between near-ambient (mean CFU±1SE; 0–5 mm, 1.89±0.72, 5–10 mm, 1.89±0.59 mm) and reduced UV-B (0–5 mm, 2.58±0.54 mm; 5–10 mm, 1.82±0.50 mm; F 1,9=0.74, P=0.41). Colony growth of C. herbarum under lamp UV-B was less inhibited than most other species (ca. 18%; Table 2). Aspergillus isolated at the 0- to 5-mm depth in the final field season was less abundant in the peatland under near-ambient (mean CFU±1SE; 0.30±0.10) than under reduced UV-B (0.70±0.19, F 1,9=6.94, P=0.01), and in the growth chamber it was strongly inhibited by lamp UV-B (ca. 50%; Table 2). Overall, there was no consistent UV-B effect on white or colored yeasts in the peatland, but changes in yeast abundance under the UV-B treatments at the two depths lead to a significant interaction between UV-B and depth (Fig. 5g, h). At the 0- to 5-mm depth there was a tendency for more yeasts under near-ambient UV-B than under reduced UV-B, whilst at the 5- to 10-mm depth the opposite tendency was apparent (Fig. 5g, h). Aureobasidium abundance was unaffected by UV-B treatment in the peatland (Fig. 5f). Lamp UV-B inhibited Aureobasidium growth (Table 2) and accelerated sporulation by approximately 2 days.

Discussion

To the best of our knowledge, ours is the only Southern Hemisphere peatland microfungal community to have been studied (see also Searles et al. 2001). All the species we encountered are widespread and frequently isolated from Northern Hemisphere peatlands, e.g., in Sweden (Nilsson et al. 1992), Canada (Thormann et al. 2003), and Italy (Dal Vesco 1975). Penicillium thomii and Mortierella species often dominate microfungal communities near the Sphagnum surface (Zabawski 1967; Maciejowska-Pokacka 1971; Dickinson and Maggs 1974; Thormann et al. 2003), and this was also the case in our peatland. In line with Searles et al. (2001), M. alpina continued to be the most common species, despite favoring the less acidic areas of the peatland (see also Dal Vesco 1975; Fritze and Bååth 1993).

The response of fungi to our UV-B treatments was species specific (Figs. 4, 5). Overall fungal abundance was largely unaffected (Fig. 4), but diversity was slightly lower (Fig. 2) under near-ambient UV-B. These results may indicate that whilst some of the less common species declined under near-ambient UV-B, others, whose growth was relatively unaffected by UV-B, were able to compensate and proliferate (Fig. 5). Additionally, those species that were most abundant under near-ambient UV-B, particularly at the 5- to 10-mm depth, may have been able to exploit the increased ionic concentration and altered Sphagnum capitulum morphology.

Sampling of the microfungal community over three field seasons revealed more effects of near-ambient UV-B than did the initial census of Searles et al. (2001). Often long-term experiments show responses to UV-B that are not apparent in studies of shorter duration (Björn et al. 1999; Day et al. 2001; Phoenix et al. 2001; Pancotto et al. 2003). The detection of UV-B effects on microfungal community composition after several years of treatments is not surprising, given that more changes in the plant community were detected after 6 years of UV-B treatments (Robson et al. 2003) than during the first 3 (Searles et al. 1999, 2002). This is particularly true for those fungal species that may respond indirectly to changes in the Sphagnum microenvironment brought about by UV-B.

Effects of UV-B on the Sphagnum microenvironment and potential consequences for peatland fungi

More water was held at the Sphagnum surface under near-ambient than reduced UV-B (Fig. 3). We consider that this was due to the more compressed and densely packed Sphagnum capitula under near-ambient UV-B (Robson et al. 2003). Moisture is known to be an important influence on microfungal community composition in peatlands (Nilsson et al. 1992). Thus, the larger wetter capitula are likely to have provided greater opportunity for fungal colonization under near-ambient UV-B.

The capitulum water was more acidic and had greater ionic conductivity under near-ambient UV-B. These effects of UV-B were not detected over the entire field season, and were not correlated with the volume of water held by the capitulum. Increased ionic conductivity is typically correlated with the leaching of sodium, magnesium and calcium during the Sphagnum growing season, and release of nitrogen and phosphorus during Sphagnum senescence in the autumn (Gerdol 1991; Bragazza and Gerdol 1999). Niemi et al. (2002b) attributed increased concentration of calcium and magnesium ions to increased Sphagnum membrane permeability under supplemental UV-B radiation. The greater abundance of M. alpina and other species under near-ambient UV-B at 5- to 10-mm depth, and an amelioration of direct UV-B effects for M. alpina at 0- to 5-mm depth in our peatland (Fig. 5a), may be due to the nutritional benefit gained through increased Sphagnum membrane permeability under near-ambient UV-B. Mortierella are known to break down sugars in initial phases of decomposition in peatlands, and have been shown to respond positively to increased leaching from the Sphagnum capitula (Deacon 1997; Thormann et al. 2003).

Decreases in pH and increases in nutrient content in the acrotelm, where most decomposition occurs (Williams and Yabitt 2003), may provide a more favorable environment for the majority of peatland microfungi (Zabawski 1967; Szumigalski and Bayley 1996). Leaching from the Sphagnum capitulum is the most significant mass loss during the first phase of decomposition (Thormann et al. 2002), so increased leaching under near-ambient UV-B could have important implications for decomposition processes.

Effects of UV-B on fungal species within the microfungal community

The active decomposer species P. thomii (Thormann et al. 2002, 2003) was the second most abundant species in our microfungal community after M. alpina. Its abundance remained unchanged by UV-B during our three-field-season sampling period (1999–2002) and in the initial sample from the peatland in January 1999 (P.S. Searles, unpublished data). Similarly, P. thomii on senesced Gunnera magellanica leaves prior to decomposition in a nearby heath was equally abundant under near-ambient and reduced UV-B (Pancotto et al. 2003).

Several investigators have reported Cladosporium herbarum to be quite tolerant of UV-B. The occurrence of C. herbarum was not significantly reduced by supplemental UV-B (30% above ambient) on the lamina of living oak leaves (Newsham et al. 1997a), though it did decrease during their decomposition (Newsham et al. 1997b). In Tierra del Fuego, C. herbarum abundance was not affected by near-ambient UV-B on living Carex leaves (Searles et al. 2001), and it increased on G. magellanicum leaves under near-ambient UV-B prior to decomposition (Pancotto et al. 2003). In line with these results, the abundance of C. herbarum in our peatland was unaffected by near-ambient UV-B, and under lamp UV-B in a growth chamber its colony growth was less inhibited than most other species studied.

It has been suggested that dark pigmentation in C. herbarum and other fungi confers protection from UV-B (Pancotto et al. 2003). In Antarctica, pigmentation of some fungal species in isolated cultures increased in response to ambient UV radiation (Hughes et al. 2003). Also in Antarctica, fewer unpigmented mycelia and conidia were present in the open than under “closed plastic cloches” that reduced UV and increased temperature by 6°C, whereas there were no differences between treatments in the abundance of pigmented soil fungi (Onofri et al. 2000). However, increased solar and supplemental UV-B do not always favor pigmented species of fungi. Supplemental UV-B radiation shifted competitive advantage from unpigmented towards pigmented fungi in only two out of six pair-wise tests of soil fungal species (Duguay and Klironomos 2000). It is difficult to make generalizations about fungal species responses to UV-B as, even within genera, fungi are known to behave differently (Moody et al. 1999). This was the case with the two Mortierella species that we encountered (Fig. 5a, b). However, most species of Penicillium studied (apart from P. frequentans) appear to be tolerant of UV-B. This is illustrated by the apparent lack of effect of UV-B radiation on P. thomii abundance in our peatland, also of supplemental UV-B radiation from lamps on P. brecompactum in a sub-Arctic peatland (Gehrke et al. 1995), and on Penicillium species living on oak leaves (Newsham et al. 1997a). Penicillium are generally considered to have high tolerance of environmental stress (Zabawski 1967; Domsch et al. 1980). Some decomposer fungi have been shown to initially respond to UV-B, but these responses were transitory (Gehrke et al. 1995; Newsham et al. 1997a). This may indicate that some species are able to acclimate to higher UV-B radiation.

Conclusions

Changes in the peatland microfungal community under long-term near-ambient UV-B were small and species specific. This was surprising given that there were considerable reductions in growth of fungal cultures under our lamp UV-B treatments. Our peatland UV-B treatments did increase the ionic conductivity, acidity and the volume of water held by the Sphagnum capitulum. Perhaps, given that solar UV-B is attenuated quickly at the peatland surface, microenvironmental changes brought about by UV-B were an important influence on the peatland microfungal community. However, this remains to be explicitly tested.

Overall fungal diversity near the surface of the Sphagnum was low under both treatments, making subtle effects on community difficult to detect. The small decrease in richness recorded may be due to direct inhibition by solar UV-B of less common fungal species, or increased dominance of the most common species in the peatland near-surface microfungal community. This decrease in diversity did not influence the overall fungal abundance, and it appears that most of the fungal species at the peatland surface are sufficiently tolerant of, or protected against, current solar UV-B radiation in Tierra del Fuego. All of the fungi encountered are relatively ubiquitous, and known to live in environments where the UV-B flux is significantly higher than it is in Tierra del Fuego. Alternatively, it is possible that these fungi in Tierra del Fuego have already adapted to the increased solar UV-B radiation that they receive after over 2 decades of ozone depletion in this region.