Short-term responses of phytoplankton to nutrient enrichment and planktivorous fish predation in a temperate South American mesotrophic reservoir

The benthic macroinvertebrate community is an important component of stream diversity, because its members are fundamental connectors among the different trophic levels of running waters. In this study, we assessed alpha and beta diversities of benthic macroinvertebrates in three stream sites and four microhabitats: (i) moss in the air-water interface; (ii) submerged roots of terrestrial plants; (iii) leaf litter deposited in pools; (iv) stones in riffles. We constructed rarefaction curves and compared species richness among microhabitats for each stream site. Additionally, we evaluated which factor, stream site, or microhabitat, was most important in determining variation in assemblage structure, i.e., beta diversity. There was no significant difference among microhabitats in terms of taxa richness evaluated by rarefaction curves. Using partial Constrained Correspondence Analysis (pCCA), we found that microhabitat was most important in determining community composition, accounting for 42.02% of the total variation. Stream sites accounted for 22.27%. In accordance with the pCCA, exploratory multivariate methods (ordination and classification) revealed four distinct groups, corresponding to the four microhabitats, independent of stream sites. Our results indicated that differences among environmental conditions are much more important in the determination of stream assemblage structure than are differences in spatial location. Accordingly, adjacent microhabitats in a single stream site harbor macroinvertebrate assemblages more dissimilar than those found in a single microhabitat at different stream sites. Handling editor: D. Dudgeon     

Comparison of fungal communities associated with spruce seedling roots and bryophyte carpets on logs in an old-growth subalpine coniferous forest in Japan

Fungal communities associated with plant tissues were compared between two bryophyte species dominating decaying logs (Scapania bolanderi and Pleurozium schreberi), and roots of spruce seedlings growing on the bryophytes and in the ground soil, to evaluate the contribution of fungal communities to seedling regeneration. Using high-throughput DNA sequencing, a total of 1233 fungal operational taxonomic units (OTUs) were detected. Saprotrophic Ascomycota were dominant in bryophytes, whereas ectomycorrhizal (ECM) Basidiomycota were dominant in spruce roots. Fungal communities were significantly different between the two bryophyte species. In addition, fungal communities of spruce seedlings were significantly affected by the substrates on which they were growing. Some ECM fungi were detected from both of the bryophytes and the spruce seedlings growing on them; however, the dominant OTU identities differed between the two bryophyte systems. The possible effects of functional differences between dominant fungal OTUs on spruce seedling regeneration are discussed.     

ABA-induced tolerance to ion leakage during rehydration following desiccation in the moss Atrichum androgynum

A simple ion leakage assay was used to investigate the effect ofabscisic acid (ABA) pretreatment on desiccation tolerance in the mossAtrichum androgynum. Results from experiments involvingtheapplication of the protein synthesis inhibitor cyclohexamide during desiccationor rehydration suggested that pretreatment with ABA does not facilitate theproduction of protection or repair proteins duringdesiccation or rehydration. Rather, ABA induces the synthesis of these proteinsduring pretreatment. The ABA-induced increase in tolerance was much less ifplants were pretreated in the dark. Exposure to red light could not substitutefor white light, suggesting that ABA action does not require phytochrome.Desiccating intact long stem segments or long segments separated into apicalandbasal parts had no effect on the desiccation tolerance of either plant part.This suggests that no movement of signals or protective molecules occurs fromthe stem bases to the apices during desiccation. Basal stem segments were muchmore sensitive to desiccation than apical regions, suggesting during senescencea breakdown in mechanisms that protect mosses from injury occurs. Theimplications of these findings for the water relations of A.androgynum are discussed.     

Field research on the effects of UV-B filters on terrestrial Antarctic vegetation

Patches of vegetation of six common species growing on Léonie Island (67°35 S, 68°20 W), Antarctic Peninsula region were covered with either UV-B transparent perspex screens or UV-B absorbing screens. Uncovered plots served as a control. Temperature and relative humidity were monitored during the austral summer under and outside the screens. The mean effective PSII quantum efficiency showed significant differences among the species, but not between the UV-B treatments. It was concluded that the temperature and the moisture status of the vegetation obscured any possible influence of UV-B treatment on the tteffective PSII quantum efficiency. he usefulness of various UV-B exclusion and supplementation methods used to study the influence of UV-B in the field is discussed.     

Diversification of ftsZ During Early Land Plant Evolution

The plastid division proteins FtsZ are encoded by a small nuclear gene family in land plants. Although it has been shown for some of the gene products that they are imported into plastids and function in plastid division, the evolution and function of this gene family and their products remain to be unraveled. Here we present two new ftsZ genes from the moss Physcomitrella patens and compare the genomic structure of members of the two plant ftsZ gene families. Comparison of sequence features and phylogenetic analyses confirm the presence of two clusters of paralogues in land plants and demonstrate that these genes were duplicated before the divergence of mosses, ferns and seed plants.     

Age and size at maturity in mountain and lowland populations of the expanding moss <Emphasis Type="Italic">Pogonatum dentatum</Emphasis>

The moss Pogonatum dentatum has expanded its distribution in Fennoscandia from mountainous areas into the lowlands. This recent expansion appears to be associated with changes in important life-history parameters in female shoots. We examined shoot age and size at first production of sex organs and mature spores in P. dentatum to investigate this phenomenon. Female shoots produced mature spores for the first time in the lowlands in their second year but in their third year in the mountains. However, sex organs were produced by second year plants in both areas. There was no size difference between the mountain and lowland female shoots at the time of spore production. Among mountain females reproducing for the first time, 41% of the shoots branched, making them potentially ‘iteroparous’. Branching was not observed among lowland females. Male shoots showed no difference in production of sex organs, and were produced by second year shoots in both areas. Female shoots in the lowlands have earlier spore production and exhibit ‘semelparous’ behaviour by not producing branches. This suggests that the lowland phenotypes of P. dentatum are more ‘invasive’ than the mountain phenotypes. Earlier studies showing high rates of diaspore establishment in lowland areas also support this observation.     

Impacts of warming and fertilization on nitrogen-fixing microbial communities in the Canadian High Arctic

The impacts of simulated climate change (warming and fertilization treatments) on diazotroph community structure and activity were investigated at Alexandra Fiord, Ellesmere Island, Canada. Open Top Chambers, which increased growing season temperatures by 1-3 degrees C, were randomly placed in a dwarf-shrub and cushion-plant dominated mesic tundra site in 1995. In 2000 and 2001 20N:20P2O5:20K2O fertilizer was applied at a rate of 5 gm(-2) year(-1). Estimates of nitrogen fixation rates were made in the field by acetylene reduction assays (ARA). Higher rates of N fixation were observed 19-35 days post-fertilization but were otherwise unaffected by treatments. However, moss cover was significantly positively associated with ARA rate. NifH gene variants were amplified from bulk soil DNA and analyzed by terminal restriction fragment length polymorphism analysis. Non-metric multidimensional scaling was used to ordinate treatment plots in nifH genotype space. NifH gene communities were more strongly structured by the warming treatment late in the growing season, suggesting that an annual succession in diazotroph community composition occurs.     

Effects of biological soil crusts on emergence of desert vascular plants in North China

Biological soil crusts are a universal and common feature in arid and semi-arid regions and their appearance profoundly affects soil surface properties which may greatly change the seed dispersal, germination and establishment. To date, only a handful of experiments have exerted to investigate the effects of crusts on vascular plants and the conclusions from these studies are variable. In this study, we investigate the influences of two different crusts universally spreading in southeastern part of the Tengger Desert with four chronosequences (24, 41, 50 years old in artificial vegetation area and natural vegetation crusts) on vascular plants. Crusts were placed at three different sites to simulate different environmental factors (wind velocity and soil crust moisture), we set two soil moisture regimes for crusts to investigate how vascular plants respond under different moisture regimes in crusts. Emergence densities of vascular plants were significantly higher in moss crust than in algae crust. With the development of crusts, seed emergence increased in moss crust while decreased in algae crust. As for effects of moisture, our results showed that soil moisture had a significant effect on seed emergence in both types of crusts at all developing phases. Crusts with higher moisture had more seedlings than those with lower moisture. The above results indicated that the appearance of crusts changed the surface soil properties, which had greatly influenced the entrapment and lodgement of seeds in the study area, thus subsequently influence seed emergence through affecting natural factors.     

Actin filament array during side branch initiation in protonema cells of the mossFunaria hygrometrica: an actin organizing center at the plasma membrane

Summary With an improved method to visualize the actin filament system it is possible to detect a small, peculiar accumulation of actin filaments under the prospective area of side branch formation inFunaria protonema cells. It consists of a ring-like configuration of actin filaments from which filaments radiate, preferentially along the plasma membrane. During the transition to tip growth the arrangement becomes loosened and eventually disappears whereas the filaments are concentrated in inner regions of the cytoplasm with a maximum in the apical dome.     

Ultrastructural analysis of cell component distribution in the apical cell ofCeratodon protonemata

Summary A distinctive feature of tip-growing plant cells is that cell components are distributed differentially along the length of the cell, although most ultrastructural analyses have been qualitative. The longitudinal distribution of cell components was studied both qualitatively and quantitatively in the apical cell of dark-grown protonemata of the mossCeratodon. The first 35 m of the apical cell was analyzed stereologically using transmission electron microscopy. There were four types of distributions along the cell's axis, three of them differential: (1) tubular endoplasmic reticulum was evenly distributed, (2) cisternal endoplasmic reticulum and Golgi vesicles were distributed in a tip-to-base gradient, (3) plastids, vacuoles, and Golgi stacks were enriched in specific areas, although the locations of the enrichments varied, and (4) mitochondria were excluded in the tipmost 5 m and evenly distributed throughout the remaining 30 m. This study provides one of the most comprehensive quantitative, ultrastructural analyses of the distribution of cell components in the apex of any tip-growing plant cell. The finding that almost every component had its own spatial arrangement demonstrates the complexity of the organization and regulation of the distribution of components in tip-growing cells.Abbreviations CER cisternal endoplasmic reticulum - ER endoplasmic reticulum - N_d numerical density - SE standard error - S_v surface density - TEM transmission electron microscopy - TER tubular endoplasmic reticulum - V_v volume fraction