Lichen secondary metabolites affect growth of <Emphasis Type="Italic">Physcomitrella patens</Emphasis> by allelopathy

Lichen secondary metabolites can function as allelochemicals and affect the development and growth of neighboring bryophytes, fungi, vascular plants, microorganisms, and even other lichens. Lichen overgrowth on bryophytes is frequently observed in nature even though mosses grow faster than lichens, but there is still little information on the interactions between lichens and bryophytes.In the present study, we used extracts from six lichen thalli containing secondary metabolites like usnic acid, protocetraric acid, atranorin, lecanoric acid, nortistic acid, and thamnolic acid. To observe the influence of these metabolites on bryophytes, the moss Physcomitrella patens was cultivated for 5 weeks under laboratory conditions and treated with lichen extracts. Toxicity of natural mixtures of secondary metabolites was tested at three selected doses (0.001, 0.01, and 0.1 %). When the mixture contained substantial amounts of usnic acid, we observed growth inhibition of protonemata and reduced development of gametophores. Significant differences in cell lengths and widths were also noticed. Furthermore, usnic acid had a strong effect on cell division in protonemata suggesting a strong impact on the early stages of bryophyte development by allelochemicals contained in the lichen secondary metabolites.Biological activities of lichen secondary metabolites were confirmed in several studies such as antiviral, antibacterial, antitumor, antiherbivore, antioxidant, antipyretic, and analgetic action or photoprotection. This work aimed to expand the knowledge on allelopathic effects on bryophyte growth.     

The role of nurse functional types in seedling recruitment dynamics of alternative states in rangelands

In arid ecosystems, recruitment dynamics are limited by harsh environmental conditions and greatly depend on the net outcome of the balance between facilitation and competition. This outcome can change as a consequence of degradation caused by livestock overgrazing. Also, distinct plant species may show a differential response to a common neighbour under the same environmental conditions. Therefore, ecosystem degradation could affect the net balance of plant-plant interactions, which can also depend on the functional traits of potential nurse species. The aim of this study is to assess the influence of alternative degradation states on (i) the density of seedlings of perennial species emerging in four microsite types, and on (ii) the relative interaction intensity (RII) between seedlings and potential nurses belonging to three functional types (deep- and shallow-rooted shrubs, and tussock grasses). During three years, we recorded seedling density of perennial species in four alternative degradation states in grass-shrubby steppes from northwestern Patagonia. The density of emerged seedlings of perennial species decreased sharply as degradation increased, showing non-linear responses in most microsites. Seedling density underneath deep-rooted shrubs was higher than underneath shallow-rooted shrubs and tussock grasses. Also, deep-rooted shrubs were the only functional type that recorded seedling emergence in highly degraded states. Deep-rooted shrubs had facilitative effects on the seedlings emerging and surviving underneath them, independently of ecosystem degradation. In contrast, RII between shallow-rooted shrubs and recently emerged seedlings, switched from positive effects in the less degraded states, to negative effects in the most degraded state. Tussock grasses recorded the weakest intensity of facilitative interactions with recently emerged seedlings, switching to competitive interactions as degradation increased. Our results suggest that species with key functional traits should be considered in management and restoration plans for rangelands with different degradation levels, since they have a strong influence in the net outcome of plant-plant interactions and in the recruitment dynamics of arid ecosystems.     

Vascular Plants Facilitated Bryophytes in a Grassland Experiment

In grassland communities vascular plants and bryophytes form two distinct layers. In order to understand the factors responsible for plant community structure, more information about interactions between these plant groups is needed. Often negative correlations between vascular plant and bryophyte covers have been reported, suggesting competition. Here we tested experimentally whether different grassland vascular plant species (Trifolium pratense, Festuca pratensis, Prunella vulgaris) had different influences on the cover of two bryophyte species (Rhytidiadelphus squarrosus, Brachythecium rutabulum). In a two-year garden pot experiment one bryophyte species and one vascular plant species were planted per pot. Bryophytes were planted at a constant density, vascular plants in four densities. The cover of both bryophyte species increased with increasing vascular plant cover, showing the facilitative effect of vascular plants through creating better microclimate, e.g., optimising temperature. Bryophyte responses to vascular plant species were species-specific. Festuca had significantly positive effects on both bryophyte species in the second year, and Trifolium on Brachythecium in both years, whereas Prunella had no significant effect on bryophytes. The facilitative effect of vascular plants was stronger at the second experimental year. In summary, the biotic effects between bryophytes and grassland vascular plants are species-specific and positive interactions are prevailing at low vascular plant densities.     

Bryophyte and lichen diversity: A comparative study

Abstract We describe the regional species richness, variation in species richness and species turnover of bryophytes and lichens from 36 sites in lowland forests of southeastern Australia. The analyses subdivided the two major taxa into their constituent sub-groups: mosses, liverworts, and crustose, fruticose and foliose lichens. They also explored correlations between selected environmental variables and patterns of diversity. On a regional scale, there were 77 species of bryophytes and 69 species of lichens, giving a total of approximately one-third of the total number of vascular plant species in the region. Mean species richness was higher for lichens than bryophytes. Also, the two taxa were negatively correlated because lichens favoured dry sites and bryophytes favoured moist ones. Species turnover was greater for bryophytes than lichens, largely due to the distribution of liverwort species. Foliose lichens showed higher levels of turnover than crustose lichens. Multiple regression and canonical correspondence analysis showed that both taxa and all sub-groups responded to the same three variables: vascular plant cover, time since last fire and topographic position. Other variables, including time since logging and intensity of logging, explained little variation in bryophyte or lichen diversity. The data suggest that the strategies for the conservation of bryophyte and lichen biodiversity will be different, to reflect the different patterns of species richness and species turnover.     

Laccase and tyrosinase activities in lichens

Phenoloxidase activity was found in lichenized ascomycetes belonging to different taxonomic groups. Most of the epigeic and epilithic lichens of the order Peltigerales were found to possess both laccase and tyrosinase activities; the lichens of the order Lecanorales possessed only laccase activity, which was an order of magnitude lower than that of Peltigerales. Water-soluble phenoloxidases were present only in peltigerous lichens: activity that could be washed out from intact thalli comprised 10% of that released from disrupted thalli. The activity of the peltigerous lichens and the release of soluble phenoloxidases into the medium increased when the thalli were rehydrated quickly. In some of the lichens tested, the phenoloxidase activity was stimulated by desiccation-rehydration cycles. The oxidases discovered may play an important role in the phenolic metabolism of lichens and be involved in the biochemical reaction of humus synthesis during primary soil formation, which may be a previously unknown geochemical function of these symbiotic microorganisms.     

Welcome to the neighbourhood: interspecific genotype by genotype interactions in Solidago influence above- and belowground biomass and associated communities

Intra- and interspecific plant-plant interactions are fundamental to patterns of community assembly and to the mixture effects observed in biodiversity studies. Although much research has been conducted at the species level, very little is understood about how genetic variation within and among interacting species may drive these processes. Using clones of both Solidago altissima and Solidago gigantea, we found that genotypic variation in a plant's neighbours affected both above- and belowground plant traits, and that genotype by genotype interactions between neighbouring plants impacted associated pollinator communities. The traits for which focal plant genotypic variation explained the most variation varied by plant species, whereas neighbour genotypic variation explained the most variation in coarse root biomass. Our results provide new insight into genotypic and species diversity effects in plant-neighbour interactions, the extended consequences of diversity effects, and the potential for evolution in response to competitive or to facilitative plant-neighbour interactions.     

Stress‐Driven Changes in the Strength of Facilitation on Tree Seedling Establishment in West African Woodlands

The strength of competitive and facilitative interactions in plant communities is expected to change along resource gradients. Contrasting theoretical models predict that with increasing abiotic stress, facilitative effects are higher, lower, or similar than those found under more productive conditions. While these predictions have been tested in stressful environments such as arid and alpine ecosystems, they have hardly been tested for more productive African woodlands. We experimentally assessed the strength of tree seedling facilitation by nurse trees in mesic and dry woodlands in Benin, West Africa. We planted seedlings of the drought‐sensitive Afzelia africana and the drought‐tolerant Khaya senegalensis under three microsite conditions (closed woodland, woodland gap, and open fields). Seedling survival was greater within woodlands compared with open fields in both the mesic and dry woodlands. The relative benefits in seedling survival were larger at the dry site, especially for the drought‐sensitive species. Nevertheless, plant interactions became neutral or negative during the dry season in the drier woodland, indicating that the net positive effects may be lost under very stressful abiotic conditions. We conclude that facilitation also occurs in the relatively more productive conditions of African woodlands. Our results underscore the role of environmental variation in space and time, and the stress tolerance of species, in explaining competitive and facilitative interactions within plant communities. Abstract in French is available at http://www.blackwell‐synergy.com/loi/btp .     

Laccase and tyrosinase activities in lichens

Phenoloxidase activity was found in lichenized ascomycetes belonging to different taxonomic groups. Most of the epigeic and epilithic lichens of the order Peltigerales were found to possess both laccase and tyrosinase activities; the lichens of the order Lecanorales possessed only laccase activity, which was an order of magnitude lower than that of Peltigerales. Water-soluble phenoloxidases were present only in peltigerous lichens: activity that could be washed out from intact thalli comprised 10% of that released from disrupted thalli. The activity of the peltigerous lichens and the release of soluble phenoloxidases into the medium increased when the thalli were rehydrated quickly. In some of the lichens tested, the phenoloxidase activity was stimulated by desiccation-rehydration cycles. The oxidases discovered may play an important role in the phenolic metabolism of lichens and be involved in the biochemical reaction of humus synthesis during primary soil formation, which may be a previously unknown geochemical function of these symbiotic microorganisms.     

Vertical concentration gradients of heavy metals in Cladonia lichens across different parts of thalli

Certain lichens of the genus Cladonia are effective heavy-metal-tolerant colonisers of strongly contaminated and disturbed sites. Among them, Cladonia cariosa, Cladonia pyxidata and Cladonia rei are the major components of specific cryptogamic assemblages proven to be bioindicators of soil pollution. This study examines the bioaccumulation capacity and heavy metal accumulation pattern of these species in the context of element concentration levels in various parts of their thalli at various vertical distances from the ground. The content of Zn, Pb, Cd, As and Cu in primary squamules, lower and upper parts of secondary thalli (podetia), and fruiting bodies (apothecia), as well as the corresponding substrate, was analysed using the AAS method. The substrate turned out to be the main source of heavy metals in the examined Cladonia lichens. Element accumulation in particular parts of thalli greatly depends on metal enrichment in the immediate vicinity while Cu/Zn ratios for both substrate and lichen samples were very low and comparable within the species. Concentration levels in thalli usually decrease significantly with distance from the substrate. The exception is copper, which content was frequently higher in apothecia than in the upper parts of podetia. Low bioaccumulation factors calculated for the examined Cladonia specimens classified these lichens as weak accumulators of heavy metals. Even given an extremely high level of contaminants in the substrate, the upper parts of thalli are not greatly affected. Consequently, fruticose and erect growth form, in combination with low accumulation capacity and a remarkable decrease in metal content along a vertical gradient, may be an important attribute of Cladonia lichens in the colonisation of a highly contaminated substrate. The content of elements differs significantly between particular parts of Cladonia thalli; this should be taken into account whenever burdens of heavy metals are used as indicators in biomonitoring studies.     

Evaluation of Swedish woodland key habitats using red-listed bryophytes and lichens

All Swedish forest land is at present being surveyed with the aim of mapping woodland key habitats which have an estimated number of ca. 70 000. An untested requirement of these habitats is that they should contain red-listed species. In order to investigate if they fulfill their aim, an inventory of nationally red-listed bryophytes and lichens was performed in ca. 120 randomly selected woodland key habitats distributed throughout the country. The species were recorded in line transects, covering the whole surface of the habitats. The mean number of red-listed species per habitat was 0.7 for bryophytes and 1.4 for lichens with 11 species at the richest site and 71% of the sites having at least one species. Nemoral forests and coniferous forests were the most species-rich habitat types. The habitats of northern Sweden were more species rich than the southern ones. There were weak correlations between presence of bryophytes and lichens. The study shows that a majority of the woodland key habitats contain red-listed bryophyte and lichen species. Further studies are needed in order to record more organism groups and to compare the woodland key habitats with the surrounding matrix forests.     

Insight into the pattern of heavy-metal accumulation in lichen thalli

BackgroundHeavy metals that pass through the plasmalemma are expected to influence on lichen metabolic processes; however, lichens may tolerate high concentrations of metals by sequestrating them extracellularly. Heavy metal accumulation level fundamentally determine the success of lichens in the colonisation of polluted sites; however, the proportions between extra- and intracellular metal concentrations in lichen thalli are still poorly recognized. In this study metal accumulation patterns of selected toxic trace elements, i.e. Pb, Cd, and micronutrients, i.e. Zn, Cu and Ni, in Cladonia cariosa thalli were recognised in relation to extra- and intracellular fractions.MethodsThe intracellular and total concentrations of Zn, Pb, Cd, Cu and Ni in lichen thalli collected from eleven variously polluted sites were determined by means of atomic absorption spectrometry. Additionally, organic carbon and total nitrogen contents as well as pH of soil substrate were measured.ResultsThe accumulation patterns differed between studied metal elements; the major part of Zn, Pb and Cd loads was accumulated extracellularly, whereas Cu and Ni accumulation was mostly intracellular. Like toxic trace elements, Zn was accumulated mainly extracellularly at high polluted sites. The non-linear models most reliably reflect relationships between intracellular and extracellular metal contents in C. cariosa thalli. The intracellular contents of Zn, Pb, Cd and Cu increased slower at higher than at lower extracellular concentrations. Moreover, at higher total concentrations of elements in the thalli, their extracellular proportions were markedly increased.ConclusionThe results suggest that in the face of extreme Zn-enrichment, lichens demonstrate the ability to accumulate the excess of Zn outside the cells. Therefore, it can be concluded that metal accumulation depend not only on the element but also on its abundance in the environment and direct availability for lichens. The studied species showed a defence against excessive intracellular accumulation when a given element is in excess. Such capability may facilitate the colonization of extremely polluted sites by certain pioneer lichens.     

Impact of climate change on alpine vegetation of mountain summits in Norway

Climate change is affecting the composition and functioning of ecosystems across the globe. Mountain ecosystems are particularly sensitive to climate warming since their biota is generally limited by low temperatures. Cryptogams such as lichens and bryophytes are important for the biodiversity and functioning of these ecosystems, but have not often been incorporated in vegetation resurvey studies. Hence, we lack a good understanding of how vascular plants, lichens and bryophytes respond interactively to climate warming in alpine communities. Here we quantified long-term changes in species richness, cover, composition and thermophilization (i.e. the increasing dominance of warm-adapted species) of vascular plants, lichens and bryophytes on four summits at Dovrefjell, Norway. These summits are situated along an elevational gradient from the low alpine to high alpine zone and were surveyed for all species in 2001, 2008 and 2015. During the 15-year period, a decline in lichen richness and increase in bryophyte richness was detected, whereas no change in vascular plant richness was found. Dwarf-shrub abundance progressively increased at the expense of lichens, and thermophilization was most pronounced for vascular plants, but occurred only on the lowest summits and northern aspects. Lichens showed less thermophilization and, for the bryophytes, no significant thermophilization was found. Although recent climate change may have primarily caused the observed changes in vegetation, combined effects with non-climatic factors (e.g. grazing and trampling) are likely important as well. At a larger scale, alpine vegetation shifts could have a profound impact on biosphere functioning with feedbacks to the global climate.     

Growth and nitrogen relations in the mat-forming lichens Stereocaulon paschale and Cladonia stellaris

BACKGROUND AND AIMS: Mat-forming lichens in the genera Stereocaulon and Cladonia have ecosystem-level effects in northern boreal forests. Yet the factors affecting the productivity of mat-forming lichens are not known. The aim of the presented work was to investigate whether mat-forming lichens adapted to low N availability employ N-conserving mechanisms similar to those of vascular plants in nutrient-poor ecosystems. Specifically, the following questions were asked: (a) Do lichens translocate N from basal areas to apical growth areas? (b) Are the quantities of N translocated of ecological significance. (c) Is lichen growth dependent on tissue N concentration [N]. METHODS: Two different, but complementary, field experiments were conducted using the mat-forming N2-fixing Stereocaulon paschale and non-fixing Cladonia stellaris as model species. First, N translocation was investigated by feeding lichens with Na(15)NO3 either directly to the apex (theoretical sink) or to the basal part (theoretical source) and observing the redistribution of (15)N after a growth period. Secondly, growth and variation in [N] in thalli of different lengths was measured after a growth period. KEY RESULTS: (15)N fed to lower parts of lichen was translocated towards the growing top, but not vice versa, indicating physiologically dependent translocation that follows a sink-source relationship. In the growth experiment where thalli were cut to different lengths, the significant decrease in [N] in apices of short vs. longer thalli after a growth period is consistent with internal relocation as an ecologically important source of N. CONCLUSIONS: The presented results demonstrate that internal recycling of N occurs in both species investigated and may be ecologically important in these mat-forming lichens under field conditions. The higher nitrogen use efficiency and relative growth rate in C. stellaris in comparison with S. paschale probably enable C. stellaris to dominate the ground cover vegetation in dry boreal coniferous forests under undisturbed conditions.     

Productivity–diversity patterns in arctic tundra vegetation

Productivity has long been argued to be a major driver of species richness patterns. In the present study we test alternative productivity–diversity hypotheses using vegetation data from the vast Eurasian tundra. The productivity–species pool hypothesis predicts positive relationships at both fine and coarse grain sizes, whereas the productivity–interaction hypothesis predicts unimodal patterns at fine grain size, and monotonic positive patterns at coarse grain size. We furthermore expect to find flatter positive (productivity–species pool hypothesis) or more strongly negative (productivity–interaction hypothesis) relationships for lichens and bryophytes than for vascular plants, because as a group, lichens and bryophytes are better adapted to extreme arctic conditions and more vulnerable to competition for light than the taller‐growing vascular plants. The normalised difference vegetation index (NDVI) was used as a proxy of productivity. The generally unimodal productivity–diversity patterns were most consistent with the productivity–interaction hypothesis. There was a general trend of decreasing species richness from moderately to maximally productive tundra, in agreement with an increasing importance of competitive interactions. High richness of vascular plants and lichens occurred in moderately low productive tundra areas, whereas that of bryophytes occurred in the least productive tundra habitats covered by this study. The fine and coarse grain richness trends were surprisingly uniform and no variation in beta diversity along the productivity gradient was seen for vascular plants or bryophytes. However, lichen beta diversity varied along the productivity gradient, probably reflecting their sensitivity to habitat conditions and biotic interactions. Overall, the results show evidence that productivity–diversity gradients exist in tundra and that these appear to be largely driven by competitive interactions. Our results also imply that climate warming‐driven increases in productivity will strongly affect arctic plant diversity patterns.     

Nitrate sequestration by corticolous macrolichens during winter precipitation events

Nitrogen is an essential nutrient in the biogeochemistry of forested ecosystems. The influence of canopy lichens on the winter biogeochemistry of nitrate in broadleaved deciduous forests is examined and it is hypothesized that nitrate sequestration will not differ between winter precipitation events. Rejection of this hypothesis would mean that meteorological conditions of winter precipitation events have a detectable influence on nitrate sequestration by canopy lichens and nitrate input to the forest floor. Canopy lichens of the genus Parmelia were found to influence winter nitrate stemflow inputs to forest soils differentially. Epiphytic lichens on an individual Carya glabra Mill. (pignut hickory) canopy tree, centrally located within the stand of an open deciduous forest, actively sequestered nitrate leached from the tree's woody frame, lowering aqueous stemflow inputs at the tree base. The quantities of nitrate sequestered by corticolous lichens during the 2 February 1999 mixed-precipitation event were significantly greater than those during all other precipitation events examined. Greater rates of nitrate uptake during the 2 February 1999 event may be attributed to (1) its intermediate rain intensity, which would have soaked the lichen thalli in a nutrient-rich bath, and (2) an air temperature range between -2 degrees C and 8 degrees C that would have increased viscosity and surface tension of stemflow drainage, thereby decreasing stemflow velocity and increasing the contact time of stemflow water on the lichen thalli. Other precipitation events were either too cold to promote metabolic activity by canopy lichens or too warm and intense for an optimal contact time of stemflow with lichen thalli, resulting in lower quantities of nitrate sequestered. Meteorological conditions of winter precipitation events have been documented to influence sequestration of nitrate by corticolous lichens and decrease aqueous stemflow inputs to the forest floor of broadleaved deciduous forests.     

Potential alteration of precipitation chemistry by epiphytic lichens

Summary Epiphytic lichen growth is abundant on the boles and branches of balsam fir trees at high elevations in New Hampshire. These lichens absorb elements needed for growth from solutions flowing over their surfaces and from direct impaction of water droplets. This study describes how epiphytic lichens and fir needles altered the chemistry of simulated rain water solutions under laboratory conditions. Experiments showed: 1) lichens absorbed ammonium and nitrate from solution; the rate of uptake increased with increasing temperature of the solution, 2) lichens lost calcium, magnesium, and hydrogen to the solution, 3) lichen thalli also initially lost potassium, but in time, net movement was reversed back into the thallus, 4) cation movement increased with increasing temperature, and 5) fir needles responded in a manner similar to that of the lichens, but the amount of change was much less. From these results it seems that epiphytic lichens have potential ecological importance in altering the chemistry of throughfall and stemflow.     

Processes underlying the effect of mycorrhizal symbiosis on plant-plant interactions

Mycorrhizal symbiosis has important implications for the diversity and productivity of plant communities. However, our mechanistic understanding of its influence on the outcome of plant-plant interactions is still expanding. In this review we propose a framework that might be useful to efficiently approach the effects of mycorrhizal fungi (MF) on plant-plant interactions. We propose several scenarios that can theoretically result in different outcomes of plant-plant interactions based on the combination of two processes: the diversity of resources provided by MF taxa to their host (resource dissimilarity) and contrasting ways of distributing those resources (resource distribution). Then, we illustrate our arguments with different ecological contexts where certain combinations of these two processes are prone to occur. The proposed framework suggests testable hypotheses that can contribute to elucidate relevant processes underlying the effects of mycorrhizal symbiosis on plant-plant interactions.     

Variation in the effects of vegetation and litter on recruitment across productivity gradients

1 We tested predictions about how the effect of vegetation and litter on seedling establishment varies among sites and herbaceous community types (sand barrens, prairies, fens). For both vegetation and litter, we also separated direct interactions from indirect interactions and interaction modifications along the gradient.
2 Although the intensity of the effects varied across sites, the direct effects of vegetation or litter alone were consistently facilitative along the productivity gradient. Predominance of facilitative effects may be due to the focus on the seedling establishment phase.
3 However, inclusion of indirect interactions and interaction modifications caused the net effects of both vegetation and litter to become largely negative. While one layer of biomass may be advantageous to ameliorate some moisture stress, the addition of another layer may be disadvantageous if this layer limits light proportionally more than it relieves moisture stress.
4 One exception to this pattern occurred at high productivity when the net effect of vegetation, even in the presence of litter, remained facilitative. The net effect of vegetation was competitive at low productivity and grew increasingly facilitative with productivity. Thus, indirect effects of litter may alter interaction patterns across this gradient.     

Species richness of vascular plants, bryophytes and lichens in dry grasslands: The effects of environment, landscape structure and competition

We studied the relative importance of local habitat conditions and landscape structure for species richness of vascular plants, bryophytes and lichens in dry grasslands on the Baltic island of Öland (Sweden). In addition, we tested whether relationships between species richness and vegetation cover indicate that competition within and between the studied taxonomic groups is important. We recorded species numbers of vascular plants, bryophytes and lichens in 4 m² plots (n=452), distributed over dry grassland patches differing in size and degree of isolation. Structural and environmental data were collected for each plot. We tested effects of local environmental conditions, landscape structure and vegetation cover on species richness using generalized linear mixed models. Different environmental variables explained species richness of vascular plants, bryophytes and lichens. Environmental effects, particularly soil pH, were more important than landscape structure. Interaction effects of soil pH with other environmental variables were significant in vascular plants. Plot heterogeneity enhanced species richness. Size and degree of isolation of dry grassland patches significantly affected bryophyte and lichen species richness, but not that of vascular plants. We observed negative relationships between bryophyte and lichen species richness and the cover of vascular plants. To conclude, effects of single environmental variables on species richness depend both on the taxonomic group and on the combination of environmental factors on a whole. Dispersal limitation in bryophytes and lichens confined to dry grasslands may be more common than is often assumed. Our study further suggests that competition between vascular plants and cryptogams is rather asymmetric.     

Photosynthetic CO2-use efficiency in lichens and their isolated photobionts: the possible role of a CO2-concentrating mechanism

The CO₂ dependence of net CO₂ assimilation was examined in a number of green algal and cyanobacterial lichens with the aim of screening for the algal/cyanobacterial CO₂-concentrating mechanism (CCM) in these symbiotic organisms. For the lichens Peltigera aphthosa (L.) Willd., P. canina (L.) Willd. and P. neopolydactyla (Gyeln.) Gyeln., the photosynthetic performance was also compared between intact thalli and their respective photobionts, the green alga Coccomyxa PA, isolated from Peltigera aphthosa and the cyanobacterium Nostoc PC, isolated from Peltigera canina. More direct evidence for the operation of a CCM was obtained by monitoring the effects of the carbonic-anhydrase inhibitors acetazolamide and ethoxyzolamide on the photosynthetic CO₂use efficiency of the photobionts. The results strongly indicate the operation of a CCM in all cyanobacterial lichens investigated and in cultured cells of Nostoc PC, similar to that described for free-living species of cyanobacteria. The green algal lichens were divided into two groups, one with a low and the other with a higher CO₂-use efficiency, indicative of the absence of a CCM in the former. The absence of a CCM in the low-affinity lichens was related to the photobiont, because free-living cells of Coccomyxa PA also apparently lacked a CCM. As a result of the postulated CCM, cyanobacterial Peltigera lichens have higher rates of net photosynthesis at normal CO₂ compared with Peltigera aphthosa. It is proposed that this increased photosynthetic capacity may result in a higher production potential, provided that photosynthesis is limited by CO₂ under natural conditions.