Related Articles

Background and Aims

A meta-analysis of global change experiments in arctic tundra sites suggests that plant productivity and the cover of shrubs, grasses and dead plant material (i.e. litter) will increase and the cover of bryophytes will decrease in response to higher air temperatures. However, little is known about which effects these changes in vegetation structure will have on seedling recruitment of species and invasibility of arctic ecosystems.

Methods

A field experiment was done in a bryophyte-dominated, species-rich subarctic heath by manipulating the cover of bryophytes and litter in a factorial design. Three phases of seedling recruitment (seedling emergence, summer seedling survival, first-year recruitment) of the grass Anthoxanthum alpinum and the shrub Betula nana were analysed after they were sown into the experimental plots.

Key Results

Bryophyte and litter removal significantly increased seedling emergence of both species but the effects of manipulations of vegetation structure varied strongly for the later phases of recruitment. Summer survival and first-year recruitment were significantly higher in Anthoxanthum. Although bryophyte removal generally increased summer survival and recruitment, seedlings of Betula showed high mortality in early August on plots where bryophytes had been removed.

Conclusions

Large species-specific variation and significant effects of experimental manipulations on seedling recruitment suggest that changes in vegetation structure as a consequence of global warming will affect the abundance of grasses and shrubs, the species composition and the susceptibility to invasion of subarctic heath vegetation.

There is widespread concern that anthropogenic global warming will trigger Arctic climate tipping points. The Arctic has a long history of natural, abrupt climate changes, which together with current observations and model projections, can help us to identify which parts of the Arctic climate system might pass future tipping points. Here the climate tipping points are defined, noting that not all of them involve bifurcations leading to irreversible change. Past abrupt climate changes in the Arctic are briefly reviewed. Then, the current behaviour of a range of Arctic systems is summarised. Looking ahead, a range of potential tipping phenomena are described. This leads to a revised and expanded list of potential Arctic climate tipping elements, whose likelihood is assessed, in terms of how much warming will be required to tip them. Finally, the available responses are considered, especially the prospects for avoiding Arctic climate tipping points.

Ultraviolet (UV) radiation has become an important stress factor in polar regions due to anthropogenically induced ozone depletion. Although extensive research has been conducted on adaptations of polar organisms to this stress factor, few studies have focused on semi-terrestrial algae so far, in spite of their apparent vulnerability. This study investigates the effect of UV on two semi-terrestrial arctic strains (B, G) and one Antarctic strain (E) of the green alga Zygnema, isolated from Arctic and Antarctic habitats. Isolates of Zygnema were exposed to experimentally enhanced UV A and B (predominant UV A) to photosynthetic active radiation (PAR) ratio. The pigment content, photosynthetic performance and ultrastructure were studied by means of high-performance liquid chromatography (HPLC), chlorophyll a fluorescence and transmission electron microscopy (TEM). In addition, phylogenetic relationships of the investigated strains were characterised using rbcL sequences, which determined that the Antarctic isolate (E) and one of the Arctic isolates (B) were closely related, while G is a distinct lineage. The production of protective phenolic compounds was confirmed in all of the tested strains by HPLC analysis for both controls and UV-exposed samples. Moreover, in strain E, the content of phenolics increased significantly (p = 0.001) after UV treatment. Simultaneously, the maximum quantum yield of photosystem II photochemistry significantly decreased in UV-exposed strains E and G (p < 0.001), showing a clear stress response. The phenolics were most probably stored at the cell periphery in vacuoles and cytoplasmic bodies that appear as electron-dense particles when observed by TEM after high-pressure freeze fixation. While two strains reacted moderately on UV exposure in their ultrastructure, in strain G, damage was found in chloroplasts and mitochondria. Plastidal pigments and xanthophyll cycle pigments were investigated by HPLC analysis; UV A- and UV B-exposed samples had a higher deepoxidation state as controls, particularly evident in strain B. The results indicate that phenolics are involved in UV protection of Zygnema and also revealed different responses to UV stress across the three strains, suggesting that other protection mechanisms may be involved in these organisms.

Background

Many studies investigating the ecosystem effects of global climate change have focused on arctic ecosystems because the Arctic is expected to undergo the earliest and most pronounced changes in response to increasing global temperatures, and arctic ecosystems are considerably limited by low temperatures and permafrost. In these nutrient limited systems, a warmer climate is expected to increase plant biomass production, primarily through increases in shrubs over graminoids and forbs. But, the influence of vertebrate and invertebrate herbivores has been largely absent in studies investigating the effects of vegetation responses to climate change, despite the fact that herbivory can have a major influence on plant community composition, biomass and nutrient cycling. Here, we present results from a multi-annual field experiment investigating the effects of vertebrate herbivory on plant biomass response to simulated climate warming in arctic Greenland.

Results

The results after four years of treatments did not give any clear evidence of increased biomass of shrubs in response climate warming. Nor did our study indicate that vertebrate grazing mediated any increased domination of shrubs over other functional plant groups in response to warming. However, our results indicate an important role of insect outbreaks on aboveground biomass. Intense caterpillar foraging from a two-year outbreak of the moth Eurois occulta during two growing seasons may have concealed any treatment effects. However, there was some evidence suggesting that vertebrate herbivores constrain the biomass production of shrubs over graminoids and forbs.

Conclusion

Although inconclusive, our results were likely constrained by the overwhelming influence of an unexpected caterpillar outbreak on aboveground biomass. It is likely that the role of large vertebrate herbivores in vegetation response to warming will become more evident as this experiment proceeds and the plant community recovers from the caterpillar outbreak. Due to the greater influence of invertebrate herbivory in this study, it is advisable to consider both the effect of invertebrate and vertebrate herbivores in studies investigating climate change effects on plant communities.

Triple staining of γ-tubulin, microtubules, and nuclei reveal that three types of MTOCs initiate spindles in bryophytes. Polar organizers in liverworts and plastid MTOCs in hornworts are unique and nuclear envelope MTOCs in mosses appear like those in seed plants.

Background and aims

As remnants of the earliest land plants, the bryophytes (liverworts, mosses and hornworts) are important in understanding microtubule organization in plant cells. Land plants have an anastral mitotic spindle that forms in the absence of centrosomes, and a cytokinetic apparatus comprised of a predictive preprophase band (PPB) before mitosis and a phragmoplast after mitosis. These microtubule arrays have no counterpart in animal cells and the nature of the plant microtubule organizing centre (MTOC) remained an enigma for many years until antibodies to γ-tubulin, an essential component of the MTOC in all eukaryotes, became available for tracing the origin of microtubule arrays.

Methodology

We used immunofluorescence techniques to colocalize γ-tubulin, microtubules and chromosomes in mitotic cells of a representative liverwort, moss and hornwort to study the organization of microtubules during mitotic cell division.

Principal results

The future division site is marked by a PPB in all taxa but the MTOCs initially generating the half spindles differ: polar organizers in the liverwort, plastid MTOCs in the hornwort, and nuclear envelope-associated MTOCs in the moss. By mid-prophase, the forming spindles become more similar as γ-tubulin begins to spread around the polar regions of the nuclear envelope.

Conclusions

Regardless of origin, mature metaphase spindles are identical and indistinguishable from the typical anastral spindle of higher plants with broad polar regions consisting of numerous subsets of converging microtubules. A curious phenomenon of plant spindles, true of bryophytes as well as higher plants, is the movement of γ-tubulin into the metaphase spindle itself. The bipolar arrays of phragmoplast microtubules are organized by diffuse γ-tubulin located at proximal surfaces of reforming nuclear envelopes. Phragmoplast development appears similar in the three taxa and to vascular plants as well.

Greening of the Arctic due to climate warming may provide herbivores with richer food supplies, resulting in higher herbivore densities. In turn, this may cause changes in vegetation composition and ecosystem function. In 1982–1984, we studied the ecology of non-breeding moulting geese in Jameson Land, low Arctic East Greenland. By then, geese consumed most of the graminoid production in available moss fens, and it appeared that the geese had filled up the available habitat. In 2008, we revisited the area and found that the number of moulting geese and the temperature sum for June–July had tripled, while the above-ground biomass in a moss fen ungrazed by geese had more than doubled. In a goose-grazed fen, the overall plant composition was unchanged, but the frequency of graminoids had decreased and the area with dead vegetation and open spots had increased. We suggest that climate warming has lead to increased productivity, allowing for higher numbers of moulting geese. However, the reduction of vegetation cover by grazing may have longer term negative consequences for the number of geese the habitat can sustain.

The two-branched inflow of warm and saline Atlantic Water to the Arctic is the major contributor of oceanic heat to the Arctic climate system. However, while the Atlantic Water entering the Arctic through the Fram Strait retains a large part of its heat as it flows along the Arctic continental slope, the branch flowing through the shallow Barents Sea releases a substantial amount of heat to the atmosphere. Hence, the pathway of the Atlantic Water into the Arctic to a large degree determines the short term fate of its heat. Here we show events in which the relative strengths of the two branches are affected by wind-induced Ekman-transport off the northern Barents Sea shelf. The resulting decrease in sea surface height induces a cyclonic circulation anomaly along the slope encircling the northern Barents Sea shelf area, which enhances the flow through the Barents Sea while weakening the branch flowing along the Arctic continental slope.

The branched inflow of warm Atlantic Water to the Arctic has been known for more than a hundred years, yet what controls the relative strengths of the two pathways remains poorly understood. Here, the authors identify the role of atmospheric circulation over the northern Barents Sea in controlling inflow.

The Arctic marine ecosystem contains multiple elements that present alternative states. The most obvious of which is an Arctic Ocean largely covered by an ice sheet in summer versus one largely devoid of such cover. Ecosystems under pressure typically shift between such alternative states in an abrupt, rather than smooth manner, with the level of forcing required for shifting this status termed threshold or tipping point. Loss of Arctic ice due to anthropogenic climate change is accelerating, with the extent of Arctic sea ice displaying increased variance at present, a leading indicator of the proximity of a possible tipping point. Reduced ice extent is expected, in turn, to trigger a number of additional tipping elements, physical, chemical, and biological, in motion, with potentially large impacts on the Arctic marine ecosystem.

Species traits, such as breeding system, phylum and growth form and habitat characteristics are shown to influence reproductive performance of liverworts and mosses in the Brazilian Atlantic Rainforest, and drive life-history differentiation among species and populations.

Background and aims

Short life cycles and trade-offs linked to breeding systems make bryophytes good models for the study of plant reproductive strategies. Our aim was to test if differences in sexual reproductive performance of bryophytes in tropical rainforests are driven by the breeding system of the species (monoicous or dioicous) or are mainly affected by the habitat.

Methodology

The reproductive performance (sexual branches, gametangia (sex organs), fertilization and sporophyte production) of 11 species was repeatedly monitored and analysed from populations at sea-level and montane sites of a Brazilian Atlantic rainforest over 15 months.

Principal results

Monoicous species had the highest reproductive performance, particularly for sexual branches, fertilized gametangia and sporophyte production. Species at the sea-level site produced more sexual branches and had more female-biased sex ratios of gametangia than species in the montane site. Fertilizations were more frequent at the montane site, but sporophyte frequency was similar between the two sites. Fertilization tended to occur mostly in the periods of heavy rain (October to December).

Conclusions

Breeding system is not the only major influence on the reproductive performance of bryophytes. We show that habitat is also an important factor determining life-history differentiation. Female-biased sex ratios and low rates of fertilization are seen to be compensated for by high production of reproductive structures at the initial phases of the reproductive cycle.

Background

Recent decades have seen a major surge in the study of interspecific variation in functional traits in comparative plant ecology, as a tool to understanding and predicting ecosystem functions and their responses to environmental change. However, this research has been biased almost exclusively towards vascular plants. Very little is known about the role and applicability of functional traits of non-vascular cryptogams, particularly bryophytes and lichens, with respect to biogeochemical cycling. Yet these organisms are paramount determinants of biogeochemistry in several biomes, particularly cold biomes and tropical rainforests, where they: (1) contribute substantially to above-ground biomass (lichens, bryophytes); (2) host nitrogen-fixing bacteria, providing major soil N input (lichens, bryophytes); (3) control soil chemistry and nutrition through the accumulation of recalcitrant polyphenols (bryophytes) and through their control over soil and vegetation hydrology and temperatures; (4) both promote erosion (rock weathering by lichens) and prevent it (biological crusts in deserts); (5) provide a staple food to mammals such as reindeer (lichens) and arthropodes, with important feedbacks to soils and biota; and (6) both facilitate and compete with vascular plants.

Approach

Here we review current knowledge about interspecific variation in cryptogam traits with respect to biogeochemical cycling and discuss to what extent traits and measuring protocols needed for bryophytes and lichens correspond with those applied to vascular plants. We also propose and discuss several new or recently introduced traits that may help us understand and predict the control of cryptogams over several aspects of the biogeochemistry of ecosystems.

Conclusions

Whilst many methodological challenges lie ahead, comparative cryptogam ecology has the potential to meet some of the important challenges of understanding and predicting the biogeochemical and climate consequences of large-scale environmental changes driving shifts in the cryptogam components of vegetation composition.

The absorption of ultraviolet (UV) light by stratospheric ozone is crucial to the provision of an environment suitable for terrestrial life. Ultraviolet radiation is the part of the solar spectrum with wavelengths between 240 and 400 nm. Photons at wavelengths below about 320 nm can produce photochemical damage to the cells of living organisms. Data suggesting a causative role for UV exposure in squamous cell carcinoma, basal cell carcinoma, melanoma, cataract formation, and (as yet) poorly defined immune suppression comes from epidemiologic studies and, in some cases, clinical observation and experimental work. The author concentrates on the direct health effects of UV radiation, although the factors that lead to stratospheric ozone depletion also exert other powerful influences on the biosphere that will have less predictable direct and indirect effects on human health.

• Background Depletion of the stratospheric ozone layer leads to an increase in ultraviolet-B (UVB: 280–320 nm) radiation reaching the earth's surface, and the enhanced solar UVB radiation predicted by atmospheric models will result in reduction of growth and yield of crops in the future. Over the last two decades, extensive studies of the physiological, biochemical and morphological effects of UVB in plants, as well as the mechanisms of UVB resistance, have been carried out.

• Scope In this review, we describe recent research into the mechanisms of UVB resistance in higher plants, with an emphasis on rice (Oryza sativa), one of the world's most important staple food crops. Recent studies have brought to light the following remarkable findings. UV-absorbing compounds accumulating in the epidermal cell layers have traditionally been considered to function as UV filters, and to play an important role in countering the damaging effects of UVB radiation. Although these compounds are effective in reducing cyclobutane pyrimidine dimer (CPD) induction in plants exposed to a challenge exposure to UVB, certain levels of CPD are maintained constitutively in light conditions containing UVB, regardless of the quantity or presence of visible light. These findings imply that the systems for repairing DNA damage and scavenging reactive oxygen species (ROS) are essential for plants to grow in light conditions containing UVB.

• Conclusion CPD photolyase activity is a crucial factor determining the differences in UVB sensitivity between rice cultivars. The substitution of one or two bases in the CPD photolyase gene can alter the activity of the enzyme, and the associated resistance of the plant to UVB radiation. These findings open up the possibility, in the near future, of increasing the resistance of rice to UVB radiation, by selective breeding or bioengineering of the genes encoding CPD photolyase.

Profiling the transcriptional response of maize tissues to UV-B irradiation suggests that a signal is transmitted from irradiated to shielded tissue. The transcriptional response occurs rapidly, even in shielded tissue.

Background

Depletion of stratospheric ozone has raised terrestrial levels of ultraviolet-B radiation (UV-B), an environmental change linked to an increased risk of skin cancer and with potentially deleterious consequences for plants. To better understand the processes of UV-B acclimation that result in altered plant morphology and physiology, we investigated gene expression in different organs of maize at several UV-B fluence rates and exposure times.

Results

Microarray hybridization was used to assess UV-B responses in directly exposed maize organs and organs shielded by a plastic that absorbs UV-B. After 8 hours of high UV-B, the abundance of 347 transcripts was altered: 285 were increased significantly in at least one organ and 80 were downregulated. More transcript changes occurred in directly exposed than in shielded organs, and the levels of more transcripts were changed in adult compared to seedling tissues. The time course of transcript abundance changes indicated that the response kinetics to UV-B is very rapid, as some transcript levels were altered within 1 hour of exposure.

Conclusions

Most of the UV-B regulated genes are organ-specific. Because shielded tissues, including roots, immature ears, and leaves, displayed altered transcriptome profiles after exposure of the plant to UV-B, some signal(s) must be transmitted from irradiated to shielded tissues. These results indicate that there are integrated responses to UV-B radiation above normal levels. As the same total UV-B irradiation dose applied at three intensities elicited different transcript profiles, the transcriptome changes exhibit threshold effects rather than a reciprocal dose-effect response. Transcriptome profiling highlights possible signaling pathways and molecules for future research.

Background

Dryas octopetala is a widespread dwarf shrub in alpine and arctic regions that forms ectomycorrhizal (ECM) symbiotic relationships with fungi. In this study we investigated the fungal communities associated with roots of D. octopetala in alpine sites in Norway and in the High Arctic on Svalbard, where we aimed to reveal whether the fungal diversity and species composition varied across the Alpine and Arctic regions. The internal transcribed spacer (ITS) region of nuclear ribosomal DNA was used to identify the fungal communities from bulk root samples obtained from 24 plants.

Results

A total of 137 operational taxonomic units (OTUs) were detected (using 97% similarity cut off during sequence clustering) and well-known ECM genera such as Cenococcum, Cortinarius, Hebeloma, Inocybe and Tomentella occurred frequently. There was no decrease in fungal diversity with increasing latitude. The overall spatial heterogeneity was high, but a weak geographical structuring of the composition of OTUs in the root systems was observed. Calculated species accumulation curves did not level off.

Conclusions

This study indicates that the diversity of fungi associated with D. octopetala does not decrease in high latitude arctic regions, which contrasts observations made in a wide spectrum of other organism groups. A high degree of patchiness was observed across root systems, but the fungal communities were nevertheless weakly spatially structured. Non-asymptotical species accumulation curves and the occurrence of a high number of singletons indicated that only a small fraction of the fungal diversity was detected.

Continental runoff is a major source of freshwater, nutrients and terrigenous material to the Arctic Ocean. As such, it influences water column stratification, light attenuation, surface heating, gas exchange, biological productivity and carbon sequestration. Increasing river discharge and thawing permafrost suggest that the impacts of continental runoff on these processes are changing. Here, a new optical proxy was developed and implemented with remote sensing to determine the first pan-Arctic distribution of terrigenous dissolved organic matter (tDOM) and continental runoff in the surface Arctic Ocean. Retrospective analyses revealed connections between the routing of North American runoff and the recent freshening of the Canada Basin, and indicated a correspondence between climate-driven changes in river discharge and tDOM inventories in the Kara Sea. By facilitating the real-time, synoptic monitoring of tDOM and freshwater runoff in surface polar waters, this novel approach will help understand the manifestations of climate change in this remote region.

This study was conducted in the Swedish sub-Arctic, near Abisko, in order to assess the direction and scale of possible vegetation changes in the alpine–birch forest ecotone. We have re-surveyed shrub, tree and vegetation data at 549 plots grouped into 61 clusters. The plots were originally surveyed in 1997 and re-surveyed in 2010. Our study is unique for the area as we have quantitatively estimated a 19% increase in tree biomass mainly within the existing birch forest. We also found significant increases in the cover of two vegetation types—“birch forest-heath with mosses” and “meadow with low herbs”, while the cover of snowbed vegetation decreased significantly. The vegetation changes might be caused by climate, herbivory and past human impact but irrespective of the causes, the observed transition of the vegetation will have substantial effects on the mountain ecosystems.

Background

The reduced concentration of stratospheric ozone results in an increased flux of biologically damaging mid-ultraviolet radiation (UVB, 280 to 320 nm) reaching earth surfaces. Environmentally relevant levels of UVB negatively impact various natural populations of marine organisms, which is ascribed to suppressed embryonic development by increased radiation.

Methodology/Principal Findings

Inspired by strategies in the living systems generated by evolution, we induce an extra UVB-adsorbed coat on the chorion (eggshell surrounding embryo) of zebrafish, during the blastula period. Short and long UV exposure experiments show that the artificial mineral-shell reduces the UV radiation effectively and the enclosed embryos become more robust. In contrast, the uncoated embryos cannot survive under the enhanced UVB condition.

Conclusions

We suggest that an engineered shell of functional materials onto biological units can be developed as a strategy to shield lives to counteract negative changes of global environment, or to provide extra protection for the living units in biological research.

As the oldest extant lineages of land plants, bryophytes provide a living laboratory in which to evaluate morphological adaptations associated with early land existence. In this paper we examine reproductive and structural innovations in the gametophyte and sporophyte generations of hornworts, liverworts, mosses and basal pteridophytes. Reproductive features relating to spermatogenesis and the architecture of motile male gametes are overviewed and evaluated from an evolutionary perspective. Phylogenetic analyses of a data set derived from spermatogenesis and one derived from comprehensive morphogenetic data are compared with a molecular analysis of nuclear and mitochondrial small subunit rDNA sequences. Although relatively small because of a reliance on water for sexual reproduction, gametophytes of bryophytes are the most elaborate of those produced by any land plant. Phenotypic variability in gametophytic habit ranges from leafy to thalloid forms with the greatest diversity exhibited by hepatics. Appendages, including leaves, slime papillae and hairs, predominate in liverworts and mosses, while hornwort gametophytes are strictly thalloid with no organized external structures. Internalization of reproductive and vegetative structures within mucilage-filled spaces is an adaptive strategy exhibited by hornworts. The formative stages of gametangial development are similar in the three bryophyte groups, with the exception that in mosses apical growth is intercalated into early organogenesis, a feature echoed in moss sporophyte ontogeny. A monosporangiate, unbranched sporophyte typifies bryophytes, but developmental and structural innovations suggest the three bryophyte groups diverged prior to elaboration of this generation. Sporophyte morphogenesis in hornworts involves non-synchronized sporogenesis and the continued elongation of the single sporangium, features unique among archegoniates. In hepatics, elongation of the sporophyte seta and archegoniophore is rapid and requires instantaneous wall expandability and hydrostatic support. Unicellular, spiralled elaters and capsule dehiscence through the formation of four regular valves are autapomorphies of liverworts. Sporophytic sophistications in the moss clade include conducting tissue, stomata, an assimilative layer and an elaborate peristome for extended spore dispersal. Characters such as stomata and conducting cells that are shared among sporophvtes of mosses, hornworts and pteridophytes are interpreted as parallelisms and not homologies. Our phylogenetic analysis of three different data sets is the most comprehensive to date and points to a single phylogenetic solution for the evolution of basal embryophytes. Hornworts are supported as the earliest divergent embryophyte clade with a moss/liverwort clade sister to tracheophytes. Among pteridophytes, lycophytes are monophyletic and an assemblage containing ferns, Equisetum and psilophytes is sister to seed plants. Congruence between morphological and molecular hypotheses indicates that these data sets are tracking the same phylogenetic signal and reinforces our phylogenetic conclusions. It appears that total evidence approaches are valuable in resolving ancient radiations such as those characterizing the evolution of early embryophytes. More information on land plant phylogeny can be found at: http: //www.science.siu.edu/ landplants/index.html.

Background and Aims

This study explores basic physiological features and time relations of recovery of photosynthetic activity and CO2 uptake following rehydration of a desiccation-tolerant moss in relation to the full temporal sequence of cytological changes associated with recovery to the normal hydrated state. It seeks reconciliation of the apparently conflicting published physiological and cytological evidence on recovery from desiccation in bryophytes.

Methods

Observations were made of water-stress responses and recovery using infrared gas analysis and modulated chlorophyll fluorescence, and of structural and ultrastructural changes by light and transmission electron microscopy.

Key Results

Net CO2 uptake fell to zero at approx. 40 % RWC, paralleling the fluorescence parameter ΦPSII at 200 µmol m–2 s–1 PPFD. On re-wetting the moss after 9–18 d desiccation, the initially negative net CO2 uptake became positive 10–30 min after re-wetting, restoring a net carbon balance after approx. 0·3–1 h. The parameter Fv/Fm reached approx. 80 % of its pre-desiccation value within approx. 10 min of re-wetting. In the presence of the protein-synthesis inhibitors chloramphenicol and cycloheximide, recovery of Fv/Fm (and CO2 exchange) proceeded normally in the dark, but declined rapidly in the light. Though initial recovery was rapid, both net CO2 uptake and Fv/Fm required approx. 24 h to recover completely to pre-desiccation values. The fixation protocols produced neither swelling of tissues nor plasmolysis. Thylakoids, grana and mitochondrial cristae remained intact throughout the drying–re-wetting cycle, but there were striking changes in the form of the organelles, especially the chloroplasts, which had prominent lobes and lamellar extensions in the normally hydrated state, but rounded off when desiccated, returning slowly to their normal state within approx. 24 h of re-wetting. Sub-cellular events during desiccation and re-wetting were generally similar to those seen in published data from the pteridophyte Selaginella lepidophylla.

Conclusions

Initial recovery of respiration and photosynthesis (as of protein synthesis) is very rapid, and independent of protein synthesis, suggesting physical reactivation of systems conserved intact through desiccation and rehydration, but full recovery takes approx. 24 h. This is consistent with the cytological evidence, which shows the thylakoids and cristae remaining intact through the whole course of dehydration and rehydration. Substantial and co-ordinated changes in other cell components, which must affect spatial relationships of organelles and metabolic systems, return to normal on a time span similar to full recovery of photosynthesis. Comparison of the present data with recently published results suggests a significant role for the cytoskeleton in desiccation responses.

A colonisation experiment was performed in situ at 2500 m water depth at the Arctic deep-sea long-term observatory HAUSGARTEN to determine the response of deep-sea nematodes to disturbed, newly available patches, enriched with organic matter. Cylindrical tubes,laterally covered with a 500 µm mesh, were filled with azoic deep-sea sediment and 13C-labelled food sources (diatoms and bacteria). After 10 days of incubation the tubes were analysed for nematode response in terms of colonisation and uptake. Nematodes actively colonised the tubes,however with densities that only accounted for a maximum of 2.13% (51 ind.10 cm−2) of the ambient nematode assemblages. Densities did not differ according to the presence or absence of organic matter, nor according to the type of organic matter added. The fact that the organic matter did not function as an attractant to nematodes was confirmed by the absence of notable 13C assimilation by the colonising nematodes. Overall, colonisationappears to be a process that yields reproducible abundance and diversity patterns, with certain taxa showing more efficiency. Together with the high variability between the colonising nematode assemblages, this lends experimental support to the existence of a spatio-temporal mosaic that emerges from highly localised, partially stochastic community dynamics.

While bryophytes greatly contribute to plant diversity of semi-natural grasslands, little is known about the relationships between land-use intensity, productivity, and bryophyte diversity in these habitats. We recorded vascular plant and bryophyte vegetation in 85 agricultural used grasslands in two regions in northern and central Germany and gathered information on land-use intensity. To assess grassland productivity, we harvested aboveground vascular plant biomass and analyzed nutrient concentrations of N, P, K, Ca and Mg. Further we calculated mean Ellenberg indicator values of vascular plant vegetation. We tested for effects of land-use intensity and productivity on total bryophyte species richness and on the species richness of acrocarpous (small & erect) and pleurocarpous (creeping, including liverworts) growth forms separately. Bryophyte species were found in almost all studied grasslands, but species richness differed considerably between study regions in northern Germany (2.8 species per 16 m2) and central Germany (6.4 species per 16 m2) due environmental differences as well as land-use history. Increased fertilizer application, coinciding with high mowing frequency, reduced bryophyte species richness significantly. Accordingly, productivity estimates such as plant biomass and nitrogen concentration were strongly negatively related to bryophyte species richness, although productivity decreased only pleurocarpous species. Ellenberg indicator values for nutrients proved to be useful indicators of species richness and productivity. In conclusion, bryophyte composition was strongly dependent on productivity, with smaller bryophytes that were likely negatively affected by greater competition for light. Intensive land-use, however, can also indirectly decrease bryophyte species richness by promoting grassland productivity. Thus, increasing productivity is likely to cause a loss of bryophyte species and a decrease in species diversity.

Background

Major changes in climate have been observed in the Arctic and climate models predict further amplification of the enhanced greenhouse effect at high-latitudes leading to increased warming. We propose that warming in the Arctic may affect the annual growth conditions of the cold adapted Arctic charr and that such effects can already be detected retrospectrally using otolith data.

Results

Inter-annual growth of the circumpolar Arctic charr (Salvelinus alpinus, L.) was analysed in relation to climatic changes observed in the Arctic during the last two decades. Arctic charr were sampled from six locations at Qeqertarsuaq in West Greenland, where climate data have been recorded since 1990. Two fish populations met the criteria of homogeny and, consequently, only these were used in further analyses. The results demonstrate a complex coupling between annual growth rates and fluctuations in annual mean temperatures and precipitation. Significant changes in temporal patterns of growth were observed between cohorts of 1990 and 2004.

Conclusion

Differences in pattern of growth appear to be a consequence of climatic changes over the last two decades and we thereby conclude that climatic affects short term and inter-annual growth as well as influencing long term shifts in age-specific growth patterns in population of Arctic charr.

Solar ultraviolet (UV) radiation is widely known as a genotoxic environmental agent that affects Earth ecosystems and the human population. As a primary consequence of the stratospheric ozone layer depletion observed over the last decades, the increasing UV incidence levels have heightened the concern regarding deleterious consequences affecting both the biosphere and humans, thereby leading to an increase in scientific efforts to understand the role of sunlight in the induction of DNA damage, mutagenesis, and cell death. In fact, the various UV-wavelengths evoke characteristic biological impacts that greatly depend on light absorption of biomolecules, especially DNA, in living organisms, thereby justifying the increasing importance of developing biological sensors for monitoring the harmful impact of solar UV radiation under various environmental conditions. In this review, several types of biosensors proposed for laboratory and field application, that measure the biological effects of the UV component of sunlight, are described. Basically, the applicability of sensors based on DNA, bacteria or even mammalian cells are presented and compared. Data are also presented showing that on using DNA-based sensors, the various types of damage produced differ when this molecule is exposed in either an aqueous buffer or a dry solution. Apart from the data thus generated, the development of novel biosensors could help in evaluating the biological effects of sunlight on the environment. They also emerge as alternative tools for using live animals in the search for protective sunscreen products.

Solar ultraviolet radiation creates an ozone layer in the atmosphere which in turn completely absorbs the most energetic fraction of this radiation. This process both warms the air, creating the stratosphere between 15 and 50 km altitude, and protects the biological activities at the Earth's surface from this damaging radiation. In the last half-century, the chemical mechanisms operating within the ozone layer have been shown to include very efficient catalytic chain reactions involving the chemical species HO, HO2, NO, NO2, Cl and ClO. The NOX and ClOX chains involve the emission at Earth's surface of stable molecules in very low concentration (N2O, CCl2F2, CCl3F, etc.) which wander in the atmosphere for as long as a century before absorbing ultraviolet radiation and decomposing to create NO and Cl in the middle of the stratospheric ozone layer. The growing emissions of synthetic chlorofluorocarbon molecules cause a significant diminution in the ozone content of the stratosphere, with the result that more solar ultraviolet-B radiation (290–320 nm wavelength) reaches the surface. This ozone loss occurs in the temperate zone latitudes in all seasons, and especially drastically since the early 1980s in the south polar springtime—the ‘Antarctic ozone hole’. The chemical reactions causing this ozone depletion are primarily based on atomic Cl and ClO, the product of its reaction with ozone. The further manufacture of chlorofluorocarbons has been banned by the 1992 revisions of the 1987 Montreal Protocol of the United Nations. Atmospheric measurements have confirmed that the Protocol has been very successful in reducing further emissions of these molecules. Recovery of the stratosphere to the ozone conditions of the 1950s will occur slowly over the rest of the twenty-first century because of the long lifetime of the precursor molecules.

Analysis of in situ and satellite data shows evidence of different regional snow cover responses to the widespread warming and increasing winter precipitation that has characterized the Arctic climate for the past 40–50 years. The largest and most rapid decreases in snow water equivalent (SWE) and snow cover duration (SCD) are observed over maritime regions of the Arctic with the highest precipitation amounts. There is also evidence of marked differences in the response of snow cover between the North American and Eurasian sectors of the Arctic, with the North American sector exhibiting decreases in snow cover and snow depth over the entire period of available in situ observations from around 1950, while widespread decreases in snow cover are not apparent over Eurasia until after around 1980. However, snow depths are increasing in many regions of Eurasia. Warming and more frequent winter thaws are contributing to changes in snow pack structure with important implications for land use and provision of ecosystem services. Projected changes in snow cover from Global Climate Models for the 2050 period indicate increases in maximum SWE of up to 15% over much of the Arctic, with the largest increases (15–30%) over the Siberian sector. In contrast, SCD is projected to decrease by about 10–20% over much of the Arctic, with the smallest decreases over Siberia (<10%) and the largest decreases over Alaska and northern Scandinavia (30–40%) by 2050. These projected changes will have far-reaching consequences for the climate system, human activities, hydrology, and ecology.

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