Journal of Experimental Botany, Vol. 53, No. 368, pp. 455-463,
March 1, 2002
© 2002 Oxford University Press
Original Papers |
Lipid metabolism in the moss Rhytidiadelphus squarrosus (Hedw.) Warnst. from lead-contaminated and non-contaminated populations
1 Institute of Ecology of the Volga River Basin RAS, Togliatti 445003, Russia
2 Cardiff School of Biosciences, Cardiff University, PO Box 911, Cardiff CF10 3US, UK
Received 24 January 2001; Accepted 20 September 2001
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Abstract |
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Lipid metabolism and the effect of Pb2+ and Cu2+ on this process was studied in the moss Rhytidiadelphus squarrosus collected from both a lead-contaminated and a non-contaminated site. Total radiolabelling of lipids from [1-14C]acetate was similar in both populations and Cu or Pb (1 µM, 10 µM) did not cause much alteration in acute exposure experiments. However, there were significant qualitative changes. Of the major labelled neutral lipid classes, samples from the lead-polluted site showed a decrease in labelling of triacylglycerols and an increase in wax esters. Acute lead exposure caused similar effects. Cu caused a decrease in the labelling of wax esters and an increase in diacylglycerols. These data suggest that heavy metals cause a change in carbon flux through the acylation reactions associated with the Kennedy pathway. R. squarrosus obtained from the Pb-contaminated site also showed changes in polar lipid labelling compared to the uncontaminated site. The labelling of phosphatidylcholine was more than halved and replaced by increased labelling of other zwitterionic lipids. The chloroplast glycerolipids were also increasingly labelled. Acute exposure to Pb, however, caused little alteration of labelling patterns within 24 h. R. squarrosus contains high levels of polyunsaturated fatty acids (PUFAs), but moss obtained from the Pb-polluted site had significantly less PUFAs containing three or more double bonds. Such samples, when incubated with [1-14C]acetate also showed decreased PUFA labelling. By contrast, acute exposure to Pb produced different results. These data provide a foundation for examining lipid metabolism in bryophytes and the effects of pollution in this important class of organism. The results also emphasize that acute and chronic exposure to heavy metals may produce different effects and that caution must be exercised in extrapolating data from one system to another.
Key words: Bryophytes, copper, fatty acids, lead, lipids, metabolism.
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Introduction |
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TopAbstractIntroductionMaterials and methodsResults and discussionConclusionsReferences |
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Heavy metals have been shown to affect a wide range of plant cellular activities including photosynthesis, respiration, mineral nutrition, membrane structure and properties, and gene expression (Smith et al., 1985
; Jones et al., 1987; Tyler, 1990; Jones and Harwood, 1993; Brown, 1995; Maksymiec, 1997; Rama Deli and Prasad, 1999). Many of these effects can be interrelated through a general action on membrane biogenesis and integrity which in turn can occur because lipid metabolism is altered. Indeed, alterations in the activities of enzymes such as fatty acid synthase and oleoyl-ACP desaturase have been noted previously (Jones et al., 1987; Jones and Harwood, 1993). Many of the above activities are probably related to the affinity of heavy metals for sulphydryl groups, which results in inhibition of the active sites of enzymes and/or conformational modifications of macromolecules (Cumming and Taylor, 1990).
Although all plants can be affected by high levels of heavy metals, some species are quite tolerant to lower amounts. This tolerance has practical application in bioremediation and in efforts to colonize polluted sites. In order to understand the ways in which tolerance may occur, research into the role of membranes and in the metabolism of lipids (as major membrane components) is needed for several reasons: (1) alterations in the composition of the plasma membrane may change membrane permeability and, consequently, net metal ion uptake (Cumming and Taylor, 1990); (2) the level of lipid peroxidation which can alter many metabolic processes in the cell is determined, to an important extent, by the degree of fatty acid unsaturation of membrane lipids. Membrane unsaturation has been shown to be closely related to heavy metal tolerance in a number of higher plants, algae and micro-organisms (Avery et al., 1996; Howlett and Avery, 1997; Maksymiec, 1997); and (3) the rapid turnover of membrane components may represent a strategy for adaptive modification to metal stress.
One adaptive response to heavy metals can involve alterations of the composition of plasma membrane to change its permeability and metal ion uptake. Alternatively (or in addition), reactions which lead to the chemical detoxification or the physical immobilization of metals outwith membranes are two potential strategies for protecting the cell's plasma and other membranes from metal stress (Cumming and Taylor, 1990). Indeed, a number of different mechanisms may be involved in metal tolerance. For example, mechanisms of copper resistance in bacteria and fungi have been related to reduced copper transport, enhanced efflux of cupric ions, extracellular chelation or precipitation by secreted metabolites as well as intracellular complexing by metallothioneins and phytochelatins (Maksymiec, 1997).
No data are available for bryophytes of heavy metal effects on lipid metabolism or on the mechanisms for their sensitivity or tolerance at the biochemical level, despite the importance of these organisms in a wide variety of terrestrial habitats. So far, certain species of mosses and liverworts are just known to accumulate heavy metals from polluted sites without obvious detrimental effects on vitality (Burton, 1990). By contrast, other species are relatively sensitive and only tolerate low concentrations of heavy metal ions in their tissues (Tyler, 1990). It is known that bryophytes have little or no possibility of avoiding exposure and retention of heavy metals supplied from the atmosphere, water and their substratum. Thus, a main tolerance mechanism in these plants is thought to be the efficiency of cell walls and associated polysaccharides to immobilize heavy metal ions (Brown, 1995). In consequence, a good correlation was found between the content of unesterified polyuronic acids and the cation exchange capacity of various Sphagnum species (Clymo, 1963). Mannuronic acid and pectin galacturonan are reported to be present in the cell wall of liverworts and the moss Rhacocarpus purpurascent, respectively (Brown, 1984; Edelmann et al., 1998). Polymeric lipids such as cutins and suberins which contain phenolics and other metal-binding groups (Kolattukudy, 1980) are also present as cell surface constituents in Sphagnum mosses (Karunen and Kalviainen, 1988). In addition, Jackson et al. have noted the inducible occurrence of intracellular metal-chelating polypeptides (phytochelatins) in the freshwater moss Rhynchostegium riparioides exposed to elevated concentrations of Zn, Cu, Cd, and Pb under both laboratory and field conditions (Jackson et al., 1991). Thus, both external and intracellular chelation of metals can take place to assist with physiological tolerance.
Important heavy metals emitted by traffic and industry are lead and copper. Lead is not essential for plant growth and is toxic even in low concentrations. Copper is an essential micronutrient for most living organisms since it is the constituent of many metalloenzymes and proteins involved in electron transport, redox and other important reactions. But copper, when present at higher levels in its free ionic form (Cu2+), is toxic to plant cells.
Rhytidiadelphus squarrosus is a moss species, which has been frequently used to understand the effects of heavy metals on the physiology of bryophytes (Brown and Wells, 1990; Wells and Brown, 1995). Therefore, this species was chosen for the present work.
In order to understand how membrane components may be involved in both the short-term cell response and potential long-term adaptive mechanisms to heavy metal exposure, the patterns of lipid labelling were studied in the presence and absence of Pb2+ and Cu2+. Samples of Rhytidiadelphus squarrosus obtained from both lead-contaminated and non-contaminated populations were used in order to assess the influence of pre-exposure to heavy metals on the results.
Based on previous data from other plant species it was anticipated that fatty acid composition (unsaturation) might be a significant target of heavy metal exposure. Moreover, because of the rapid effects of toxic metals on R. squarrosus photosynthesis (Brown and Wells, 1990) changes were sought in the metabolism of thylakoid lipids in particular. Furthermore, by assessing metabolism through radiolabelling, it was possible to detect changes more rapidly than from quantification of endogenous membrane compositions.
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Materials and methods |
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Plant material
Field-grown material from two populations of the moss Rhytidiadelphus squarrosus (Hedw.) Warnst with different lead contamination was gathered from grassland in Bute Park, Cardiff (relatively uncontaminated site) and in Priddy, Mendip Hills, Somerset, a disused lead mine (metal-contaminated site) in July and September, 1998. Other flora were removed as carefully as possible from the moss and the samples were randomized before incubations or analysis. Only freshly collected material was used (the moss was kept in the laboratory no longer than one week in moist, dark conditions).
Chemicals
Fatty acid standards were from Nu-Chek Prep. Inc. (PO Box 172, Elysian, MN56028, USA) and silica gel G plates from Merck. Complex lipid standards were from Sigma (Poole, Dorset, UK). [1-14C]acetate, Na salt (sp. act. 1.85–2.29 GBq mmol-1) was from Amersham Life Science Ltd. (Bucks HP7 9NA, UK). Other reagents were of the best available grades and were from Sigma (Poole, Dorset, UK) or from BDH (Poole, Dorset, UK).
Incubations
24 h before the experiments, the samples (2–3 cm apical segments of green tissue) were placed in high humidity in an enclosed vessel at 20 °C for an adaptation period. Moss samples (approximately 0.7 g FW) were then placed into 25 ml beakers containing 6.0 ml of 1 µM or 10 µM aqueous solution of Cu(NO3)2 or Pb(NO3)2 with 2–4 µCi [14C]acetate for 24 h at 20 °C with 200 µE m-2 s-1 continuous illumination. At the end of the incubation period, tissues were rinsed in H2O to remove excess radiolabel and metabolism was terminated by the addition of hot iso-propanol and heating at 70 °C for 30 min. This method ensures that lipid catabolic enzymes are inactivated. Lipids were extracted using the method of Garbus et al. (Garbus et al., 1963). Fatty acid methyl esters were prepared by transmethylation with 2.5% H2SO4 in dry methanol.
Lipid analysis
Total lipid extracts were separated into neutral and polar lipid fractions by column chromatography on Florisil 60–100 mesh (Sigma). Neutral lipids were eluted with 8 column volumes of chloroform, glycolipids with 25 column volumes of acetone and phosphoglycerides+betaine lipid with 10 column volumes of methanol (Christie, 1982).
Neutral lipids were separated by one-dimensional TLC on 20x20 cm silica gel G plates with double development, first with toluene:hexane:formic acid (140:60:1, by vol.) for the whole plate height followed by hexane:diethyl ether:formic acid (60:40:1, by vol.) to half height (Hansen and Rossi, 1990).
Polar lipids were separated by two-dimensional TLC on 10x10 cm silica gel G plates using chloroform:methanol:toluene:28% ammonium hydroxide (65:30:10:6, by vol.) in the first dimension and then chloroform:methanol:toluene:acetone:acetic acid:water (70:30:10:5:4:1, by vol.) in the second direction.
Plates were sprayed with 0.05% 8-analino-4-naphthosulphonic acid in methanol and viewed under UV light to reveal lipids. Identification was made by reference to authentic standards and confirmed using specific colour reagents (Kates, 1986).
Fatty acid methyl esters (FAMEs) were analysed by radio-GLC using a Unicam GCD gas chromatograph connected via an effluent splitter to a LabLogic RAGA (LabLogic, Sheffield, UK) gas flow proportional counter. Glass columns (1.5 mx4 mm internal diameter) were packed with 5% SP-2100 on 100/120 Supelcoport or 10% SP-2330 on 100/120 Supelcoport (Supelco, Bellefonte, PA, USA). The SP-2100 column was run with a temperature programme (initial temp. 210 °C for 10 min, then 4 °C min-1 to 250 °C). The SP-2330 column was run isothermally at 180 °C. Routine identification was by reference to standards and quantification (Rachel Software, LabLogic) was made using an internal standard of heptadecanoate.
Silver nitrate-TLC was carried out using FAMEs on silica gel plates impregnated with 10% AgNO3 and a solvent of hexane:diethyl ether:acetic acid (85:15:1, by vol.) with double development. Bands were revealed with a Rhodamine 6G spray (0.02% in ethanol). When bands were to be further analysed by GLC, then lipid elution and removal of Ag ions was as described previously (Henderson and Tocher, 1992).
Radioactive counting was made using Opti-Fluor (Packard Bioscience bv, Groningen, The Netherlands) scintillant and a Beckman 1209 Rackbeta liquid scintillation counter. Quench correction was by the external standard channels ratio method.
Atomic absorption spectrometry
Metal measurements in Rhytidiadelphus squarrosus from lead-polluted and non-polluted populations were determined by taking 0.2–0.3 g FW of tissue, drying and subjecting to complete digestion by boiling the samples in concentrated HNO3, followed by dilution with 1 M HNO3 and centrifugation. The clear supernatant was used for the measurement of total metal concentrations by comparison with known concentrations of metal salts (1.5, 1.0 and 0.5 mg l-1).
After treatment with elevated lead and copper concentrations, the extracellular and intracellular metal concentrations were determined using a sequential elution technique with Na2-EDTA at pH 4.5 as displacing agent (Branquinho et al., 1997). After sequential elution, the moss specimens were dried overnight at 80 °C and the intracellular metal concentration was measured after digestion with concentrated HNO3 as described above. Independent experiments by X-ray probe analysis showed that surface-associated Cu or Pb were effectively removed by the elution method. All fractions were analysed with a Varian SpectrAA-100 fitted with background corrector (Varian Instruments, Walton-on-Thames, UK), using an air/acetylene flame.
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Results and discussion |
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TopAbstractIntroductionMaterials and methodsResults and discussionConclusionsReferences |
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The existence of heavy metal tolerance in bryophytes has been demonstrated frequently (Tyler, 1990
). So, gemmae taken from populations of Marchantia polymorpha subject to atmospheric Pb pollution were more tolerant to additional Pb in laboratory experiments than gemmae taken from plants grown at sites with low Pb levels (Briggs, 1972). Brown and House, using K leakage as a measure of membrane damage, showed that a population of the liverwort Solenostoma crenulatum from a disused Cu mine was more resistant to experimentally supplied Cu than a population from a disused Pb mine (Brown and House, 1978). Further, cross metal tolerance has sometimes been observed. For example, in Funaria hygrometrica, pretreatment with Zn was found to increase tolerance to Cd (Shaw, 1987). By contrast, a metal-contaminated population of Rhytidiadelphus squarrosus tolerated Cd well, but was not co-tolerant to Cu (Wells and Brown, 1995).
Heavy metals and overall lipid metabolism
In these experiments, the lipid biochemistry of populations of R. squarrosus, which had been growing in lead-contaminated and uncontaminated regions, were examined and these populations were tested against the addition of Pb or Cu. The concentrations of lead and copper which were used for acute exposure had been shown previously to produce significant changes in photosynthesis and those of copper to give rise to membrane leakage in R. squarrosus (Brown and Wells, 1990).
Data from atomic absorption spectrometry showed that lead concentrations were 84.6±5.4 µg g-1 dry wt and 21.0±2.3 µg g-1 dry wt in the moss samples from the lead-contaminated and non-contaminated sites, respectively. The latter results were comparable to those for other bryophyte samples taken from uncontaminated sites (e.g. Atrichum undulatum, Porella platyphylla, Homalothecium sireceum: data not shown). (For a literature review on the metal concentrations in R. squarrosus at different sites see Bruning and Kreeb (1993), as well as Puckett and Burton (1981) for other bryophyte species). The conclusions of the above authors and the present data showed that the collection sites were suitable for further experiments on the lipid biochemistry of R. squarrosus.
Independent experiments were carried out under the standard incubation conditions to check that penetration of the tissues with heavy metals was achieved. During the standard 24 h of acute exposure, some 60% of the copper and over 90% of the lead were removed from the incubation solution. For both metals, the majority (50% of the original Cu and 80% of the original Pb) was found associated with the bryophyte surface. The internal concentrations, determined after sequential elution (see Materials and methods) of copper and lead in the moss were found to have risen from 12 to 15 µg g-1 and 14 to 22 µg g-1, respectively. Clearly, although the final internal concentrations were still low, they were high enough to disturb lipid metabolism (below).
Effect of Cu and Pb on neutral lipid metabolism
In order to monitor rapidly any disturbances to metabolism, radiolabelling from [1-14C]acetate was used. This precursor has been well justified for lipid labelling in plant tissues (Roughan and Slack, 1982). Total incorporation into lipids was found to be similar for the two populations of R. squarrosus (Table 1). Samples were also exposed to copper or lead simultaneously with the radiolabelled precursor. Pb only affected labelling significantly at 10 µM and for moss gathered from the unpolluted site. By contrast, copper generally reduced labelling (by 13–23%) (Table 1). The greater sensitivity of lipid metabolism to Cu is similar to other physiological parameters, such as photosynthesis or membrane damage, in R. squarrosus (Brown and Wells, 1990).
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Measurement of total lipid labelling seldom reveals the effects of environmentally-relevant pollutants which are assessed better by a qualitative examination of lipid biochemistry (Harwood, 1998
b). Thus, in order to evaluate specific effects of heavy metals on lipid metabolism, lipids were separated into neutral (mainly storage or surface lipids) and polar (mainly membrane) classes. When the neutral lipid classes were examined, samples from contaminated areas showed significantly higher relative labelling of steryl esters, wax esters and diacylglycerols than moss from areas with lower lead levels (Fig. 1). These increases in relative labelling were accompanied by a significant decrease in that of triacylglycerol (TAG). Mosses are known to contain significant levels of TAG, wax and steryl esters (Karunen, 1981). Thus, it was not surprising that these components were effectively labelled from [1-14C]acetate.
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The labelling of complex lipid fractions was examined further by exposing R. squarrosus to low concentrations of Cu or Pb. The effect of acute exposure in samples obtained from the lead-contaminated site is shown in Table 2
. Lead exposure decreased TAG labelling and caused an equivalent rise in that of wax esters. Since the total labelling was unchanged (Table 1
), these results suggest an increased flux of fatty acids (from acyl-CoAs) into wax synthesis and esterification compared to the major route of acylglycerol formation. By contrast, copper appeared to lower wax ester labelling while that of diacylglycerol was increased, although the changes were only significant at the 10% level at the higher concentration (10 µM) of copper. Similar changes in labelling patterns following copper or lead exposure were also found with the R. squarrosus obtained from the unpolluted site (data not shown).
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Since an important role for TAG and wax or steryl esters during adaptation of mosses to severe environmental conditions has been suggested (Karunen et al., 1988
; Hakala and Sewon, 1992), the significant changes in labelling of these components are of interest. It is also pertinent to point out that the increased labelling of wax esters and the decrease in that of TAG seen on acute lead exposure (Table 2) is also seen when comparing populations exposed chronically to lead compared to moss obtained from an unpolluted site (Fig. 1). In contrast to lead, acute copper exposure reduced the relative labelling of wax esters while increasing that of diacylglycerol. These changes in labelling patterns suggested that copper disturbed the normal utilization of acyl-CoAs by the Kennedy pathway (Harwood, 1998a) compared to wax ester formation. Taken together, these data suggest strongly that heavy metals can interact with the main lipid assembling pathway and so affect the distribution of flux between major products. This distribution is, undoubtedly, influenced by both the activity of enzymes and the availability of acyl-CoA substrates (Browse et al., 1998).
Effects on polar lipids
In agreement with the changes in neutral lipid labelling (Fig. 1), there were also significant differences in the pattern of labelling of the polar lipid classes in the two populations of R. squarrosus (Table 3). In these samples, the three ‘chloroplast’ glycosylglycerides, various phosphoglycerides and the betaine lipid, diacylglyceryltrimethylhomoserine (DGTS) were positively identified. In the lead-contaminated population, the relative labelling of all three glycosylglycerides, DGTS and phosphatidylethanolamine was raised. These increases were mainly at the expense of phosphatidylcholine radiolabelling (Table 3). In fact, it was interesting that the decrease in phosphatidylcholine labelling was largely compensated for by the increase in that of the other two zwitterionic lipids (phosphatidylethanolamine and DGTS).
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By contrast to the data with neutral lipids (Table 2
), acute exposure to copper did not change polar lipid labelling patterns (data not shown). Lead exposure, on the other hand, did cause some small alterations in the relative labelling of monogalactosyldiacylglycerol, phosphatidylglycerol and DGTS for the population from the lead-contaminated region (Table 3), but had no significant effect on that from the uncontaminated site (data not shown).
Thus, when the polar lipid classes were examined, greater effects were found following chronic lead exposure than for acute in vitro experiments (Table 3). R. squarrosus, which had grown in lead-contaminated sites showed increased labelling of typical chloroplast lipid components (e.g. galactosylglycerides: Harwood, 1980) which could, perhaps, be due to accelerated turnover of components of the thylakoid membranes in response to damage caused by metal stress. In fact, chloroplast metabolism and function has been shown to be sensitive to heavy metal pollution in a variety of lower and higher plants species (Krupa and Baszynski, 1989; Tayler, 1990; Maksymiec et al., 1992; Stefanov et al., 1993, 1995; Wells and Brown, 1995) including R. squarrosus (Brown and Wells, 1990). An interesting point to note from the results shown in Table 3 were the reciprocal changes in the labelling of phosphatidylethanolamine and phosphatidylcholine. This agreed with previous observations in the marine diatom Asterionella glacialis where similar changes were found (Jones et al., 1987). The latter suggested that heavy metal binding to cysteine and methionine and the consequent impairment of S-adenosylmethionine synthesis could account for these alterations.
Diacylglyceryltrimethylhomoserine (DGTS) is known to be an important component of cryptogamic plants, including bryophytes (Sato, 1992; Eichenberger, 1993), but its physiological role is still uncertain. DGTS has been suggested to be a good substitute for phosphatidylcholine in membranes (Sato, 1992; Dembitsky, 1996) and, moreover, the N-methyltransferase involved in the formation of DGTS is distinct from that used in the methylation of phosphatidylethanolamine to phosphatidylcholine (Hofmann and Eichenberger, 1996). Thus, the simplest explanation of these data is that lead causes a decrease in the methylation of phosphatidylethanolamine in R. squarrosus. This would allow more S-adenosylmethionine to be available for DGTS formation (Hofmann and Eichenberger, 1996) and also allow substitution of one quaternary nitrogen-containing lipid for another.
Fatty acid metabolism
Because radiolabel from [1-14C]acetate is incorporated very well into the acyl chains of complex lipids (over 90% of the radioactivity in lipids in R. squarrosus: data not shown), this precursor could be used to examine fatty acid metabolism. This aspect was particularly interesting because heavy metals have often been noted to change fatty acid biosynthesis in plants (Jones et al., 1987; Maksymiec et al., 1992; Jones and Harwood, 1993).
Chronic exposure and endogenous fatty acids
The endogenous fatty acid compositions of R. squarrosus from lead-contaminated and non-contaminated areas are shown in Table 4. An interesting feature was the high proportions of the unsaturated 20:4 and 20:5 fatty acids which are typical for many bryophyte species (Anderson et al., 1972; Gellerman et al., 1972; Hansen and Rossi, 1990). When studying the levels of, for example, arachidonic acid in bryophytes Dembitsky noted that this could vary from 1.6% in the total polar lipid fatty acids of the moss Atrichum angustatum to 32% of total fatty acids of gametophores of the moss Mnium cuspidatum (Dembitsky, 1993; Gellerman et al., 1972). The ability of bryophytes to synthesize such twenty-carbon polyunsaturated fatty acids places them into an intermediate evolutionary position between algae and higher vascular plants and has taxonomical importance (Dembitsky, 1993).
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As can be seen from Table 4,
moss from the lead-contaminated site had a decreased proportion of unsaturated fatty acids containing three or more double bonds. This led to increased relative amounts of oleate, linoleate and, in particular, palmitate.
Fatty acid synthesis
When incubations were carried out with [1-14C]acetate, the main fatty acids labelled were palmitate, oleate, linoleate, and linolenate (results not shown). However, small amounts of radioactivity co-chromatographed with other endogenous fatty acids and were tentatively identified as dihomogammalinolenate, arachidonate and docosatrienoate. Because the major components of each fatty acid class were known, they were separated by silver nitrate-TLC in order to quantify radiolabelling more accurately.
For R. squarrosus collected from the polluted site there was decreased incorporation into all polyunsaturated fatty acid bands (Fig. 2). Because total labelling was unaffected (Table 1), there was relatively more labelling of the saturated and monoenic bands in moss from the lead-contaminated site. Thus, the radiolabelling results broadly agreed with the endogenous fatty acid patterns in that there was a proportional decrease in the polyunsaturated fatty acids (Fig. 2). This agreed with the reported effects of heavy metals in a variety of plant species, where they reduced formation of polyunsaturated fatty acids (Harwood, 1998b) but, interestingly, synthesis of oleate is often unaffected or actually increased (Harwood and Jones, 1989).
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In view of the above findings, it was surprising that acute lead exposure of R. squarrosus actually increased the ability of moss from the contaminated site to label polyunsaturated fatty acids (Table 5
). Even so, such mosses never achieved the levels of polyunsaturated fatty acids labelled by R. squarrosus from the uncontaminated site. Since the sensitivity of metabolic pathways can depend on altered gene expression, the internal metal concentrations in the vicinity of the enzymes concerned, as well as the interactions of membrane PUFAs in generating peroxidation products (none of which is known), it would be pointless to speculate on the mechanism behind these changes. Nevertheless, the sensitivity of polyunsaturated fatty acid formation in R. squarrosus fits well to previous data from plants (Harwood, 1998b).
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Conclusions |
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TopAbstractIntroductionMaterials and methodsResults and discussionConclusionsReferences |
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The experiments reported here reveal that the moss Rhytidiadelphus squarrosus shows heavy metal sensitivity in its lipid metabolism on both an acute and a chronic time-scale. Furthermore, these effects occur at environmentally-relevant metal concentrations. Therefore, the data should provide a basis for further experiments on environmental effects on lipid metabolism in this important, but neglected, group of organisms.
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Acknowledgments |
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The financial support of the Royal Society is gratefully acknowledged. We are indebted to A Roy Perry (National Museum of Wales) and Dr Dennis Brown (University of Bristol) for help in collection and identification of moss samples. Dr D Brown is also thanked for helpful discussions.
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Notes |
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3 To whom correspondence should be addressed. Fax: +44 (0)2920874116. E-mail: Harwood{at}Cardiff.ac.uk
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References |
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