Responses of bacterial and fungal communities to an elevation gradient in a subtropical montane forest of China

Bacteria and fungi are ecologically important contributors to various functioning of forest ecosystems. In this study, we examined simultaneously the bacterial and fungal distributions in response to elevation changes of a forest. By using clone library analysis from genomic DNA extracted from forest humic clay soils, the composition and diversity of bacterial and fungal communities were determined across an elevation gradient from low via medium to high, in a subtropical forest in the Mountain Lushan, China. Our results showed that soil water content and nutrient availability, specifically total carbon, differed significantly with elevation changes. Although the soil acidity did not differ significantly among the three sites, low pH (around 4) could be an important selection factor selecting for acidophilic Acidobacteria and Alphaproteobacteria, which were the most abundant bacterial clones. As the majority of the fungi recovered, both Basidiomycota and Ascomycota, and their relative abundance were most closely associated with the total carbon. Based on the Shannon–Weaver diversity index and ∫-libshuff analysis, the soil at medium elevation contained the highest diversity of bacteria compared with those at high and low elevations. However, it is difficult to predict overall fungal diversity along elevation. The extreme high soil moisture content which may lead to the formation of anaerobic microhabitats in the forest soils potentially reduces the overall bacterial and fungal diversity.     

Effect of cattle grazing on soil salinity and vegetation composition along an elevation gradient in a temperate coastal salt marsh of Samborombón Bay (Argentina)

Salt marshes of Samborombón Bay (Argentina) have been grazed sporadically at very low stocking rates, but in the last decade, grazing intensity increased due to agriculture expansion. We investigated the effect of cattle grazing on vegetation and soil salinity on the most extended Spartina densiflora community. This community develops along an elevation gradient where the frequency and duration of tidal flooding and soil salinity increased as elevation decreased. Vegetation and soil data were collected from a national park excluded to cattle grazing for 30 years and from an adjacent commercial livestock farm continuously grazed by cattle. As elevation level decreased, plant cover, richness and diversity of functional groups and species decreased. As we expected, grazing altered soil salinity and vegetation composition in different extent along the elevation gradient. Grazing changed vegetation structure more intensively in the high elevation level because it reduced the competitive exclusion exerted by S. densiflora, allowing the increase in floristic richness. Grazing increased soil salinity and the contribution of salt-tolerant species only in the medium but not in the low elevation level probably because the higher frequency and duration of tidal flooding counterbalanced the increase in evaporation promoted by biomass removal in the low respect to the medium elevation level. While grazing may cause positive impacts for plant conservation in the high elevation level, it may cause negative consequence for livestock production because of the reduction in forage quality along the entire elevation gradient.     

Three Gorges Dam: Impact of Water Level Changes on the Density of Schistosome-Transmitting Snail Oncomelania hupensis in Dongting Lake Area,China

BackgroundSchistosomiasis remains an important public health issue in China and worldwide. Oncomelania hupensis is the unique intermediate host of schistosoma japonicum, and its change influences the distribution of S. japonica. The Three Gorges Dam (TGD) has substantially changed the ecology and environment in the Dongting Lake region. This study investigated the impact of water level and elevation on the survival and habitat of the snails.MethodsData were collected for 16 bottomlands around 4 hydrological stations, which included water, density of living snails (form the Anxiang Station for Schistosomiasis Control) and elevation (from Google Earth). Based on the elevation, sixteen bottomlands were divided into 3 groups. ARIMA models were built to predict the density of living snails in different elevation areas.ResultsBefore closure of TGD, 7 out of 9 years had a water level beyond the warning level at least once at Anxiang hydrological station, compared with only 3 out of 10 years after closure of TGD. There were two severe droughts that happened in 2006 and 2011, with much fewer number of flooding per year compared with other study years. Overall, there was a correlation between water level changing and density of living snails variation in all the elevations areas. The density of living snails in all elevations areas was decreasing after the TGD was built. The relationship between number of flooding per year and the density of living snails was more pronounced in the medium and high elevation areas; the density of living snails kept decreasing from 2003 to 2014. In low elevation area however, the density of living snails decreased after 2003 first and turned to increase after 2011. Our ARIMA prediction models indicated that the snails would not disappear in the Dongting Lake region in the next 7 years. In the low elevation area, the density of living snails would increase slightly, and then stabilize after the year 2017. In the medium elevation region, the change of the density of living snails would be more obvious and would increase till the year 2020. In the high elevation area, the density of living snails would remain stable after the year 2015.ConclusionThe TGD influenced water levels and reduced the risk of flooding and the density of living snails in the study region. Based on our prediction models, the density of living snails in all elevations tends to be stabilized. Control of S. japonica would continue to be an important task in the study area in the coming decade.     

The use of medium point density LiDAR elevation data to determine plant community types in Baltic coastal wetlands

This study investigates the use of medium point density LiDAR (light detection and ranging) elevation data for identifying and locating plant community types in Baltic coastal wetlands. Ground-surveyed micro-topography data obtained using real-time kinematic differential GPS were used to analyse and correct the accuracy of LiDAR sourced elevation values in Matlab R2010a. Following the application of elevation corrections, 3 interpolation techniques were applied: Inverse Distance Weighting (IDW), Ordinary Kriging (OK) and the Delaunay Triangulated Irregular Network (TIN) interpolation. Analysis showed that the TIN method produced a more robust model that adequately represented the micro-topographical variation across the wetland. The TIN elevation interpolation was then used to characterise the extent and distribution of the plant communities using known elevation preferences in one coastal wetland site, Tahu. The plant community model developed from the TIN elevation interpolation was then further ground-truthed in two separate Baltic coastal wetland sites, Matsalu and Kudani, in order to test model validity.The results showed that the accuracy of the LiDAR data varied dependent upon plant community type. In 5 of the 6 studied community types, the LiDAR data gave a mean elevation difference of 0.177 m (0.016 SD) above the dGPS elevations. With dGPS elevation corrections, the LiDAR data produced an accurate model of the location of the different plant community types, Kappa coefficient (κ) 0.63, at the initial site, Tahu. The model was found to be very accurate in one of the subsequent ground-truthed sites, κ 0.81, with a similar management history, but less accurate, κ 0.53, in a wetland more exposed to wave action and with a longer history of vegetation management.The inclusion of dGPS ground survey data was shown to considerably improve the accuracy of the LiDAR elevation data to facilitate the identification of plant community types on the basis of elevation. The approach developed in this study of Baltic coastal wetlands may provide a transferable method for rapidly and accurately mapping plant communities in other open environments.     

Effects of ammonium treatment on the protein and chlorophyll contents and the number of ribosomes in Chlamydomonas reinhardtii cells

The effects of exogenous ammonium on total protein and chlorophyll contents and the number of ribosomes in Chlamydomonas reinhardtii (Sager) green alga cells were studied. The absence of ammonium from nutrient medium resulted in a twofold drop in the number of ribosomes in the cytoplasm. The chlorophyll content of the cells grown on ammonium-free nitrate-containing medium insignificantly increased. The increase in the ribosome number in the presence of exogenous ammonium was not accompanied by the elevation of total cellular protein and did not depend on the levels of mRNA of rps6, a small ribosome subunit protein, and the 18S rRNA. Possible reasons for the effect of exogenous ammonium during ribosome biogenesis are discussed.     

Therapeutic efficacy of differentiated versus undifferentiated mesenchymal stem cells in experimental type I diabetes in rat

Selective MSCs differentiation protocol into pancreatic beta cells was conducted in the present study using exendin-4 and TGF-beta. Differentiated and undifferentiated MSCs were assessed in experimental type I diabetes in rats. Ninety female white albino rats were included in the study and divided equally (n=15/group) into 6 groups: healthy control, healthy control rats received acellular tissue culture medium, diabetic rats, diabetic rats received acellular tissue culture medium, diabetic rats received undifferentiated MSCs and diabetic rats received differentiated MSCs. Therapeutic efficacy of undifferentiated versus differentiated MSCs was evaluated via assessment of quantitative gene expressions of insulin1, insulin 2, Smad-2, Smad-3, PDX-1, PAX-4, neuroD. Blood glucose and insulin hormone levels were also assessed. Results showed that quantitative gene expressions of all studied genes showed significant decrease in diabetic rat groups. Use of undifferentiated and differentiated MSCs led to a significant elevation of expression levels of all genes with more superior effect with differentiated MSCs except smad-2 gene. As regards insulin hormone levels, use of either undifferentiated or differentiated MSCs led to a significant elevation of its levels with more therapeutic effect with differentiated MSCs. Blood glucose levels were significantly decreased with both undifferentiated and differentiated MSCs in comparison to diabetic groups but its levels were normalized 2 months after injection of differentiated MSCs. In conclusion, use of undifferentiated or differentiated MSCs exhibited significant therapeutic potentials in experimental type I diabetes in rats with more significant therapeutic effect with the use of differentiated MSCs.     

Effect of Elicitation and Changes in Extracellular pH on the Cytoplasmic and Vacuolar pH of Suspension-Cultured Soybean Cells

We have employed both ³¹P nuclear magnetic resonance spectroscopy and two intracellular fluorescent pH indicator dyes to monitor the pH of the vacuole and cytoplasm of suspension-cultured soybean cells (Glycine max Merr cv Kent). For the ³¹P nuclear magnetic resonance studies, a flow cell was constructed that allowed perfusion of the cells in oxygenated growth medium throughout the experiment. When the perfusion medium was transiently adjusted to a pH higher than that of the ambient growth medium, a rapid elevation of vacuolar pH was observed followed by a slow (approximately 30 minute) return to near resting pH. In contrast, the concurrent pH changes in the cytoplasm were usually fourfold smaller. These data indicate that extracellular pH changes are rapidly communicated to the vacuole in soybean cells without significantly perturbing cytoplasmic pH. When elicitors were dissolved in a medium of altered pH and introduced into the cell suspension, the pH of the vacuole, as above, quickly reflected the pH of the added elicitor solution. In contrast, when the pH of either a polygalacturonic acid or Verticillium dahliae elicitor preparation was adjusted to the same pH as the ambient medium, no significant change in either vacuolar or cytoplasmic pH was observed during the 35 minute experiment. These results were confirmed in experiments with pH-sensitive fluorescent dyes. We conclude that suspension-cultured soybean cells do not respond to elicitation by significantly changing the pH of their vacuolar or cytoplasmic compartments.     

秦岭大熊猫栖息地巴山木竹生物量

巴山木竹是秦岭大熊猫冬、春季食物的主要来源,其生长状况、数量和空间分布格局与大熊猫的生存与活动有着密切的关系。为了探讨不同立地因子对巴山木竹生物量的影响,确定巴山木竹生长的主导因子和适宜立地条件,在秦岭大熊猫栖息地内选取54块样地,对不同立地条件下巴山木竹生物量进行了测定。通过统计分析,结果表明:由于不同地区气候和土壤条件的差异,导致了巴山木竹生物量亦存在较大差别;巴山木竹生物量在其分布区内随海拔的升高而呈规律性变化,1500—1700 m为最适分布区域,其生物量最大;坡度越大生物量越小,陡坡不适宜竹林的生长;上坡位不利于竹林生长,而中、下坡位生物量没有显著差异;坡向对生物量的影响不明显,均适宜巴山木竹生长;在落叶阔叶和针阔混交林中,当林冠郁闭度>0.6时,竹林生长减弱。在这些立地因子中,影响巴山木竹生物量积累的主导因子是坡位和林冠郁闭度。     

Calcium and cyclic AMP involvement in the regulation of juvenile hormone biosynthesis in Diploptera punctata

We have examined the effects of brain extracts on cAMP levels in the CA as well as their effect on JH biosynthesis in vitro. Brain extracts induced a dose-dependent elevation in intracellular cAMP levels (as measured by RIA) within 10 min of treatment. Significant (P < 0.05) elevations in cAMP were observed with concentrations of extract as low as 0.01 brain equivalents. Maximal elevation (120–140% over controls) was obtained with 0.25 brain equivalents and greater. Elevated extracellular calcium (10 mM) substantially reduced the inhibitory effects of brain extracts on JH biosynthesis in vitro when compared to normal controls at 1.3 mM calcium. High calcium had relatively little effect on basal cAMP levels in the CA. In contrast, the use of calcium-free medium with EGTA caused a dramatic increase in cAMP levels. High calcium did not affect the ability of the brain extracts or the ability of the adenylate cyclase activator forskolin to elevate cAMP levels. These results provide further evidence that cAMP acts as a second messenger for a putative allatostatin from the brain. As well, these results further suggest that both calcium-dependent and calcium-independent (involving cAMP) pathways function to reduce JH biosynthesis in the CA of D. punctata.     

Elevation,Temperature, and Aquatic Connectivity All Influence the Infection Dynamics of the Amphibian Chytrid Fungus in Adult Frogs

Infectious diseases can cause population declines and even extinctions. The amphibian chytrid fungus, Batrachochytrium dendrobatidis (Bd), has caused population declines and extinctions in amphibians on most continents. In the tropics, research on the dynamics of this disease has focused on amphibian populations in mountainous areas. In most of these areas, high and low elevation sites are connected by an assemblage of streams that may transport the infectious stage of the pathogen from high to low elevations, and, also, this pathogen, which grows well at cool temperatures, may persist better in cooler water flowing from high elevations. Thus, the dynamics of disease at low elevation sites without aquatic connections to higher elevation sites, i.e., non-contiguous low elevation sites, may differ from dynamics at contiguous low elevation sites. We sampled adult common mistfrogs (Litoria rheocola) at six sites of three types: two at high (> 400m) elevations, two at low elevations contiguous with high elevation streams, and two at low elevations non-contiguous with any high elevation site. Adults were swabbed for Bd diagnosis from June 2010 to June 2011 in each season, over a total of five sampling periods. The prevalence of Bd fluctuated seasonally and was highest in winter across all site types. Site type significantly affected seasonal patterns of prevalence of Bd. Prevalence remained well above zero throughout the year at the high elevation sites. Prevalence declined to lower levels in contiguous low sites, and reached near-zero at non-contiguous low sites. Patterns of air temperature fluctuation were very similar at both the low elevation site types, suggesting that differences in water connectivity to high sites may have affected the seasonal dynamics of Bd prevalence between contiguous and non-contiguous low elevation site types. Our results also suggest that reservoir hosts may be important in the persistence of disease at low elevations.     

Shoot demography and morphology of Zostera japonica and Ruppia maritima from British Columbia,Canada

Shoot dynamics and morphology were studied in co-occuring intertidal populations of Zostera japonica Aschers. and Graebn. and Ruppia maritima L. s.l. in southwestern British Columbia. Repeated mapping and examination of cohorts of shoots (ramets) in permanent plots on a gradient in elevation showed that the appearance and loss of shoots, age structure, survivorship and leaf and rhizome dimensions were all affected by position on the gradient. For both species, the area having the greatest exposure t o air had fewer shoots and a greater percentage of shoots flowering early in the season. The duration of vegetative growth in R. maritima was the same regardless of elevation, while plants of Z. japonica at high elevation initiated and ended flowering and entered a quiescent overwintering state earlier than plants at lower elevation.     

不同海拔高程梯度下鄱阳湖典型草洲植物群落物种-多度分布格局的模型拟合

基于恒湖农场茶叶港草洲4个样带、19个样地（5 m×5 m）沿海拔高程（高高程-湖边、中高程-洲滩中间、低高程-洲滩边缘）调查得到的植被数据,用4种生态位模型拟合研究该区域不同海拔高程段植物种-多度分布格局。结果表明：（1）随着海拔高程下降,群落偶见种不断减少,且平均盖度均较低;常见种数目保持不变,其中优势种灰化薹草（Carex cinerascens Kukenth）平均盖度、平均高度持续升高,在群落中保持较高的优势地位;（2）3个高程段内生态位优先占领模型对群落物种-多度分布格局有较高的拟合优度（P < 0.05,AIC值分别为-149.41、-61.11、-25.63）,断棍模型、重叠生态位模型、优势优先模型总体上拟合效果较差（P < 0.05,但AIC值均较大）。为研究区植物群落生物多样性的保护和构建机制的探索提供了一定参考。     

RNase R is a highly unstable protein regulated by growth phase and stress

RNase R is an important exoribonuclease that participates in the degradation of structured RNAs in Escherichia coli. In earlier work, it was shown that RNase R levels increase dramatically under certain stress conditions, particularly during cold shock and stationary phase. However, the regulatory processes that lead to this elevation are not well understood. We show here that the increase in RNase R in stationary phase is unaffected by the global regulators, RpoS and (p)ppGpp, and that it occurs despite a major reduction in rnr message. Rather, we find that RNase R is a highly unstable protein in exponential phase, with a half-life of ∼10 min, and that the protein is stabilized in stationary phase, leading to its relative increase. RNase R is also stabilized during cold shock and by growth in minimal medium, two other conditions that lead to its elevation. These data demonstrate that RNase R is subject to regulation by a novel, posttranslational mechanism that may have important implications for our complete understanding of RNA metabolism.     

In vivo and in vitro effects of epofenonane on juvenile hormone esterase activity in tissues of last stadium larvae of Trichoplusia ni

Topical application of the juvenoid, epofenonane, to last stadium postwandering larvae of Trichoplusia ni caused a precocious elevation of juvenile hormone esterase (JHE) activity that was tissue speific and time dependent. This increase in enzyme activity over controls was most dramatic in the hemolymph, whereas increases in the fat body were lower. Antibodies raised against JHE reacted on Western blots with a fat body and hemolymph protein present in epofenonane treated and untreated last stadium day 3 larvae. The abundance of this protein, which comigrated with JHE, closely coincided with the temporal increases in JHE catalytic activity that occurred in response to treatment in vivo with epofenonane.The presence of epofenonane (5–10,000 nM) in the medium at the start of fat body incubations failed to shift the temporal appearance of JHE activity or boost activity levels significantly over those of controls. If larvae were treated in vivo with epofenonane before fat body tissue was removed, only a small, but significant increase in JHE activity was found in vitro. The rate of enzyme secretion was insufficient to account for the rapid increases in enzyme activity that occur in the hemolymph in response to epofenonane, even though tissue held in vitro was deemed viable by monitoring lactate dehydrogenase activity in the medium, fat body intracellular ATP, and the incorporation of [³⁵S]methionine into fat body protein. Fat body tissue removed from various aged last stadium larvae released enzyme at different rates in vitro.     

Biochemical status of in vitro regenerated Lilium bosniacum and Lilium cattaniae plantlets

Lilium cattaniae (Vis.) Vis. and Lilium bosniacum (G. Beck) Beck ex Fritsch, endemic species of Balkan Dinaric Alps, were micropropagated from seeds collected from their natural habitats. The relationship between peroxidase activity, photosynthetic status and differentiation of Lilium cattaniae and L. bosniacum in vitro was investigated. Peroxidase activity recorded for somatic embryos of Lilium cattaniae obtained on Murashige and Skoog (MS) medium containing 9.05 mM 2.4-dichlorophenoxyacetic acid (2.4-D) and 4.44 mM N6-bezyladenin (BA), was about two times higher than for any other treatment. Photosynthetic status of plantlets obtained through regeneration was explant-specific and generally higher for plantlets regenerated from basal leaf explants than from bulb explants. The accumulation of anthocyanin was detected in some regenerated shoots and more often in plantlets obtained through regeneration from bulb explants. High frequency of somatic embryo formation was recorded for L. cattaniae on MS medium containing 9.05 mM 2.4-D and 4.44 mM BA. The peroxidase activity for L. bosniacum plantlets obtained through regeneration showed treatment-specific elevations. We consider that basal leaf parts are suitable for successful regeneration of these two lilies and that elevation in peroxidase activity is a good indicator of somatic embryogenesis in both lilies.     

Haemosporidian prevalence,parasitaemia and aggregation in relation to avian assemblage life history traits at different elevations

The transmission of vector-borne protozoa such as parasites of the Order Haemosporida is dependent on both biotic and abiotic factors such as host life history traits and environmental conditions. This study aimed to identify the variables that determine haemosporidian prevalence, parasitaemia and aggregation within the context of elevation and avian life history traits in Central Veracruz, Mexico. We sampled 607 birds from 88 species; we used microscopy and the mtDNA cytochrome b gene to detect parasites. We found an overall prevalence of 32.3%. Haemosporidian prevalence was 21.6% in tropical sub-deciduous forest (at sea level), 38% in tropical deciduous forest (265 m above sea level (asl)), 19.4% in montane cloud forest (1630 m asl), and 51.7% in pine-oak forest (2790 m asl). The prevalence of each parasite genus was strongly influenced by elevation (a proxy of habitat type). Plasmodium showed the highest prevalence at low elevation. Haemoproteus increased in prevalence with elevation. Leucocytozoon displayed the highest prevalence at the highest elevation (pine-oak forest). Haemoproteus spp. and Leucocytozoon spp. prevalences were higher in open cup than in closed nests. Haemoproteus prevalence and haemosporidian parasitaemia were lower in solitary birds than birds with pairing and gregarious behavior. Haemosporidian aggregation decreased with elevation, yielding the significantly lowest values at the pine-oak forest. Elevation distribution patterns of prevalence for each genus were similar to those previously reported in other geographical areas (e.g., South America, Europe).     

Understanding the Linkage between Elevation and the Activated-Sludge Bacterial Community along a 3,600-Meter Elevation Gradient in China

To understand the relationship between elevation and bacterial communities in wastewater treatment plants (WWTPs), bacterial communities in 21 municipal WWTPs across China, located 9 to 3,660 m above sea level (masl), were investigated by 454 pyrosequencing. A threshold for the association of elevation with bacterial community richness and evenness was observed at approximately 1,200 masl. At lower elevations, both richness and evenness were not significantly associated with elevation. At higher elevations, significant declines with increased elevations were observed for community richness and evenness. The declining evenness trend at the phylum level was reflected by distinct trends in relative abundance for individual bacterial phyla. Betaproteobacteria, Bacteroidetes, and Firmicutes displayed significant increases, while most other phyla showed declines. Spearman correlation analysis indicated that the community richness and evenness at high elevations were more correlated with elevation than with any other single environmental variable. Redundancy analysis indicated that the contribution of elevation to community composition variances increased from 3% at lower elevations to 11% at higher elevations whereas the community composition variance at higher elevations remained much more explained by operational variables (39.2%) than by elevation. The influent total phosphorus concentration, food/microorganism ratio, and treatment process were the three shared dominant contributors to the community composition variance across the whole elevation gradient, followed by effluent ammonia nitrogen and temperature at higher elevations.     

The influence of water level fluctuations on the potential for convective flow in the emergent macrophytes Typha domingensis and Phragmites australis

This paper examines the influence of the amplitude of water level fluctuations and elevation on the gas space anatomy and potential for convective flow in the emergent macrophytes Typha domingensis Pers. and Phragmites australis (Cav.) Trin. ex Steud. Plants were grown under a range of amplitudes of cyclic water level fluctuations: static, ±15, ±30 and ±45 cm. The water level of each treatment fluctuated around an initial and average depth of 60 cm. Within each amplitude treatment, plants were grown at three elevations: sediment surface 20, 40 and 60 cm above the base of the pond.The gas space anatomy of T. domingensis showed a range of modifications in response to experimental treatments. Lacunal cross-sectional area of the leaves increased with decreasing elevation as a product of changes in both fractional porosity and total cross-sectional area. Lacunal cross-sectional area of leaves was maintained across the gradient in amplitude through changes in fractional porosity despite decreasing total cross-sectional area. As a result of these adaptations in gas space anatomy, specific resistance in T. domingensis decreased with decreasing elevation but was unaffected by amplitude. In contrast, no unequivocal anatomical modifications were detected in P. australis and the specific resistance was constant across treatments.Estimates of the total resistance to convection in the above-ground parts of the plant suggests the adaptations in the gas space anatomy by T. domingensis largely compensated for increases in the pathlength of convection due to decreasing elevation. Total resistance increased significantly with increasing amplitude. P. australis had a low specific resistance which minimised the effect of increasing amplitude on total resistance but, lacking the ability to adapt gas space anatomy, the total resistance was strongly affected by elevation. The differential impact of elevation and amplitude suggests the two species have contrasting aptitudes for aeration across gradients in elevation and amplitude. T. domingensis is better suited to tolerate a range of static water depths whereas P. australis is better suited to fluctuating water levels especially when growing at high elevations.     

Tree diversity patterns in successive vegetation types along an elevation gradient in the Mountains of Eastern Mexico

Tree species richness changes along elevation gradients in response to underlying environmental conditions. Our hypothesis was that richness is associated with climatic variables and decreases with elevation. The objective was to identify trends in species, genus and family richness, diversity and vegetation structure in relation to climate variables along an elevation gradient with successive types of forest in Veracruz, Mexico. Trees were identified and measured in 0.1 ha at 15 sites located from 140 to 4000 m a.s.l. Generalized linear models were used to fit richness, diversity, basal area and density as a function of elevation; the best model was selected using Akaike’s Information Criterion. Multivariate analyses were used to explore climatic variables associated to composition of groups of sites along the gradient. Along the entire elevation gradient, species, genus and family richness decreased unimodally, and diversity decreased monotonically. Richness was positively correlated with temperature but not with precipitation. Basal area increased monotonically and highest basal area was associated with high humidity and certain tree species (Quercus and Abies). Ordinations indicated three groups of sites: lower elevation dry forest associated with temperature seasonality, mid-elevation cloud forest associated with precipitation-related variables, and coniferous forest at the top of the gradient associated with elevation. Our study shows that different plant communities are associated with certain climatic conditions and harbour different tree species, genera and families. The results support the hypothesis that species richness is associated with climate, and decreases with elevation.     

Effect of methionine and vitamin B-12 on the activities of methionine biosynthetic enzymes in metJ mutants of Escherichia coli K12

The effects of supplementation of growth medium with high concentrations of methionine (5 mm) and/or vitamin B₁₂ (10 nm) on the activities of five enzymes of the methionine regulon were measured in wild-type Escherichia coli K12, a metJ prototrophic and three metJ methionine auxotrophic derivatives. Growth on vitamin B₁₂ causes lowering of the activities of the non-B₁₂ methyltransferase while growth on methionine causes elevation of its activity in all four metJ mutants. The previous observation that this enzyme is not repressed by vitamin B₁₂ addition in metH mutants together with our observation that vitamin B₁₂ causes repression in mutants (metF) unable to synthesize the donor for homocysteine methylation supports the model of Kung et al. (10) that the holo-B₁₂-methyltransferase functions as a repressor of synthesis of the non-B₁₂-methyltransferase. Growth on methionine causes lowering of cystathionase activity, and growth on vitamin B₁₂ results in elevation of cystathionase activity in a metJ prototroph and one metJ auxotroph. The metJmetA strain (RG326) has a higher than normal level of cystathionase while the metJmetF strain (RG191) has lower than normal cystathionase activity. These results indicate the existence of a metJ independent system that modulates the activity of cystathionase possibly in response to changes in concentration of unidentified metabolite(s).