Kristin Palmqvist

Growth and vitality of epiphytic lichens

Abstract We tested the hypothesis that changed microclimate at induced forest edges causes reduced growth of epiphytic lichens. Two foliose, green algal lichens were transplanted to the lower canopy of a mature Picea abies forest at six distances (2, 6.25, 12.5, 25, 50 and 100 m) from a clearcut. The biomass growth in Platismatia glauca (6.2% in 16 months) was 41% higher than in Lobaria pulmonaria (4.4%). We found no growth reduction near the forest edge. In contrast, the highest growth in both species occurred within 12 m from the edge. Further, fluorescence and chlorophyll measurements showed that lichen vitality was unaffected by distance from edge. The light intensity was 4.3 times higher at the edge than in the interior during the growing season, but there were only minor differences in air temperature and relative humidity. Monitoring of thallus water content revealed clear differences in both number and length of wetting and drying cycles. However, the total time with water content sufficient for photosynthetic activity was only slightly higher at the edge. The data thus indicate that our gradient in microclimate was too small to significantly affect lichen growth, and that lichens are largely metabolically inactive when large edge-interior contrasts in microclimate occur. Lichen response to forest edge microclimate results from intricate interactions among several biotic and abiotic factors. Linking data on lichen growth, microclimate and thallus water content with physiological measurements provides a framework for future studies of the mechanisms behind abiotic edge effects.

Growth of epiphytic old forest lichens across climatic and successional gradients

This paper aims to assess the influence of canopy cover on lichen growth in boreal forests along a regional forest gradient. Biomass and area gain, and some acclimation traits, were assessed in the ...

Predicting lichen hydration using biophysical models.

Two models for predicting the hydration status of lichens were developed as a first step towards a mechanistic lichen productivity model. A biophysical model included the water potential of the air, derived from measurements of air temperature, relative humidity and species-specific rate constants for desiccation and rehydration. A reduced physical model, included only environmental parameters, assuming instantaneous equilibration between the lichen and the air. These models were developed using field and laboratory data for three green algal lichens: the foliose epiphytic Platismatia glauca (L.) W. Culb., the fruticose epiphytic Alectoria sarmentosa (Ach.) Ach. and the fruticose, terricolous and mat-forming Cladina rangiferina (L.) Weber ex Wigg. The models were compared and validated for the same three species using data from a habitat with a different microclimate. Both models predicted the length and timing of lichen hydration periods, with those for A. sarmentosa and P. glauca being highly accurate—nearly 100% of the total wet time was predicted by both the biophysical and physical models. These models also predicted an accurate timing of the total realized wet time for A. sarmentosa and P. glauca when the lichens were wet. The model accuracy was lower for C. rangiferina compared to the epiphytes, both for the total realized wet time and for the accuracy of the timing for the hydration period. These results demonstrate that the stochastic and continually varying hydration status of lichens can be simulated from biophysical data. Further development of these models to also include water-related activity, light and temperature conditions during the hydration events will then be a potent tool to assess potential lichen productivity in landscapes and habitats of various microclimatic conditions.

Predicting CO2 gain and photosynthetic light acclimation from fluorescence yield and quenching in cyano-lichens

Modulated chlorophylla fluorescence is useful for eco-physiological studies of lichens as it is sensitive, non-invasive and specific to the photobiont. We assessed the validity of using fluorescence yield to predict CO2 gain in cyano-lichens, by simultaneous measurements of CO2 gas exchange and chlorophylla fluorescence in five species withNostoc-photobionts. For comparison, O2 evolution and fluorescence were measured in isolated cells ofNostoc, derived fromPeltigera canina (Nostoc PC). At irradiances up to the growth light level, predictions from fluorescence yield underestimated true photosynthesis, to various extents depending on species. This reflected the combined effect of a state transition in darkness, which was not fully relaxed until the growth light level was reached, and a phycobilin contribution to the minimum fluorescence yield (Fo). Above the growth light level, the model progressively overestimated assimilation, reflecting increased electron flow to oxygen under excess irradiance. In cyanobacteria, this flow maintains photosystem II centres open even up to photoinhibitory light levels without contributing to CO2 fixation. Despite this we show that gross CO2 gain may be predicted from fluorescence yield also in cyanolichens when the analysis is made near the acclimated growth light level. This level can be obtained even when measurements are performed in the field, since it coincides with a minimum in non-photochemical fluorescence quenching (NPQ). However, the absolute relation between fluorescence yield and gross CO2 gain varies between species. It may therefore be necessary to standardise the fluorescence prediction for each species with CO2 gas exchange.

Anthropogenic nitrogen deposition enhances carbon sequestration in boreal soils

It is proposed that carbon (C) sequestration in response to reactive nitrogen (Nr ) deposition in boreal forests accounts for a large portion of the terrestrial sink for anthropogenic CO2 emissions. While studies have helped clarify the magnitude by which Nr deposition enhances C sequestration by forest vegetation, there remains a paucity of long-term experimental studies evaluating how soil C pools respond. We conducted a long-term experiment, maintained since 1996, consisting of three N addition levels (0, 12.5, and 50 kg N ha(-1) yr(-1) ) in the boreal zone of northern Sweden to understand how atmospheric Nr deposition affects soil C accumulation, soil microbial communities, and soil respiration. We hypothesized that soil C sequestration will increase, and soil microbial biomass and soil respiration will decrease, with disproportionately large changes expected compared to low levels of N addition. Our data showed that the low N addition treatment caused a non-significant increase in the organic horizon C pool of ~15% and a significant increase of ~30% in response to the high N treatment relative to the control. The relationship between C sequestration and N addition in the organic horizon was linear, with a slope of 10 kg C kg(-1) N. We also found a concomitant decrease in total microbial and fungal biomasses and a ~11% reduction in soil respiration in response to the high N treatment. Our data complement previous data from the same study system describing aboveground C sequestration, indicating a total ecosystem sequestration rate of 26 kg C kg(-1) N. These estimates are far lower than suggested by some previous modeling studies, and thus will help improve and validate current modeling efforts aimed at separating the effect of multiple global change factors on the C balance of the boreal region.

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.

Characterisation of inorganic carbon fluxes, carbonic anhydrase(s) and ribulose-1,5-biphosphate carboxylase-oxygenase in the green unicellular alga Coccomyxa

Processes involved in the uptake and fixation of dissolved inorganic carbon (DIC) were characterised for Coccomyxa, the green algal primary photobiont of the lichen Peltigera aphthosa and compared with the freeliving alga Chlamydomonas reinhardtii Dangeard (WT cc 125+). A mass-spectrometer disequilibrium technique was used to quantify fluxes of both HCOinf3sup−and CO2 in the two algae, while activities of carbonic anhydrases (CAs) were examined in intact cells by measuring 18O exchange from doubly labelled CO2 (13C18O18O) to water and by using CA inhibitors. The CO2-fixation kinetics of intact Coccomyxa cells were also compared with the carboxylation efficiency of its isolated and purified primary carboxylating enzyme, ribulose-1,5-bisphosphate carboxylaseoxygenase (Rubisco). The two algae were found to be significantly different in their modes of acquiring CO2 for photosynthesis. Chlamydomonas was able to actively transport both HCOinf3sup−and CO2 from the external medium, while Coccomyxa clearly favoured CO2 as a substrate. Both algae were found to possess external as well as internal CAs, but the relative amounts of these as well as their overall significance for the functioning of photosynthesis differed. In Coccomyxa, the internal CA activity was significantly higher than in Chlamydomonas and also predominated over the external activity. In Chlamydomonas, both transport and fixation of DIC were severely inhibited by ethoxyzolamide, an inhibitor of external and internal CAs as well as the DIC-transporting system, while this inhibitor only caused a minor inhibition of photosynthesis in Coccomyxa. These results thus give strong support for earlier indirect observations of the absence of a CO2concentrating mechanism in Coccomyxa. In addition, Coccomyxa was found to possess a Rubisco with a higher carboxylation efficiency than Chlamydomonas, having a Km (CO2) of 12 +3 μM CO2 and a CO2/O2 specificity factor (Sc/o) of 83 +2, and it may hence be concluded that the absence of the CO2-concentrating mechanism is positively correlated with a more efficient Rubisco in this alga.

Ammonium uptake in the nitrophytic lichen Xanthoria parietina and its effects on vitality and balance between symbionts

Ammonium uptake in the nitrophytic lichen Xanthoria parietina (L.) Th. Fr. and its effects on vitality and balance between symbionts

Photobiont-related differences in carbon acquisition among green-algal lichens

The photosynthetic properties of a range of lichens (eight species) containing green algal primary photobionts of either the genus Coccomyxa, Dictyochloropsis or Trebouxia were examined with the aim of obtaining a better understanding for the different CO2 acquisition strategies of lichenized green algae. Fast transients of light/dark-dependent CO2 uptake and release were measured in order to screen for the presence or absence of a photosynthetic CO2-concentrating mechanism (CCM) within the photobiont. It was found that lichens with Trebouxia photobionts (four species) were able to accumulate a small pool of inorganic carbon (DIC; 70–140 nmol per mg chlorophyll (Chl)), in the light, which theoretically may result in, at least, a two to threefold increase in the stromal CO2 concentration, as compared to that in equilibrium with ambient air. The other lichens (four species), which were tripartite associations between a fungus, a cyanobacterium (Nostoc) and a green alga (Coccomyxa or Dictyochloropsis) accumulated a much smaller pool of DIC (10–30 nmol·(mg Chl)−1). This pool is most probably associated with the previously documented CCM of Nostoc, inferred from the finding that free-living cells of Coccomyxa did not show any signs of DIC accumulation. In addition, the kinetics of fast CO2 exchange for free-living Nostoc were similar to those of intact tripartite lichens, especially in their responses to the CCM and the carbonic anhydrase (CA) inhibitor ethoxyzolamide. Trebouxia lichens had a higher photosynthetic capacity at low and limiting external CO2 concentrations, with an initial slope of the CO2-response curve of 2.6–3.9 μmol·(mg Chl)−1·h−1·Pa−1, compared to the tripartite lichens which had an initial slope of 0.5–1.1 μmol-(mg Chl)−1·h−1·-Pa−1, suggesting that the presence of a CCM in the photobiont affects the photosynthetic performance of the whole lichen. Regardless of these indications for the presence or absence of a CCM, ethoxyzolamide inhibited the steady-state rate of photosynthesis at low CO2 in all lichens, indicating a role of CA in the photosynthetic process within all of the photobionts. Measurements of CA activity in photobiont-enriched homogenates of the lichens showed that Coccomyxa had by far the highest activity, while the other photobionts displayed only traces or no activity at all. As the CCM is apparently absent in Coccomyxa, it is speculated that this alga compensates for this absence with high internal CA activity, which may function to reduce the CO2-diffusion resistance through the cell.

Lichen responses to nitrogen and phosphorus additions can be explained by the different symbiont responses.

• Responses to simulated nitrogen (N) deposition with or without added phosphorus (P) were investigated for three contrasting lichen species - the N-sensitive Alectoria sarmentosa, the more N-tolerant Platismatia glauca and the N(2) -fixing Lobaria pulmonaria- in a field experiment. • To examine whether nutrient limitation differed between the photobiont and the mycobiont within the lichen, the biomass responses of the respective bionts were estimated. • The lichenized algal cells were generally N-limited, because N-stimulated algal growth in all three species. The mycobiont was P-limited in one species (A. sarmentosa), but the growth response of the mycobionts was complex, as fungal growth is also dependent on a reliable carbon export from the photobiont, which may have been the reason for the decrease of the mycobiont with N addition in P. glauca. • Our findings showed that P availability was an important factor when studying effects of N deposition, as P supply can both mitigate and intensify the negative effects of N on epiphytic lichens.