Microclimates and water budget of algae,lichens and a moss on some nunataks in Queen Maud Land

During the Norwegian Antarctic Expedition of 1984–1985, land parties worked in the area of Mühlig-Hofmannfjella and Gjelsvikfjella in Queen Maud Land (5° 20E, 1° 37W, 1000–1600 m a.s.l.). The nunataks in this area, which represents one of the climatic limits for terrestrial life on earth, is among those areas absorbing the highest quantity of shortwave radiation during a period of 24 h in summer. In spite of this the air temperature never, or very seldom, exceeds 0° C. The limiting factor for photosynthesis over most of the summer was water availability. Melt-water plays an important role in spring. During rest of the growth season, water from condensation is probably the most important water source for plants. On calm nights the rate of condensation during 6 h may reach 0.5 mm, which constitutes only 10%–30% of daily potential evapotranspiration. Plants situated in narrow clefts or shielded by stone blocks have the highest rate of potential photosynthesis. These locations are shielded from direct solar radiation most of the time, but the radiation from surrounding stone surfaces is higher than from the atmosphere and heat loss by turbulence is smaller than for exposed locations. These locations also probably have the highest rate of actual photosynthesis.     

Selenium concentrations in the human thyroid gland

Recently, we found that prediagnostic serum selenium concentration was significantly lower for cases developing thyroid cancer (n=43) than for controls. We assumed that redistribution of serum selenium into the affected tissue took place in the prediagnostic period. The present study was carried out to determine the physiological concentration of selenium in the thyroid, since very few data are available in the literature. The concentrations of selenium in the thyroid (n=45) and liver samples from Norwegians who had died because of acute illness or accidents were determined by hydride generation atomic absorption spectrometry. The mean selenium concentration was found to be 0.72±0.44 μg/g in the thyroid and 0.45±0.11 μg/g in the liver tissue. The surprisingly high concentration of selenium in apparently normal thyroids indicates that selenium has important functions in this organ. The remarkably broad range, together with the observation that no significant correlation exists between thyroid and liver concentrations, suggest that factors other than the selenium status are important determinants for the selenium concentration in the thyroid gland. This observation is consistent with our hypothesis that in carcinogenesis, prediagnostic processes influence the serum-/thyroid-ratio of selenium.     

COMPETITION BETWEEN STAURASTRUM LUETKEMUELLERII (CHLOROPHYCEAE) AND MICROCYSTIS AERUGINOSA(CYANOPHYCEAE) UNDER VARYING MODES OF PHOSPHATE SUPPLY1

The desmid Staurastrum luetkemuellerii Donat et Ruttner and the cyanobacterium Microcystis aeruginosa Kütz. were grown in mixed cultures with various phosphate (P_i) additions. One pulse of P_i each day (semi-continuous cultures) favored M. aeruginosa whereas S. luetkemuellerii was favored when the same quantity of P_i was supplied continuously (chemostats). Both species coexisted under P limitation provided that the nutrient was supplied in an appropriate mode. The ability of each species to compete for P depended on their P_i uptake characteristics and their capability to retain the accumulated P_i. High affinity in uptake at low P_i concentrations contributed considerably to the growth eficiency of S. luetkemuellerii under continuous supply of P_iM. aeruginosa was, however, consistently superior to S. luetkemuellerii in accuniulatiug the newly added P, but had a high rate of P_i release. In both -types of cultures, a net high of P went from M. aeruginosa to S. luetkemuellerii. The kinetic characteristics of the two species were used to simulate the outcome of competition experiments. Simulations agreed with the experimental data f both uptake and P_i release were considered in the model. The zlariable P*(the concentration of P_i at which the net uptake is equal to μ·Q_P is a function of uptake and release of P_i but could not explain the chemostat results. S. luetkemuellerii was the winner in many experiments even if its P*was higher thou that of M. aeruginosa. Thus, in the present case P_c (the concentration at which the net uptake is zero) was a better predictor of the ability to compete for P_i under steady state as well as transient conditions in the P_i supply.     

Prolonging the hydration and active metabolism from light periods into nights substantially enhances lichen growth

This study investigates how hydration during light and dark periods influences growth in two epiphytic old forest lichens, the green algal Lobaria pulmonaria and the cyanobacterial L. scrobiculata. The lichens were cultivated in growth chambers for 14 days (200 μmol m^?1 s^?2; 12 h photoperiod) at four temperature regimes (25/20 °C, 21/16 °C, 13/8 °C, and 6/1 °C; day/night temperatures) and two hydration regimes (12 h day-time hydration; 12 h day-time + 12 h night-time hydration). Growth was highly dynamic, showing that short-term growth experiments in growth cabinets have a high, but largely unexplored potential in functional lichen studies. The highest measured growth rates were not far from the maximal dry matter gain estimated from published net photosynthetic CO₂ uptake data. For the entire data set, photobiont type, temperature, hydration regime and specific thallus mass accounted for 46.6 % of the variation in relative growth rate (RGR). Both species showed substantially higher relative growth rates based on both biomass (RGR) and thallus area (RT_AGR) when they were hydrated day and night compared to hydration in light only. Chronic photoinhibition was substantial in thalli hydrated only during the day time and kept at the highest and lowest temperature regimes, resulting in exponential increases in RGR with increasing maximal PSII efficiency (F _v/F _m) in both species. However, the depression in F _v/F _m was stronger for the cyanolichen than for the cephalolichen at extreme temperatures. The growth-stimulating effect of night-time hydration suggests that nocturnal metabolic activity improves recovery of photoinhibition and/or enhances the conversion rate of photosynthates into thallus extension.     

Seed viability and germination success of Acacia tortilis along land‐use and aridity gradients in the Eastern Sahara

Our study focuses on the keystone species Acacia tortilis and is the first to investigate the effect of domestic ungulates and aridity on seed viability and germination over an extensive part of the Eastern Sahara. Bruchids infest its seeds and reduce their viability and germination, but ingestion by ruminant herbivores diminishes infestation levels and enhances/promotes seed viability and germination. The degree of these effects seems to be correlated with animal body mass. Significantly reduced numbers of wild ruminant ungulates have increased the potential importance of domestic animals and pastoral nomadism for the functionality of arid North African and Middle Eastern ecosystems. We sampled seeds (16,543) from A. tortilis in eight areas in three regions with different aridity and land use. We tested the effect of geography and sampling context on seed infestation using random effects logistic regressions. We did a randomized and balanced germination experiment including 1193 seeds, treated with different manure. Germination time and rates across geography, sampling context, and infestation status were analyzed using time‐to‐event analyses, Kaplan–Meier curves and proportional hazards Cox regressions. Bruchid infestation is very high (80%), and the effects of context are significant. Neither partial infestation nor adding manure had a positive effect on germination. There is a strong indication that intact, uningested seeds from acacia populations in the extremely arid Western Desert germinate more slowly and have a higher fraction of hard seeds than in the Eastern Desert and the Red Sea Hills. For ingested seeds in the pastoralist areas we find that intact seeds from goat dung germinate significantly better than those from camel dung. This is contrary to the expected body‐mass effect. There is no effect of site or variation in tribal management.     

Lichen palatability depends on investments in herbivore defence

Lichens are well-suited organisms for experimental herbivory studies because their secondary compounds, assumed to deter grazing, can be non-destructively extracted. Thalli of 17 lichen species from various habitats were cut in two equal parts; compounds were extracted from one part by acetone, the other served as a control. These two pieces were offered as a paired choice to the generalist herbivore snail Cepaea hortensis. Control thalli of all lichens were consumed at a low rate regardless of their investments in acetone-extractable lichen compounds; naturally compound-deficient lichen species were not preferred compared to those with high contents. However, for extracted thalli, there was a highly significant positive correlation between rate of consumption and the extracted compound contents. These data imply that herbivore defence has evolved in different directions in different lichens. Studied members of Parmeliaceae, common in oligotrophic habitats, have high contents of carbon-rich acetone-soluble compounds; these lichens became highly palatable to snails subsequent to acetone rinsing. Extracted lichen compounds were applied to pieces of filter paper and fed to snails. Extracts from members of the Parmeliaceae significantly deterred feeding on paper. Such data suggest that generalist herbivores may have shaped evolution in the widespread and highly diverse Parmeliaceae towards high investments in lichen compounds. On the other hand, lichens belonging to the Physciaceae and Teloschistales, common in nutrient-enriched habitats, are deficient in, or have low concentrations of, lichen compounds. Such lichens did not become more palatable after acetone rinsing. The orange anthraquinone compound parietin, restricted to the Teloschistales, and which has previously been found to protect against excess light, did not deter grazing.     

The lichens <Emphasis Type="Italic">Xanthoria elegans</Emphasis> and <Emphasis Type="Italic">Cetraria islandica</Emphasis> maintain a high protection against UV-B radiation in Arctic habitats

This study reports UV screening pigments in the upper cortices of two widespread lichens collected in three sun-exposed locations along a latitudinal gradient from the Arctic lowland to alpine sites of the Central European Alps. Populations from the Alps receive 3–5 times higher UV-B irradiance than their Arctic counterparts from Svalbard because of latitudinal and altitudinal gradients in UV-B irradiance. In Cetraria islandica, the screening capacity of melanin in the upper cortices was assessed by direct measurements of cortical transmittance (250–1,000 nm). A comparison of cortical transmittances in brown sun-exposed and pale shade-adapted forest C. islandica thalli showed that fungal melanins strongly absorb both UV-B and photosynthetically active radiation (PAR). For Xanthoria elegans cortical UV-B absorbing pigments, mainly the orange parietin, were extracted and quantified. Field experiments with extracted, parietin-deficient X. elegans thalli cultivated under various filters showed that UV-B was essential for the induction of parietin synthesis. The parietin resynthesis in these parietin-deficient samples increased with decreasing latitude of their location in which they had been sampled, which may imply that the synthesis of pigments is habitat specific. However, no latitudinal gradient in cortical screening capacity was detected for any of the two species investigated in the field. This implies that Arctic populations maintain a high level of screening pigments in spite of low ambient UV-B, and that the studied lichen species presumably may tolerate an increase in UV-B radiation due to the predicted thinning of the ozone layer over polar areas     

UV triggers the synthesis of the widely distributed secondary lichen compound usnic acid

Synthesis of the cortical dibenzofuran derivative usnic acid and the medullary depsidone salazinic acid was studied in Xanthoparmelia stenophylla thalli from which the compounds had been removed by acetone rinsing prior to a 21-day field experiment with UV absorbing and transmitting screens. Natural levels of ultraviolet radiation clearly induced the re-synthesis of usnic acid. The re-synthesis was boosted by the addition of ribitol, the carbohydrate delivered from the Trebouxia photobiont to the mycobiont. Salazinic acid was also weakly induced by UV. Re-synthesis was relatively low, up to 2.5 and 3.1% of start values for usnic and salazinic acid, respectively. However, given that the natural content of both compounds was high, constituting 12% of thallus dry weight, the absolute amounts of lichen compounds re-synthesised were not so small. We also studied the extractability of nine extracellular lichen compounds in three species X. stenophylla, Hypogymnia tubulosa, and Vulpicida pinastri, and found two distinct fractions of cortical compounds, one major that was completely extractable from living lichens and one minor that was only extractable with grinding. Medullary compounds were completely extracted without grinding. These findings did not influence the relative differences between treatments in our experiment, but may be of importance for future assessments of, e.g., quantitative studies of extracellular lichen compounds.     

Curling during desiccation protects the foliose lichen Lobaria pulmonaria against photoinhibition

This study aims to assess the photoprotective potential of desiccation-induced curling in the light-susceptible old forest lichen Lobaria pulmonaria by using chlorophyll fluorescence imaging. Naturally curled thalli showed less photoinhibition-induced limitations in primary processes of photosynthesis than artificially flattened specimens during exposures to 450 μmol m⁻² s⁻¹ in the laboratory after both 12- (medium dose treatment) and 62-h duration (high dose treatment). Thallus areas shaded by curled lobes during light exposure showed unchanged values of measured chlorophyll fluorescence parameters (F _V/F _M, Φ_{PS II}), whereas non-shaded parts of curled thalli, as well as the mean for the entire flattened thalli, showed photoinhibitory limitation after light treatments. Furthermore, the chlorophyll fluorescence imaging showed that the typical small-scale reticulated ridges on the upper side of L. pulmonaria caused a spatial, small-scale reduction in damage due to minor shading. Severe dry-state photoinhibition readily occurred in flattened and light-treated L. pulmonaria, although the mechanisms for such damage in a desiccated and inactive stage are not well known. Natural curling is one strategy to reduce the chance for serious photoinhibition in desiccated L. pulmonaria thalli during high light exposures.     

Copepods act as a switch between alternative trophic cascades in marine pelagic food webs

A recent meta‐analysis indicates that trophic cascades (indirect effects of predators on plants via herbivores) are weak in marine plankton in striking contrast to freshwater plankton ( Shurin et al. 2002 , Ecol. Lett., 5, 785–791). Here we show that in a marine plankton community consisting of jellyfish, calanoid copepods and algae, jellyfish predation consistently reduced copepods but produced two distinct, opposite responses of algal biomass. Calanoid copepods act as a switch between alternative trophic cascades along food chains of different length and with counteracting effects on algal biomass. Copepods reduced large algae but simultaneously promoted small algae by feeding on ciliates. The net effect of jellyfish on total algal biomass was positive when large algae were initially abundant in the phytoplankton, negative when small algae were dominant, but zero when experiments were analysed in combination. In contrast to marine systems, major pathways of energy flow in Daphnia‐dominated freshwater systems are of similar chain length. Thus, differences in the length of alternative, parallel food chains may explain the apparent discrepancy in trophic cascade strength between freshwater and marine planktonic systems.