Light use efficiency of dry matter gain in five macro-lichens: relative impact of microclimate conditions and species-specific traits

Relations between irradiance (I) and lichen growth were investigated for five macro‐lichens growing at two sites in Sweden. The lichens represented different mycobiont–photobiont associations, two morphologies (foliose, fruticose) and two life forms (epiphytic, terricolous). The lichens were transplanted at two geographically distant sites in Sweden (1000 km apart) from Sept 1995 to Sept 1996 in their typical microhabitats, where microclimate and growth were followed. Between April/May and Sept 96, the terricolous species had a dry matter gain of 0·2 to 0·4 g (g DW)^–1 and the epiphytes 0·01 to 0·02 g (g DW)^–1. When related to area, growth amounted to 30 to 70 g m^?2 for the terricolous species and to 1 to 4 g m^?2 for the epiphytes. There was a strong correlation between growth and intercepted irradiance when the lichens were wet (I_wet), with 0·2 to 1·1 g lichen dry matter being produced per MJ solar energy. Across the 10 sets of transplants, light use efficiencies of dry matter yield (e) ranged between 0·5 and 2%, using an energy equivalent of 17·5 kJ g^?1 of lichen dry matter. The higher productivity of the terricolous species was due to longer periods with thallus water contents sufficient for metabolic activity and because of the higher mean photon flux densities of their microhabitat. A four‐fold difference in photosynthetic capacity among the species was also important. It is concluded that lichen dry matter gain was primarily related to net carbon gain during metabolically active periods, which was determined by light duration, photon flux density and photosynthetic capacity.     

Rapid breakdown of exogenous extracellular hydrogen peroxide by lichens

All organisms, even highly stress‐tolerant lichens, produce a variety of reactive oxygen species (ROS) during and after stress. Furthermore, the cell walls of some lichens in Suborder Peltigerineae contain laccases, and therefore can produce quinone radicals that can break down to yield ROS. While the extracellular ROS produced by these enzymes probably play important roles in the biology of these lichens, they may also be potentially harmful and need to be rapidly broken down. To test this, rates of breakdown of exogenously supplied H₂O₂ were measured in a range of lichen species. Considerable diversity existed in rates of H₂O₂ breakdown but rates were on average almost double in members of Suborder Peltigerineae. While all lichens tested appeared to lack extracellular peroxidases and catalases, enzymes normally involved in breaking down H₂O₂, extracellular tyrosinase activity could be readily detected in the Peltigerineae. A role for tyrosinases in H₂O₂ breakdown was supported by the results from experiments involving inhibitors, and demonstration of the simultaneous release into an incubation solution of tyrosinase activity and the ability to breakdown H₂O₂. Rates of breakdown were very high, and tyrosinase appeared to break down H₂O₂ by a catalase‐like mechanism. However, significant rates of breakdown of H₂O₂ also occurred in species that did not possess cell wall redox enzymes. These species probably took up the exogenously supplied H₂O₂ intracellularly and then broke it down by the usual catalases and peroxidases. The importance of H₂O₂ degradation is discussed in terms of its possible role in defence against the harmful effects of ROS.     

Chlorophyll a Fluorescence Emission,Xanthophyll Cycle Activity,and Net Photosynthetic Rate Responses to Ozone in Some Foliose and Fruticose Lichen Species

The lichens Parmelia quercina, Parmelia sulcata, Evernia prunastri, Hypogymnia physodes, and Anaptychia ciliaris were exposed to ozone (O₃) in controlled environment cuvettes designed to maintain the lichens at optimal physiological activity during exposure. Measurements of gas exchange, modulated chlorophyll (Chl) fluorescence, and pigment analysis were conducted before and after exposure to 300 mm³ (O₃) m^–3, 4 h per d for 14 d. No changes in the efficiency of photosystem 2 (PS2) photochemistry, the reduction state of Q_A, or the electron flow through PS2, measured by Chl fluorescence, were detected in any of the five lichen species studied. Additionally, neither photosynthetic CO₂ assimilation nor xanthophyll cycle activity or photosynthetic pigment concentration were affected by high O₃ concentrations. Thus the studied lichen species have significant capacities to withstand oxidative stresses induced by high concentration of O₃.     

Heterogeneity of Chlorophyll Fluorescence over Thalli of Several Foliose Macrolichens Exposed to Adverse Environmental Factors: Interspecific Differences as Related to Thallus Hydration and High Irradiance

Spatial heterogeneity of chlorophyll (Chl) fluorescence over thalli of three foliose lichen species was studied using Chl fluorescence imaging (CFI) and slow Chl fluorescence kinetics supplemented with quenching analysis. CFI values indicated species-specific differences in location of the most physiologically active zones within fully hydrated thalli: marginal thallus parts (Hypogymnia physodes), central part and close-to-umbilicus spots (Lasallia pustulata), and irregulary-distributed zones within thallus (Umbilicaria hirsuta). During gradual desiccation of lichen thalli, decrease in Chl fluorescence parameters (F_O - minimum Chl fluorescence at point O, F_P - maximum Chl fluorescence at P point, ₂ - effective quantum yield of photochemical energy conversion in photosystem 2) was observed. Under severe desiccation (>85 % of water saturation deficit), substantial thalli parts lost their apparent physiological activity and the resting parts exhibited only a small Chl fluorescence. Distribution of these active patches was identical with the most active areas found under full hydration. Thus spatial heterogeneity of Chl fluorescence in foliose lichens may reflect location of growth zones (pseudomeristems) within thalli and adjacent newly produced biomass. When exposed to high irradiance, fully-hydrated thalli of L. pustulata and U. hirsuta showed either an increase or no change in F_O, and a decrease in F_P. Distribution of Chl fluorescence after the high irradiance treatment, however, remained the same as before the treatment. After 60 min of recovery in the dark, F_O and F_P did not recover to initial values, which may indicate that the lichen used underwent a photoinhibition. The CFI method is an effective tool in assessing spatial heterogeneity of physiological activity over lichen thalli exposed to a variety of environmental factors. It may be also used to select a representative area at a lichen thallus before application of single-spot fluorometric techniques in lichens.     

Biphasic regulation of superoxide radical levels in Mn-depleted and photoactivated photosystem II

In the past decades, it has become clear that superoxide radical (O₂ ^·?) can be generated from photosystem II (PSII) during photosynthesis. Depending on the extent of its accumulation, O₂ ^·? plays an important role in plant physiology and pathology. The photoinhibition/repair cycle is a typical process in PSII which is mainly responsible for the survival of plants under the photoinihibition condition. It is therefore of significant importance to determine O₂ ^·? production in this cycle, and then explore how O₂ ^·? is controlled by PSII within a normal physiological level. With this in mind, we herein investigate the variation of the O₂ ^·? levels in PSII under Mn-depleted and photoactivated conditions mimicking the photoinhibition/repair cycle in vitro. The effect of intrinsic SOD-like component on the O₂ ^·? levels was also studied. Results show that PSII has the ability to regulate the O₂ ^·? levels in these two processes by simultaneously modulating the O₂ ^·? generation activity and intrinsic SOD-like activity. This finding could shed new lights on the photoprotective property of PSII against O₂ ^·? and other reactive oxygen species.     

Decreased superoxide anion radical production by rat alveolar macrophages following inhalation of ozone or nitrogen dioxide

$\text{In vivo}$ exposure of rats to ozone or nitrogen dioxide results in a dose-dependent decrease in superoxide anion radical production (O₂^?·) by alveolar macrophages isolated from the exposed animals. When alveolar macrophages from ozone-exposed animals were stimulated with phorbol myristate acetate (PMA, a non-phagocytic stimulus of O₂^?· production) the decrease in O₂^?· production ranged from 85.9% of control at 3.2 ppm-hrs ozone to 7% of control at 10.5 ppm-hrs. In a similar fashion, O₂^?· production by PMA-stimulated macrophages from NO₂-exposed rates ranged from 78% of control at 18.3 ppm-hrs NO₂ down to 14.5% of control at 51 ppm-hrs. Since the viability of the alveolar macrophages obtained from ozone or nitrogen dioxide-exposed animals was 88% or better in all cases as judged by both Trypan blue exclusion and lactate dehydrogenase release, the decreased ability of these cells to produce superoxide anion radical cannot be attributed to a pollutant effect on cell viability. This diminution in superoxide anion radical production by alveolar macrophages from the pollutant-exposed animals might account, in part, for the ability of these 2 air pollutants to potentiate bacterial infections in laboratory animals.     

Australian mulga ecosystems –13C and 15N natural abundances of biota components and their ecophysiological significance

Samples of recently produced shoot material collected in winter/spring from common plant species of mulga vegetation in eastern and Western Australia were assayed for ¹³C and ¹⁵N natural abundance. ¹³C analyses showed only three of the 88 test species to exhibit C₄ metabolism and only one of seven succulent species to be in CAM mode. Non-succulent winter ephemeral C₃ species showed significantly lower mean δ¹³C values (– 28·0‰) than corresponding C₃-type herbaceous perennials, woody shrubs or trees (– 26·9, – 25·7 and – 26·2‰, respectively), suggesting lower water stress and poorer water use efficiency in carbon acquisition by the former than latter groups of taxa. Corresponding values for δ¹⁵N of the above growth and life forms lay within the range 7·5–15·5‰. δ¹⁵N of soil NH₄⁺ (mean 19·6‰) at a soft mulga site in Western Australia was considerably higher than that of NO₃^– (4·3‰). Shoot dry matter of Acacia spp. exhibited mean δ¹⁵N values (9·10 ± 0·6‰) identical to those of 37 companion non-N₂-fixing woody shrubs and trees (9·06 ± 0·5‰). These data, with no evidence of nodulation, suggested little or no input of fixed N₂ by the legumes in question. However, two acacias and two papilionoid legumes from a dune of wind-blown, heavily leached sand bordering a lake in mulga in Western Australia recorded δ¹⁵N values in the range 2·0–3·0‰ versus 6·4–10·7‰ for associated non-N₂-fixing taxa. These differences in δ¹⁵N, and prolific nodulation of the legumes, indicated symbiotic inputs of fixed N in this unusual situation. δ¹⁵N signals of lichens, termites, ants and grasshoppers from mulga of Western Australia provided evidence of N₂ fixation in certain termite colonies and by a cyanobacteria-containing species of lichen. Data are discussed in relation to earlier evidence of nitrophily and water availability constraints on nitrate utilization by mulga vegetation.     

The effect of nitrogen on growth and key thallus components in the two tripartite lichens, Nephroma arcticum and Peltigera aphthosa

Relationships between growth, nitrogen and concentration of unique biont components were investigated for the tripartite lichens Nephroma arcticum (L.) Torss. and Peltigera aphthosa (L.) Willd. Nitrogen availability was manipulated during 4 summer months by removing cephalodia and their associated N₂ fixation activity, or by weekly irrigation with NH₄NO₃. Chlorophyll and ribulose 1·5‐biphosphate carboxylase/oxygenase (Rubisco), and chitin and ergosterol were used as photobiont and mycobiont markers, respectively. Nitrogen concentrations were similar in older and newer parts of the same thallus, varying between 2 and 5 g m^?2, with P. aphthosa having higher concentrations than N. arcticum. Both chlorophyll (Chl a) and chitin were linearly correlated with thallus N, but N. arcticum invested more in fungal biomass and had lower Chl a concentrations in comparison with P. aphthosa at equal thallus N. During the 4 months, control and N‐fertilized thalli of N. arcticum increased in area by 0·2 m² m^?2 and P. aphthosa by 0·4 m² m^?2. Thallus expansion was significantly inhibited in samples without cephalodia, but there was no effect on lichen weight gain. Mean relative growth rate (RGR; mg g^?1 d^?1) was 3·8 for N. arcticum and 8·4 for P. aphthosa, when time (d) reflected the lichen wet periods. RGR was 2–3 times lower when based on the whole time, i.e. when including dry periods. The efficiency (e) of converting incident irradiance into lichen biomass was positively and linearly correlated with thallus Chl a concentration to the same extent in both species. The slower growth rates of N. arcticum, in comparison with P. aphthosa, could then be explained by their lower nitrogen and Chl a concentrations and a subsequently lower light energy conversion efficiency. Functional and dynamic aspects of resource allocation patterns of the two lichens are discussed in relation to the above findings.     

Reactive oxygen species in the killing of Pseudomonas aeruginosa by human leukocytes

Pseudomonas aeruginosa (PA) infects hosts with compromised host defenses. An important defense mechanism is the generation of reactive oxygen species (ROS) by white blood cells (WBCs). What roles do ROS play in host defense against PA? Human WBCs killed PA in vitro, and they generated a respiratory burst as measured by the production of H₂O₂. ROS efficiently killed PA; in acellular assays, less than 10mm of H₂O₂ or OCl^- eliminated all bacteria in 90 min. However, WBCs with suppressed production of ROS (caused by hypoxia) killed PA normally. In addition, none of the antioxidants vitamin C, N-acetylcysteine, superoxide dismutase, or catalase affected PA killing by WBCs. Thus, PA stimulates WBCs to produce ROS, which can kill the bacteria, but disturbances of WBC ROS production do not interfere with the killing of PA. WBCs have robust, redundant mechanisms for PA elimination.     

Primary production and respiration of the phytoplankton of the Rivers Thames and Kennet at Reading

During the period April 1967-ApriI 1968 the phytoplankton production and respiration of the River Thames and its tributary, the River Kennet, were measured at approxi-mately 2-week intervals using the light and dark bottle technique. Concentrations of chlorophyll and pheopigment were determined weekly. On fourteen occasions sets of light and dark bottles were rotated in a specially designed apparatus, and production and respiration values obtained were found to be 1·38 ± 0·31 times higher than in stationary bottles at identical depths over the same period. There was little horizontal, vertical or diurnal variation in chlorophyll concentration showing that the water was well mixed. Peaks of chlorophyll were found in spring, summer and autumn in the Thames (max. 219 mg/m³) but there was very little variation in the Kennet (max. 38·2 mg/m³). In both rivers lowest concentrations were found during winter. Pheo-pigment concentration was low in both rivers for most of the period although in the Kennet this represented on average 50% of the pigments present. In the Thames a peak of pheopigments(1·33–5 mg/m³) was associated with the autumnal bloom and repre-sented 61 % of the total pigments. No pheopigments were detected during the spring bloom. The average concentration of suspended organic matter was identical in both rivers but in the Thames over 25 % was due to phytoplankton and in the Kennet almost 95 % was non-algal. In the Thames, net oxygen production reached a peak in May (10·81 gO₂/m²/day) and was negative from November to February (min. ?0·45 gO₂/m²/day). In the Kennet, maximum production also occurred in May (0·85 gO₂/m2) but was negative from the middle of May until the following March. The average annual net production was 1250 and ?78 g O₂/m² in the Thames and Kennet respectively. Respiration rates showed similar fluctuations being 4·59 g O₂/m²/day in spring in the Thames to 0·09 g O₂/m²/day in November. The Kennet was almost always lower (1·05–0·34 g O₂/m2/day. The average annual respiration was almost three times higher in the Thames than in the Kennet (641–228 g O₂/m²). Various factors which might influence production are discussed. The average net efficiency ofthe Thames phytoplankton fell within ranges described from other rivers. Net efficiencies ofthe Kennet were almost always negative. In the Thames it appeared that net production could be explained as a function of solar radiation, chlorophyll concentration and euphotic depth.     

Functional characteristics of a fruticose type of lichen, Stereocaulon foliolosum Nyl. in response to light and water stress

Stereocaulon foliolosum a fruticose type of lichen under its natural habitat is subjected to low temperature, high light conditions and frequent moisture stress due its rocky substratum. To understand as to how this lichen copes up with these stresses, we studied the reflectance properties, light utilization capacity and the desiccation tolerance under laboratory conditions. S. foliolosum showed light saturation point for photosynthesis at 390 μmol CO₂ m^?2 s^?1 and the light compensation point for photosynthesis at 64 μmol CO₂ m^?2 s^?1. Our experiments show that S. foliolosum has a low absorptivity (30–35 %) towards the incident light. The maximum rates of net photosynthesis and apparent electron transport observed were 1.9 μmol CO₂ m^?2 s^?1 and 45 μmol e^? m^?2 s^?1, respectively. The lichen recovers immediately after photoinhibition under low light conditions. S. foliolosum on subjecting to desiccation results in the decrease of light absorptivity and the reflectance properties associated with water status of the thalli show a change. During desiccation, a simultaneous decrease in photosynthesis, dark respiration and quenching in the fluorescence properties was observed. However, all the observed changes show a rapid recovery on rewetting the lichen. Our study shows that desiccation does not have a severe or long-term impact on S. foliolosum and the lichen is also well adapted to confront high light intensities.     

Kinetic simulation studies on the transient formation of the oxo‐iron(IV) porphyrin radical cation during the reaction of iron(III) tetrakis‐5,10,15,20‐(N‐methyl‐4‐pyridyl)‐porphyrin with hydrogen peroxide in aqueous solution

High‐valent oxo‐iron(IV) species are commonly proposed as the key intermediates in the catalytic mechanisms of iron enzymes. Water‐soluble iron(III) tetrakis‐5,10,15,20‐(N‐methyl‐4‐pyridyl)porphyrin (Fe(III)TMPyP) has been used as a model of heme‐enzyme to catalyse the hydrogen peroxide (H₂O₂) oxidation of various organic compounds. However, the mechanism of the reaction of Fe(III)TMPyP with H₂O₂ has not been fully established. In this study, we have explored the kinetic simulation of the reaction of Fe(III)TMPyP with H₂O₂ and of the catalytic reactivity of FeTMPyP in the luminescent peroxidation of luminol. According to the mechanism that has been established in this work, Fe(III)TMPyP is oxidized by H₂O₂ to produce (TMPyP)^·+Fe(IV)=O (k1 = 4.5 × 10⁴/mol/L/s) as a precursor of TMPyPFe(IV)=O. The intermediate, (TMPyP)^·+Fe(IV)=O, represented nearly 2% of Fe(III)TMPyP but it does not accumulate in suf?cient concentration to be detected because its decay rate is too fast. Kinetic simulations showed that the proposed scheme is capable of reproducing the observed time courses of FeTMPyP in various oxidation states and the decay pro?les of the luminol chemiluminescence. It also shows that (TMPyP)^·+Fe(IV)=O is 100 times more reactive than TMPyPFe(IV)=O in most of the reactions. These two species are responsible for the initial sharp and the sustained luminol emissions, respectively. Copyright © 2003 John Wiley & Sons, Ltd.     

Non-invasive monitoring of photosynthetic activity and water content in forest lichens by spectral reflectance data and RGB colors from photographs

There is a need for non-invasive monitoring of temporal and spatial variation in hydration and photosynthetic activity of red-listed poikilohydric autotrophs. Here, we simultaneously recorded kinetics in RGB-colors (photos), reflectance spectra, water content, maximal (F_V/F_M), and effective quantum yield of PSII (Φ_PSII) during desiccation in foliose lichens differing in cortical characteristics and photobionts. The spectral absorbance peaks of chlorophyll a, phycocyanin, and phycoerythrin were clearly displayed at high hydration levels. Brightness and total RGB colors of the lichens strongly increased during desiccation. The normalized difference vegetation index (NDVI) efficiently estimated hydration level and Φ_PSII – a proxy for lichen photosynthesis – in all species, including threatened old forest lichens. Color and reflectance indices based on green wavelengths gave good estimates of water content in cephalo- and chlorolichens, but not in cyanolichens with a wider range of photosynthetic pigments. Due to species-specific characteristics, species-wise calibration is essential for non-invasive assessments of lichen functioning.     

Oxidative reactions in axenically seedling roots of olive (Olea europaea, L.)

The production of reactive oxygen species (ROS) plays important roles in the life cycle and in the stress response and defence mechanisms of plants. Various enzyme systems are involved in the formation of ROS in the apoplast, including plasmalemma NADPH oxidase and apoplastic peroxidases. The production of O ₂ ^·? and apoplastic peroxidase and exogenous NADH oxidation activities are all strongly dependent on the age of roots??the younger the root, the greater the activity. Apoplastic production of ROS is shown in the root by using specific histochemical probes, this ROS production is growing zone dependent. In the present study, using olive seedlings, differences were also observed between cultivars, especially in O ₂ ^·? production by the Verdial cultivar which was well above that of other cultivars studied. In all the cultivars, treatment of roots with methyl jasmonate (MeJA) or methyl salicylate (MeSA) increased O ₂ ^·? production. Similar results were observed for peroxidase activity, but not for the oxidation of exogenous NADH which was either unaffected (MeJA) or even partially inhibited (MeSA). A conclusion was that MeJA or MeSA induced apoplastic production of ROS does not use exogenous NADH. Treatment with diphenylene iodonium (DPI) reduced the formation of O ₂ ^·? , but affected neither peroxidase nor NADH oxidation activities. Cyanide inhibited O ₂ ^·? production and peroxidase and NADH oxidation activities. Treatment with MnCl₂ had a strong stimulatory effect on peroxidase and NADH oxidation activities, but much less on O ₂ ^·? production. Finally, azide greatly reduced all activities, but especially O ₂ ^·? production. Together, these results indicate a relationship between oxidative activities and the processes of root growth, and that those activities are also dependent on the cultivar, as well as an involvement of peroxidases and plasmalemma NADPH oxidase in apoplast ROS production which is sensitive to DPI, azide, and cyanide but relatively insensitive to MnCl₂, while exogenous NADH oxidation is linked to peroxidase activity.     

Percentage of phagocytosis,production of O2·−, H2O2 and NO,and antioxidant enzyme activities of rat neutrophils in culture

Changes in the integrity, ultrastructure, phagocytosis capacity, and production of H₂O₂, O₂^{· −}and NO₂⁻ were evaluated in cultured neutrophils. The activities of the antioxidant enzymes (catalase—CAT, superoxide dismutase—SOD and glutathione-dependent peroxidase—GSH-Px) were measured under similar conditions. The integrity of the cells remained unchanged up to 18 h. After 24 h, the number of viable cells in culture dropped by 16 per cent. The percentage of viable cells in culture was of 72 per cent even after 72 h. An ultrastructural analysis of the cells was carried out after 3, 6, 12, 24, 48, and 72 h in culture. Neutrophils started developing morphologic changes after 24 h: decreased cell volume, abundant vacuoles (mainly around the nucleus), and also the presence of autophagic vacuoles. This period was then chosen for the study of neutrophil function and antioxidant enzyme activities. Neutrophils cultured for 24 h presented reduced phagocytosis capacity. The rates of production of H₂O₂ and O₂^{· −} remained unchanged after 24 h in culture. Concomitantly, these cells were also able to produce NO in significant amounts. The production of O₂^·− in response to PMA stimulus was lowered in 24-h cultured cells. Possibly, the production of oxygen and nitrogen reactive species accomplished with a decrease in the activities of CAT and GSH-Px play a key role for the process of apoptosis which takes place in neutrophils under these conditions. © 1998 John Wiley & Sons, Ltd.     

Zn/Pb-tolerant lichens with higher content of secondary metabolites produce less phytochelatins than specimens living in unpolluted habitats

Many lichens can cope with heavy-metal stress, however, the mechanisms of lichen tolerance are still not fully understood. Some lichen secondary metabolites (depsides and depsidones), produced in lichens by the fungal symbiont and accumulated on the outer surface of its hyphae, are supposed to play an important role in the extracellular immoblilization of heavy metals. Lichen photobionts (algal partners in the symbiosis), although surrounded by the mycobiont hyphae, may also accumulate high amounts of trace metals. This can lead to physiological disruptions and morphological damage in algal cells and hence affect the lichen physiological status. We hypothesized that lichen species/specimens living in heavily polluted sites and showing HM tolerance possess a higher content of secondary metabolites than those living in unpolluted sites. Hence, their photobionts can be better protected from the excess of metal ions and need to produce less metal-complexing phytochelatins (PCn) to combat metal toxicity. Specimens of Hypocenomyce scalaris, Cladonia furcata and Lepraria spp. sampled from Zn/Pb-polluted and control sites were compared for the accumulation of Zn/Pb and secondary metabolites, as well as for their production of phytochelatins and glutathione in response to experimental Zn or Pb exposure. Generally, the lichen specimens sampled from the HM-polluted site contained higher amounts of Zn and Pb as well as lichen substances (different depsides and depsidones) than those from the control site. A strong positive correlation was found between the accumulation of secondary metabolites and Zn/Pb accumulation (R² = 0.98 and 0.63, respectively). For the first time, production of phytochelatins (PC_2-3) in response to Zn and Pb (50-200 μM) exposure was found in H. scalaris, L. elobata, L. incana and C. furcata. In both species of Lepraria also cysteine, a substrate for GSH and PCs synthesis was detected. The lichens from the polluted site produced under the same exposure conditions, or in response to higher metal concentrations, lower amounts of PCn than those sampled from the control site. It strongly suggests that less Zn and Pb ions reached the photobiont cells of the lichens containing higher amounts of secondary metabolites (lecanoric, fumarprotocetraric, stictic, constictic acids, antranorin). The results obtained support the putative role of some metabolites in heavy-metal tolerance of the lichens inhabiting metal-polluted habitats.     

Pleistocene expansion of the bipolar lichen Cetraria aculeata into the Southern hemisphere

Many boreal and polar lichens occupy bipolar distributional ranges that frequently extend into high mountains at lower latitudes. Although such disjunctions are more common among lichens than in other groups of organisms, the geographic origin of bipolar lichen taxa, and the way and time frame in which they colonized their ranges have not been studied in detail. We used the predominantly vegetative, widespread lichen Cetraria aculeata as a model species. We surveyed the origin and history of its bipolar pattern using population genetics, phylogenetic and genealogical reconstruction methods. Cetraria aculeata originated in the Northern Hemisphere and dispersed southwards during the Pleistocene. The genetic signal suggests a Pleistocene dispersive burst in which a population size expansion concurred with the acquisition of a South‐American range that culminated in the colonization of the Antarctic.     

Colonization,growth, and survival strategies of lichens on leaves in a subtropical rainforest

Rainforest leaves are a relatively short-lived habitat which is well defined in both time and space, and which is occupied by a range of specialized lichens which might be expected to show survival strategies contrasting with those of other lichens. Changes in the lichen populations of individual leaves in subtropical rainforest at Ml Glorious, Queensland, Australia were observed for 1662 days. Over 1100 days elapsed before 50% of surviving leaves showed visible lichen thalli and the probability of colonization estimated from life tables did not exceed 0.42 ± 0.20 at any time. Porina epiphylla had a relative growth rate of 3.01 × 10^?2 mm² mm^?2 week^?1, a high value for a lichen. The relative growth rate of Strigula subtilissima, however, was 6.86 × 10^?2 mm² mm^?2 week^?1, the highest rate known for any lichen. Small size and high relative growth rates indicate that lichens on leaves have the most extreme ruderal strategy yet demonstrated amongst the lichens.     

Revisiting the Ramachandran plot: hard-sphere repulsion, electrostatics, and H-bonding in the alpha-helix

What determines the shape of the allowed regions in the Ramachandran plot? Although Ramachandran explained these regions in terms of 1–4 hard‐sphere repulsions, there are discrepancies with the data where, in particular, the α_R, α_L, and β‐strand regions are diagonal. The α_R‐region also varies along the α‐helix where it is constrained at the center and the amino terminus but diffuse at the carboxyl terminus. By analyzing a high‐resolution database of protein structures, we find that certain 1–4 hard‐sphere repulsions in the standard steric map of Ramachandran do not affect the statistical distributions. By ignoring these steric clashes (N···H_i+1 and O_i?1···C), we identify a revised set of steric clashes (C^β···O, O_i?1···N_i+1, C^β···N_i+1, O_i?1···C^β, and O_i?1···O) that produce a better match with the data. We also find that the strictly forbidden region in the Ramachandran plot is excluded by multiple steric clashes, whereas the outlier region is excluded by only one significant steric clash. However, steric clashes alone do not account for the diagonal regions. Using electrostatics to analyze the conformational dependence of specific interatomic interactions, we find that the diagonal shape of the α_R and α_L‐regions also depends on the optimization of the N···H_i+1 and O_i?1···C interactions, and the diagonal β‐strand region is due to the alignment of the CO and NH dipoles. Finally, we reproduce the variation of the Ramachandran plot along the α‐helix in a simple model that uses only H‐bonding constraints. This allows us to rationalize the difference between the amino terminus and the carboxyl terminus of the α‐helix in terms of backbone entropy.     

Further evidence that activation of net photosynthesis by dry cyanobacterial lichens requires liquid water

Abstract: In contrast to green algal lichens, cyanobacterial species of different families, growth forms and habitats proved to be unable to attain positive net CO₂ assimilation when the dry thalli were treated with air of high relative humidity; they needed liquid water for the reactivation of their photosynthetic apparatus. Identical behaviour is shown by all of the 47 lichen species with cyanobacterial photobionts, from six different genera, studied so far. This suggests a widely distributed, if not general, characteristic of cyanobacterial lichens. The difference in performance between both groups of photobionts was maintained when the lichen thallus was macerated. Furthermore, cultures of Chroococcidiopsis were unable to make use of water vapour hydration for positive net photosynthesis, and were similar in this respect to some free-living aerophilic cyanohacteria tested earlier. Possible physiological implications as well as ecological consequences for water-relation-dependent habitat selection of green-algal and cyanobacterial lichens are discussed.