Nonrandom variation of gas exchange within arctic lichens

THOMAS H. NASH 111, THOMAS J. MOSER, A N D STEVEN 0. LINK Departrnetlt of Botany and Microbiology, Arizona State University, Tempe, A Z , U.S.A. 8528i

Received September 10, 1979

NASH, T. H., 111, T. J . MOSEK, and S. 0 . LINK. 1980. Nonrandom variation of gas exchange within arctic lichens. Can. J. Bot. 58: 1181-1 186.

Nonrandom variation in gas exchange of the three arctic lichens Clndotzia rangifcrina (L.) Wigg., Cladonia stellaris (Opiz) Brodo, and Parrnelia separata Th. Fr. is documented through a combination of field and laboratory studies. The highest rates of both photosynthesis and respiration occur in the youngest, actively growing sections of the thalli and both parameters decrease progressively into the older sections. For the two Clador~ia species gross photosyn- thesis is shown to vary directly as a function of chlorophyll a and b concentration as found in a vertical prsfile through the lichen mat.

NASH, T. H., 111, T. J . MOSEK et S. 0. LINK. 1980. Nonrandom variation of gas exchange within arctic lichens. Can. J . Bot. 58: 1181-1 186.

Une variation non aleatoire des echanges gazeux chez trois lichens arctiques, Cladonia rangiferina (L.) Wigg., Cladonia stellaris (Opiz) Brodo et Permelia separata Th. Fr., est documentee par une combinaison de travaux sur le terrain et au laboratoire. Les taux de photosynthkse et de respiration sont les plus eleves dans les parties les plus jeunes du thalle, qui sont en croissance active, et ces taux diminuent progressivement dans les parties plus LgCes. Chez les deux espkces de Cladonia, la photosynthkse brute vane directement en fonction de la concentration de chlorophylle a et b que l'on trouve dans un profil vertical a travers le tapis lichenique.

[Traduit par le journal]

Introduction Lichens are prominent components of many

ecosystems, particularly those of high latitudes. Knowledge of lichen photosynthetic and prod- uctivity patterns is particularly important in areas where lichens are important components of food chains, such as those involving Rangiffr popula- tions (Bunnell et al. 1975). In recent years numer- ous papers dealing with lichen photosynthesis have been published (Lechowicz and Adams 1974; Kal- lio and Karenlampi 1975; Kershaw and Smith 1978; Kappen et al. 1979, etc.). Most of these studies have employed infrared gas analyzer techniques which require the use of fairly large lichen samples (gram quantities). These studies, with the impor- tant exception of a series of careful studies by Karenlampi (1970a, 19706, 1971), thus fail to ac- count for potential sources of systematic gas ex- change variation within lichen thalli. In our studies at Anaktuvuk Pass, Alaska (Moser and Nash 1978), we have found marked gradients in gross photo- synthesis of Cetraria cucullata (Bell.) Ach. with depth in the lichen mat.

The present study illustrates the occurrence of marked gradients in photosynthesis, respiration, and chlorophyll concentration down the fruticose thalli lengths in Cladonia stellaris (Opiz) Brodo (=C. alpestris) and Cladonia rangiferina (L.) Wigg. This confirms and extends earlier observa-

tions made by Karenlampi (1970a) for C. stellaris. In addition, this study reveals the occurrence of a marked photosynthetic gradient in the foliose lichen, Parmelia separata Th. Fr.

Methods These studies were conducted at Anaktuvuk Pass, Alaska

(68"08'3OU N, 151°45' W) in mid-July 1978 and in the laboratory of Professor 0 . L. Lange in Wiirzburg, West Germany in Oc- tober 1978. At Anaktuvuk Pass a portable CO, porometer sys- tem modified from Shimshi (1969) was used to expose the lichens to I4CO2 gas. Before exposure the lichens were saturated by submersion in distilled water for 0.5 h at 12°C for the Cladonia species and at 15.5"C for the Parmelia species. Exposures were made using a 30-s pulse of 14C02 under ambient solar radiation (0.46 cal cm-2 min-I (1 cal = 4.184 J) for the Cladot~ia species and 1.03 cal cm-2 min-I for the Parmelia species). The upper portion (2.0-2.5 cm) of two specimens of C . stellaris and three specimens of C . rangiferina were taken from the dense lichen mats (i = 7 cm deep) which characterize the region (Moser et al. 1979). To allow for a simpler diagrammatical representation the robust thalli of C . stellaris were partially pruned. After exposure of each 2-cm portion, they were quickly divided into segments (Fig. I), which were then oven-dried for 24 h at 1WC. In addi- tion to the Cladonia exposures, 20-mm radial strips of the com- mon saxicolous lichen, Parmelia separata were also exposed sequentially to the 14C02. After exposure each strip was divided into 5-mm segments and these were oven-dried. The dried lichen material was shipped to Arizona State University where the pieces were subsequently combusted in an oxygen rich atmo- sphere by an in-vial combustion technique (Tieszen et al. 1974). The assimilated I4CO2 in each lichen segment was determined by standard liquid scintillation techniques.

Additional air-dried material of the two Cladonia species was

0008-4026/80/101181-06$01 .W/O 01980 National Research Council of Canada/Conseil national de recherches du Canada

Can

. J. B

ot. D

ownl

oade

d fr

om w

ww

.nrc

rese

arch

pres

s.co

m b

y U

nive

rsity

of

Sask

atch

ewan

on

05/2

6/12

For

pers

onal

use

onl

y.

CAN. J . BOT. VOL. 58, 1980

.--__

FIG. I . Photosynthetic activity (disintegrations per minute per milligram) of the upper portion (2.0-2.5cm) of two specimens of C. stellaris (top) and three specimens of C. rangiferitla (bottom).

air mailed to Wiirzburg. These samples were stored dry in a freezer until the experiments were run in October. Prior to experimentation the samples were preincubated at 10°C with a 12-h light-12-h dark photoperiod for 5 days with periodic moist- ening by spraying. All gas exchange measurements were made at 10°C with photosynthesis measured under 15OpE m-2 s-I (Osram lamp-HQL 1000 W). In general, the IRGA procedures followed those of Lange (1969) as they are standardly applied at Wiirzburg. The lichen material was prewetted to eliminate the effects of resaturation respiration (Farrar and Smith 1973) and was brought to approximately 80% of full saturation (e.g., 180% saturated wlw) for each set of measurements. At 10°C gas ex- change (both respiration and net photosynthesis) for these lichens exhibits a flat (nonvarying) response over a wide range of water content values (T. H. Nash, unpublished results). Con- sequently our results should be unbiased by any sources of variation in water content. Gas exchange measurements re- ported herein are equilibration measurements as determined from a series of repeated, steady-state measurements obtained over 30-40 min. As the air line passed through a water vapor trap, drying was negligible during the measurement period.

After the gas exchange measurements were completed, the lichen material was air dried in the laboratory and the chlorophylls were extracted with 80% acetone following the procedures used by Turk et al. (1974). To prevent lichen acid induced degradation of the chlorophylls, the 80% acetone solu- tion was made with 5 mM Tris buffer. Extinctions ofthe extracts were determined on a spectrophotometer (Zeiss DMR 21) at the wavelength required to calculate total chlorophyll concentration (a + b) according to the equations developed by Arnon (1949).

Results Analysis of the Cladonia thalli exposed at

Anaktuvuk Pass revealed a clear gradient in 14C02 incorporation from top to bottom and secondarily from outside to inside (Fig. 1). A Kolmogarov- Smirnov one sample test (Zar 1974) demonstrated that the disintegrations per minute per milligram variation was highly significant (a = 0.01) when the sample segments were arranged in order of position

Can

. J. B

ot. D

ownl

oade

d fr

om w

ww

.nrc

rese

arch

pres

s.co

m b

y U

nive

rsity

of

Sask

atch

ewan

on

05/2

6/12

For

pers

onal

use

onl

y.

from top to bottom. Subsequently regression equa- tions were developed where the disintegrations per minute per milligram values were related (i) to the vertical distance (u ) from the center of the exposed piece to the base of the whole thalli and (ii) to the horizontal distant (h) from the center of the ex- posed piece to the central stalk. The joint regres- sion equation for both species combined was:

[ I ] log,, dpm/mg = 2.32 + 0.900 + 0.15h

NASH ET AL. 1183

where the amount of variation (R2) explained by the regression was 84% and the regression was highly significant (a = 0.01; F = 139.1 with 2 and 53 degrees of freedom). Analysis of the residuals demonstrated that the underlying assumptions of constant error variance, independence, and normal distribution of error terms were reasonable to make. Separate regressions were calculated for the two Cladonia species, but no significant difference was found between the two equations when the beta coefficients were compared using the appro- priate joint F-test cited in Neter and Wasserman (1974). Consequently the joint regression equation adequately characterizes the whole data set.

For Parmelia separata a clear gradient in gross photosynthesis from the thalli margins inward was found (Fig. 2). For the data as plotted a regression was calculated (see line on Fig. 2) as follows:

[2] gross photosynthesis (GP) = 2.33 (10-0.051d)

where d is the distance from the edge to the middle of each section. The regression explained 71% of the variation (r2) and was highly significant (a = 0.01, F = 101.8 with 1 and 42 degrees of freedom and a SEM of 0.036). Further analysis of these data led to the conclusion that length of submersion was also a significant variable. All 11 thalli had been submerged simultaneously but were removed se- quentially beginning at a 0.5-h submersion time. A trend of decreased photosynthetic values with time was evident in the data. This observation presuma- bly corresponds to the observed depression of photosynthesis in the arctic lichen Cladonia alpes- tris at saturated (or supersaturated?) conditions (Kershaw 1975). Accordingly a second multivariate regression was calculated where gross photosyn- thesis was related to distance (4 and time (t):

This new regression model explained 83% of the variation (R2) and each factor in the equation was highly significant (a = 0.01). The "t" value for distance was - 12.97 and for time was -5.34 with 44 degrees of freedom. For this model the SEM was reduced to 0.021. Thus water content needs to be

I (loll !a 25 7.5 12.5 17.5 (3 DISTANCE ( m m )

FIG. 2. Gross photosynthesis (milligrams CO, per gram per hour) of Parmelia separata along distance gradients from the thallus edge inward. Each confidence interval (a = 0.05) is constructed by multiplying the t distribution value for n = 1 1 times the appropriate standard error. The equation for the fitted regression line is given in the text.

carefully monitored when gas exchange is mea- sured at saturated conditions.

In the laboratory a clear gradient in metabolic activity (both respiration and photosynthesis) was found from the tip of the Cladonia lichens to their bases (Table 1). This gradient is most pronounced in photosynthesis where essentially 100% of the activity is restricted to the top half of the lichen mat (0-4 cm). In contrast almost four fifths of the respi- ration of C. stellaris and two thirds of the respira- tion in C. rangiferina is concentrated in the top half of the lichen. Furthermore, partitioning of the top half of the lichens into progressively smaller sec- tions demonstrates that peak activity occurs in the tips (0-0.5 cm) for both photosynthesis and respi- ration.

The underlying differences in absolute rates of gas exchange are reflected in the relative percent- age of total respiration and photosynthesis found in each depth section (Table 2). In this case the abso- lute rates (Table 1) are scaled against the biomass present in each of the sections to provide the per- centage estimates. Most of the total respiratory and photosynthetic activity is concentrated in the upper portion of each species. Approximately 30% of the respiratory activity is found in the upper 1 cm and

Can

. J. B

ot. D

ownl

oade

d fr

om w

ww

.nrc

rese

arch

pres

s.co

m b

y U

nive

rsity

of

Sask

atch

ewan

on

05/2

6/12

For

pers

onal

use

onl

y.

CAN. J. BOT. VOL. 58. 1980

TABLE 1 . Cladonfa partitioning experiment: absolute gas exchange rate (milligrams C 0 2 per gram per hour ovendry weight) as a function of depth of sample within the lichen mat

Estimated Depth in Net gross mat, cm Respiration photosynthesis photosynthesis

Cladonia stellaris (=C. alpestris) - 0 1 1 (78%) + O . 32(100%) +0.43(97%) - 0.03(22%) - 0.02(0%) + 0.01 (3%) -0.19(68%) + 0 .64(82z ) + O . 83(78%) -0.09(32%) +0.14(18%) +0.23(22%) -0.22(61%) +0.93(67%) + 1.15(66%) -0.14(39%) +0.46(33%) + O . 60(34%) - 0.30(55%) + 1.43(57%) + 1 .74(56%) - 0.25(45%) + 1.09(43%) + 1.34(44%)

Cladonia rangijerina -0.14(67%) +0.58(100%) +0.72(100%) - 0.07(33%) - 0.08(0%) - 0.01 (0%) -0.28(71%) +1.23(80%) +1.51(78.3%) - 0.12(29%) + O . 30(20%) +0.42(21.7%) - 0 3 1 6 + 1.94(68%) +2.26(67.2%) - 0.20(39%) + O . 90(32%) + 1.10(32.8%) - 0.40(54%) + 2.63(57%) + 3.03G6.573 -0.33(46%) +2.00(43%) +2.34(43.5%)

TABLE 2. Cladonia partitioning experiment: percentage photo- synthesis and respiration as a function of the depth of sample

within the lichen mat"

Photosynthesis Depth in Respiration Chlorophylls mat, cm % Net, % Gross, % a + b, mg/g

Cladonia stellaris (=C. alpestris) 0-i! 17.7 31.3 28.2 0.1035(42%) f 1 14.7 23.7 21.8 0.0453(15%) 1-2 20.5 27.0 25.9 0.0338(29%) 2-4 25.1 18 .O 21.2 0.0090(14%) 4-8 + 22.0 0.0 2.9 0 . OOOO(O%)

Cladonia rangijeritra w 15.4 31.0 29.7 0.2684(28.1%) +- 1 12.9 23.6 22.9 0.2155(20.2%) 1-2 18.8 25.4 25.7 0.1071(22,3%) 2 4 19.7 20.0 21.7 0.0256(13.7%) 4-8 + 33.0 0 .0 0 .0 0.0207(15.7%)

T h e estimates are scaled to remove the measured differences due to different sample weights of the different sized segments. Chlorophyll estimates were determined by the techniaue of T i i r ~ ct a / . (1974). The percentage chlorophylls estimates are not based on the milligram pei gram figures given to the left. but rather are based on a scaline (not shown) nced t o reflect the absolute quantity of chlorophylls per dip~h';angi~-;~fleciin~ different weights. Thus these percentage values are directly comparable to the other percentage values in the table.

approximately 50% in the upper 2 cm. In the case of photosynthesis over 50% of the activity is found in the upper 1 cm and over 75% of the activity in the upper 2 cm.

The underlying chemical-physical basis for the observed differences in photosynthetic rates is the corresponding differences in chlorophyll a + b concentrations (Table 2). In terms of milligrams

r2 = .98"

y = -0.07 t 11.43~

.i

0.2 0.3 0 CHLOROPHYLLS a+b (mg/g)

= 2-01 C. stellaris k

CHLOROPHYLLS a+b (mg&

FIG. 3. Gross photosynthesis of C. srellaris and C. ran- giferirla, each as a function of chlorophyll a + b concentrations.

chlorophylls per gram lichen there is a clear gra- dient with the highest concentrations in the top 0.5 cm and the lowest concentrations occurring in the bottom half of the lichens. Furthermore the photosynthetic activity of either species can be

Can

. J. B

ot. D

ownl

oade

d fr

om w

ww

.nrc

rese

arch

pres

s.co

m b

y U

nive

rsity

of

Sask

atch

ewan

on

05/2

6/12

For

pers

onal

use

onl

y.

NASH ET AL. 1185

clearly predicted on the basis of chlorophyll con- food source. Photosynthetic measurements over centrations (Fig. 3). In the case of Cladonia stel- the course of a growing season showed that the laris the regression line is significant at the ct = bases of the Cetraria were consistently inactive and 0.05% level and in the case of C. rnngferinn the that the midportion of the thallus almost always regression line is significant at the ct = 0.01% level. exhibited lower photosynthetic activity than the

Discussion Clearly none of the lichens is a homogeneous

entity with respect to gas exchange. In Clarlonia respiration rate estimates vary by a factor of 10 in the case of C. stellnris and by a factor of over 5 in the case of C. rangferinn. For photosynthesis the variability is even greater in Clacioniae and varies by a factor of seven in P. sepnrata. From these gradients it is evident that lichens are most active physiologically in a relatively small portion of their biomasses (e.g., tips in the Clndonin and edges in P. separata). There is a steady progression of senescence in the older portion (lower portions) of the lichens, reflected by a progressive decrease in both photosynthesis and respiration.

The results amplify the fact that is difficult to compare absolute gas exchange rates established by different authors either for different species or for the same species. For example, the highest ab- solute rates of net photosynthesis measured in the laboratory of 1.43 mg CO, g-' h-I for C. stellaris and 2.63 mg CO, g-I h-I for C. rnngferinn exceed the highest literature reports (Kallio and Karen- lampi 1975) by factors of 1.5 and 5.3, respectively. Thus analogous dry-weight samples are not sufficient for standardization of results. Even if temperature and water content are standardized, very different results may be found by lack of at- tention to chlorophyll distribution patterns. Where it is desirable to make comparisons of the absolute magnitude of gas exchange rates, it may thus be necessary to provide chlorophyll data as well.

Furthermore, comparisons among different lichen species will require examination of photo- synthetic rates as a function of chlorophyll con- centration for each species. The b, coefficients for the two equations given in Fig. 3 are significantly different. We do not fully understand why the dif- ferences are present, but it may be related to the different branching patterns of the two Clndorzia species or due to variation in the depth of the algae in relation to the surface. These possibilities and other potential sources of variation should be in- vestigated in future studies.

The observations with the two Clndonia species are consistent with earlier field observations on photosynthetic patterns of Cetrarin cucullata at Anaktuvuk Pass (Moser and Nash 1978). The Cet- raria species occurs concomitantly with Cladonia in mats which are utilized by caribou as a winter

tips. The results are also generally consistent with

those of Karenlampi (1970a) who studied Clarlorlin alpestris (= C. stellnris) populations from northern Finland. He showed that there was a consistent trend of decreasing chlorophyll concentrations from tip to base in different sized classes of C. alpestris (presumably reflecting different age- classes). Apparently Karenlampi did not make di- rect gas exchange measurements on different seg- ments of C. alpestris (as was done in the present study), but rather made measurements on whole thalli from the different size classes. It can be in- ferred from his graphs of photosynthesis versus chlorophyll content (Fig. 6 and 8 in his 1970a paper) that he would have obtained similar gas exchange values as those found herein. Our observations thus confirm Karenlampi's earlier observations on C. alpestris made at a very different locality and extend the results to a second species.

Although it is clear that photosynthesis varies directly as a function of chlorophyll concentration, the manner in which chlorophylls vary on a per alga cell basis is not established by our study. The ob- served gradients may result from (i) a decrease in the number of alga cells per gram lichen from new growth to old growth or (ii) a decrease in the chlorophyll concentration per alga cell. It would not be surprising if future studies demonstrate that both factors are important in determining such gra- dients.

Acknowledgments The assistance of a stipendium granted to the

senior author by the Alexander von Humboldt Stiftung is gratefully acknowledged as is the use of laboratory facilities of Professor Dr. 0. L. Lange. Financial assistance from the U.S. Department of Energy Contract No. W-7405-Eng-36 with the Uni- versity of California's Los Alamos Scientific Labo- ratory and the U.S. National Science Foundation grant DEB 77-25432 are also gratefully acknowl- edged. For the field studies use of the U.S. Naval Arctic Research Laboratory facilities at Anak- tuvuk Pass was indispensable. We also thank Pro- fessors L. Karenlampi, L. Kappen, and 0. L. Lange for critically reading portions of the manu- script.

ARNON, D. I . 1949. Copper enzymes in isolated chloroplasts. Polyphenoloxydase in Beta vulgaris. Plant Physiol. 24: 1-15.

BUNNELL, F. L., S. F. MACLEAN, JR. , and J. BROWN. 1975.

Can

. J. B

ot. D

ownl

oade

d fr

om w

ww

.nrc

rese

arch

pres

s.co

m b

y U

nive

rsity

of

Sask

atch

ewan

on

05/2

6/12

For

pers

onal

use

onl

y.

1186 CAN. J. BOT. VOL. 58, 1980

Barrow, Alaska, U.S.A. 111 Structure and function of tundra ecosystems. Ecol. Bull. (Stockholm), 20: 73-124.

FARRAR, T., and D. C. SMITH. 1973. Resaturation respiration in lichens. New Phytol. 72: 155-162.

KALLIO, P., and L. KARENLAMPI. 1975. Photosynthesis in mosses and lichens. In Photosynthesis and productivity in different environments. Edited by J. P. Cooper. Cambridge University Press, Cambridge.

KAPPEN, L., 0 . L. LANGE, E.-D. SCHULZE, M. EVENARI, and U. BUSCHBOM. 1979. Ecophysiological investigations on lichens of the Negev Desert. VI. Annual course of the photo- synthetic production of Rarnalina rnaciformis (Del.) Bory. Flora (Jena), 168: 85- 108.

KARENLAMPI, L. 1970a. Distribution ofchlorophyll in thelichen Cladonia alpestris. Rep. Kevo Subarct. Res. Stn. 7: 1-8.

1970b. Morphological analysis of the growth and prod- uctivity of the lichen Cladonia alpestris. Rep. Kevo Subarct. Res. Stn. 7: 9-15.

1971. Studies on the relative growth rate of some fruti- cose lichens. Rep. Kevo Subarct. Res. Stn. 7: 33-39.

KERSHAW, K. A. 1975. Studies on lichen-dominated systems. XIV. The comparative ecology of Alectoria rlitidula and Cladina alpestris. Can. J. Bot. 53: 2608-2613.

KERSHAW, K. A., and M. M. SMITH. 1978. Studies on lichen- dominated systems. XXI. The control of seasonal rates of net photosynthesis by moisture, light, and temperature in Stereocaulonpc~schnle. Can. J. Bot. 56: 2825-2830.

LANGE, 0. L. 1969. Experimentell-okologische Unter-

suchungen an Flechten der Negev-Wiiste. I. CO, Gaswechsel von Rarnalina rnaciformis (Del.) Bory unter kontrollierten Bedingungen im Laboratorium. Flora (Jena) Abt. B, 158: 324-359.

LECHOWICZ, M. J., and M. S. ADAMS. 1974. Ecology of Clador~ia lichens. 11. Comparative physiological ecology of C . rnitis, C . rangiferina, and C . uncialis. Can. J. Bot. 52: 41 1-422.

MOSER, T. J., and T. H. NASH, 111. 1978. Photosynthetic pat- terns of Cetraricl cucullata (Bell.) Ach. at Anaktuvuk Pass, Alaska. Oecologia, 34: 37-43.

MOSER, T. J., T. H. NASH, 111, and J. W. THOMSON. 1979. Lichens of Anaktuvuk Pass, Alaska, withemphasis on impact of caribou grazing. Bryologist, 82: 393-408.

NETER, J., and W. WASSERMAN. 1974. Applied linear statistical models. Richard D. Irwin Inc., Homewood, IL.

SHIMSHI, D. 1969. A rapid field method for measuring photo- synthesis with labelled carbon dioxide. J. Exp. Bot. 20: 381-401.

TIESZEN, L. L., D. A. JOHN SEN,^^^ M. M. CALDWELL. 1974. A portable system for the measurement of photosynthesis using 14-carbon dioxide. Photosynthetica, 8: 151-160.

TURK, R., V. WIRTH, and 0. L. LANGE. 1974. C0,- Gaswechsel-Untersuchungen zur SO2-Resistenz von Flechten. Oecologia, 15: 33-64.

ZAR, J. H. 1974. Biostatistical analysis. Prentice-Hall Inc., En- glewood Cliffs, NJ.

Can

. J. B

ot. D

ownl

oade

d fr

om w

ww

.nrc

rese

arch

pres

s.co

m b

y U

nive

rsity

of

Sask

atch

ewan

on

05/2

6/12

For

pers

onal

use

onl

y.