Plants as Sources of Airborne Bacteria, Including Ice Nucleation-Active Bacteria

Vertical wind shear and concentration gradients of viable, airborne bacteria were used to calculate the upward flux of viable cells above bare soil and canopies of several crops. Concentrations at soil or canopy height varied from 46 colony-forming units per m³ over young corn and wet soil to 663 colony-forming units per m³ over dry soil and 6,500 colony-forming units per m³ over a closed wheat canopy. In simultaneous samples, concentrations of viable bacteria in the air 10 m inside an alfalfa field were fourfold higher than those over a field with dry, bare soil immediately upwind. The upward flux of viable bacteria over alfalfa was three- to fourfold greater than over dry soil. Concentrations of ice nucleation-active bacteria were higher over plants than over soil. Thus, plant canopies may constitute a major source of bacteria, including ice nucleation-active bacteria, in the air.     

Toxicity of Smoke to Epiphytic Ice Nucleation-Active Bacteria

Wheat straw smoke aerosols and liquid smoke condensates reduced significantly both the viability and the ice-nucleating activity of Pseudomonas syringae pv. syringae and Erwinia herbicola in vitro and on leaf surfaces in vivo. Highly significant reductions in numbers of bacterial ice nuclei on the surface of both corn and almond were observed after exposure to smoke aerosols. At −5°C, frost injury to corn seedlings colonized by ice nucleation-active bacteria was reduced after exposure to smoke aerosols. Effects on −9°C ice nuclei, although significant, were less than on ice nuclei active at −5°C. These results suggest that smoke from wildfires or smudge pots may reduce plant frost susceptibility and sources of ice nuclei important in other natural processes under some conditions.     

Measurement of Ice Nucleation-Active Bacteria on Plants and in Precipitation by Quantitative PCR

Ice nucleation-active (INA) bacteria may function as high-temperature ice-nucleating particles (INP) in clouds, but their effective contribution to atmospheric processes, i.e., their potential to trigger glaciation and precipitation, remains uncertain. We know little about their abundance on natural vegetation, factors that trigger their release, or persistence of their ice nucleation activity once airborne. To facilitate these investigations, we developed two quantitative PCR (qPCR) tests of the ina gene to directly count INA bacteria in environmental samples. Each of two primer pairs amplified most alleles of the ina gene and, taken together, they should amplify all known alleles. To aid primer design, we collected many new INA isolates. Alignment of their partial ina sequences revealed new and deeply branching clades, including sequences from Pseudomonas syringae pv. atropurpurea, Ps. viridiflava, Pantoea agglomerans, Xanthomonas campestris, and possibly Ps. putida, Ps. auricularis, and Ps. poae. qPCR of leaf washings recorded ∼10⁸ina genes g⁻¹ fresh weight of foliage on cereals and 10⁵ to 10⁷ g⁻¹ on broadleaf crops. Much lower populations were found on most naturally occurring vegetation. In fresh snow, ina genes from various INA bacteria were detected in about half the samples but at abundances that could have accounted for only a minor proportion of INP at −10°C (assuming one ina gene per INA bacterium). Despite this, an apparent biological source contributed an average of ∼85% of INP active at −10°C in snow samples. In contrast, a thunderstorm hail sample contained 0.3 INA bacteria per INP active at −10°C, suggesting a significant contribution to this sample.     

从空间分布特征认识珍稀植物领春木的种群动态

领春木(Euptelea pleiosperrnum)系第三纪孑遗植物和东亚特有种,目前已被列为国家Ⅲ级重点保护植物.基于空间定位数据以最近邻体距离统计研究了神农架地区领春木的空间分布特征,比较幼苗(DBH≤2.5cm)、幼树(2.5～7.5cm)和成树(＞7.5cm)各径级(代表各生活史阶段)形成的时间序列上的空间格局差异,进而探讨空间格局与立苗、补员、种内竞争等种群动态过程的相互关系.结果显示,在邻域尺度上,领春木的空间格局呈聚集态;幼苗(或幼树)的大小与其距离最近幼树(或成树)的远近没有相关性,幼树(或成树)周围一定距离以内出现同等大小个体的概率约等于幼苗(或幼树)出现的概率,且幼树与最近幼苗(或成树与最近幼树)的平均距离与幼树之间(或成树之间)的平均最近邻体距离没有显著差异;任意个体的大小、任意个体与相应最近邻体的大小之和与相应的最近邻体距离均为显著的正相关关系,但幼树间的最近邻体距离并不大于幼苗随机死亡产生的最近邻体距离,成树间最近邻体距离也不大于幼苗+幼树随机死亡产生的最近邻体距离.这些结果表明,领春木的聚集分布可能与种子散布、生境异质性对立苗格局的作用有关;已定植的大个体可能不限制其邻域内小个体的布局与生长,但是长期的补员过程与邻体间的相互作用不无关系;邻体间存在一定程度的竞争作用,但是竞争强度并未充分激化至发生距离依赖的死亡.     

Sea Ice Microbial Communities: Distribution, Abundance, and Diversity of Ice Bacteria in McMurdo Sound, Antarctica, in 1980

An abundant and diverse bacterial community was found within brine channels of annual sea ice and at the ice-seawater interface in McMurdo Sound, Antarctica, in 1980. The mean bacterial standing crop was 1.4 × 10¹¹ cells m⁻² (9.8 mg of C m⁻²); bacterial concentrations as high as 1.02 × 10¹² cells m⁻³ were observed in ice core melt water. Vertical profiles of ice cores 1.3 to 2.5 m long showed that 47% of the bacterial numbers and 93% of the bacterial biomass were located in the bottom 20 cm of sea ice. Ice bacterial biomass concentration was more than 10 times higher than bacterioplankton from the water column. Scanning electron micrographs showed a variety of morphologically distinct cell types, including coccoid, rod, fusiform, filamentous, and prosthecate forms; dividing cells were commonly observed. Approximately 70% of the ice bacteria were free-living, whereas 30% were attached to either living algal cells or detritus. Interactions between ice bacteria and microalgae were suggested by a positive correlation between bacterial numbers and chlorophyll a content of the ice. Scanning and transmission electron microscopy revealed a close physical association between epibacteria and a dominant ice alga of the genus Amphiprora. We propose that sea ice microbial communities are not only sources of primary production but also sources of secondary microbial production in polar ecosystems. Furthermore, we propose that a detrital food web may be associated with polar sea ice.     

烟草内生细菌种群动态研究

经过对 7个田间种植的烟草品种不同栽培时期、不同组织内生细菌种群动态研究表明 ,不同品种内生细菌种群有一定程度差异。同一品种中有的内生细菌为常住菌群 ,有的为暂居菌群 ,带菌量根中最高 ,茎次之 ,叶中最低。在整个生育期中 ,7个品种内生细菌数量表现出从种子到出苗期大幅增加 ,从出苗期到十字期又大幅度下降 ,随后从缓苗期到伸根期再一次急剧增加并维持在一个较高水平。通过对烟草这一重要经济作物内生细菌种群动态变化研究 ,可为烟草病虫害生物防治和促生增产研究提供理论基础。     

Different Population Dynamics of Microcebus murinus in Primary and Secondary Deciduous Dry Forests of Madagascar

The goal of this study was to identify causes for lower population densities of mouse lemurs (Microcebus murinus) in secondary than in primary dry deciduous forests of western Madagascar. Variations in the abundance of Microcebus murinus are linked to the capacity to enter energy-saving torpor during the dry season. Under natural conditions in primary forest, Microcebus murinus can maintain daily torpor (and possibly hibernation) as long as body temperatures remain below 28°C. Females are more likely to hibernate than males, resulting in skewed sex ratios of captured Microcebus murinus in the primary forest. In the secondary forest the sex ratio of subjects captured during the dry season is more balanced than in primary forest, indicating that fewer females go into torpor in secondary than in primary forest. Secondary forests have fewer large standing or fallen trees that might provide holes as shelter for Microcebus murinus. Ambient temperatures are higher in secondary than in primary forests and higher outside than inside tree holes. These high ambient temperatures might hinder the ability of Microcebus murinus to maintain torpor for prolonged periods in secondary forests. Mouse lemurs from secondary forest have lower body mass than in primary forest. The year-to-year recapture rate is zero in secondary forest and thus significantly lower than in primary forest. This indicates that survival rates are lower in secondary than in primary forests. Thus, secondary forests may be of limited value as buffer zones or even corridors for mouse lemurs.     

Survival of Ice Nucleation-Active and Genetically Engineered Non-Ice-Nucleating Pseudomonas syringae Strains after Freezing

The survival after freezing of ice nucleation-active (INA) and genetically engineered non-INA strains of Pseudomonas syringae was compared. Each strain was applied to oat seedlings and allowed to colonize for 3 days, and the plants were subjected to various freezing temperatures. Plant leaves were harvested before and after freezing on two consecutive days, and bacterial populations were determined. Populations of the INA wild-type strain increased 15-fold in the 18 h after the oat plants incurred frost damage at −5 and −12°C. Plants colonized by the non-INA strain were undamaged at −5°C and exhibited no changes in population size after two freeze trials. As freezing temperatures were lowered (−7, −9, and −12°C), oat plants colonized by the non-INA strain suffered increased frost damage concomitant with bacterial population increases following 18 h. At −12°C, both strains behaved identically. The data show a relationship between frost damage to plants and increased bacterial population size during the following 18 h, indicating a potential competitive advantage of INA strains of P. syringae over non-INA strains in mild freezing environments.     

Distribution of Tetracycline Resistance Genes and Transposons among Phylloplane Bacteria in Michigan Apple Orchards

The extent and nature of tetracycline resistance in bacterial populations of two apple orchards with no or a limited history of oxytetracycline usage were assessed. Tetracycline-resistant (Tc^r) bacteria were mostly gram negative and represented from 0 to 47% of the total bacterial population on blossoms and leaves (versus 26 to 84% for streptomycin-resistant bacteria). A total of 87 isolates were screened for the presence of specific Tc^r determinants. Tc^r was determined to be due to the presence of Tet B in Pantoea agglomerans and other members of the family Enterobacteriacae and Tet A, Tet C, or Tet G in most Pseudomonas isolates. The cause of Tc^r was not identified in 16% of the isolates studied. The Tc^r genes were almost always found on large plasmids which also carried the streptomycin resistance transposon Tn5393. Transposable elements with Tc^r determinants were detected by entrapment following introduction into Escherichia coli. Tet B was found within Tn10. Two of eighteen Tet B-containing isolates had an insertion sequence within Tn10; one had IS911 located within IS10-R and one had Tn1000 located upstream of Tet B. Tet A was found within a novel variant of Tn1721, named Tn1720, which lacks the left-end orfI of Tn1721. Tet C was located within a 19-kb transposon, Tn1404, with transposition genes similar to those of Tn501, streptomycin (aadA2) and sulfonamide (sulI) resistance genes within an integron, Tet C flanked by direct repeats of IS26, and four open reading frames, one of which may encode a sulfate permease. Two variants of Tet G with 92% sequence identity were detected.     

Diversity and Population Dynamics of Methanogenic Bacteria in a Granular Consortium

Upflow anaerobic sludge blanket bioreactor granules were used as an experimental model microbial consortium to study the dynamics and distribution of methanogens. Immunologic methods revealed a considerable diversity of methanogens that was greater in mesophilic granules than in the same granules 4 months after a temperature shift from 38 to 55°C. During this period, the sizes of the methanogenic subpopulations changed with distinctive profiles after the initial reduction caused by the shift. Methanogens antigenically related to Methanobrevibacter smithii PS and ALI, Methanobacterium hungatei JF1, and Methanosarcina thermophila TM1 increased rapidly, reached a short plateau, and then fell to lower concentrations that persisted for the duration of the experiment. A methanogen related to Methanogenium cariaci JR1 followed a similar profile at the beginning, but it soon diminished below detection levels. Methanothrix rods weakly related to the strain Opfikon increased rapidly, reaching a high-level, long-lasting plateau. Two methanogens related to Methanobrevibacter arboriphilus AZ and Methanobacterium thermoautotrophicum ΔH emerged from very low levels before the temperature shift and multiplied to attain their highest numbers 4 months after the shift. Histochemistry and immunohistochemistry revealed thick layers, globular clusters, and lawns of variable density which were distinctive of the methanogens related to M. thermoautotrophicum ΔH, M. thermophila TM1, and M. arboriphilus AZ and M. soehngenii Opfikon, respectively, in thin sections of granules grown at 55°C for 4 months. Mesophilic granules showed a different pattern of methanogenic subpopulations.     

The Population and Evolutionary Dynamics of Homologous Gene Recombination in Bacteria

In bacteria, recombination is a rare event, not a part of the reproductive process. Nevertheless, recombination—broadly defined to include the acquisition of genes from external sources, i.e., horizontal gene transfer (HGT)—plays a central role as a source of variation for adaptive evolution in many species of bacteria. Much of niche expansion, resistance to antibiotics and other environmental stresses, virulence, and other characteristics that make bacteria interesting and problematic, is achieved through the expression of genes and genetic elements obtained from other populations of bacteria of the same and different species, as well as from eukaryotes and archaea. While recombination of homologous genes among members of the same species has played a central role in the development of the genetics and molecular biology of bacteria, the contribution of homologous gene recombination (HGR) to bacterial evolution is not at all clear. Also, not so clear are the selective pressures responsible for the evolution and maintenance of transformation, the only bacteria-encoded form of HGR. Using a semi-stochastic simulation of mutation, recombination, and selection within bacterial populations and competition between populations, we explore (1) the contribution of HGR to the rate of adaptive evolution in these populations and (2) the conditions under which HGR will provide a bacterial population a selective advantage over non-recombining or more slowly recombining populations. The results of our simulation indicate that, under broad conditions: (1) HGR occurring at rates in the range anticipated for bacteria like Streptococcus pneumoniae, Escherichia coli, Haemophilus influenzae, and Bacillus subtilis will accelerate the rate at which a population adapts to environmental conditions; (2) once established in a population, selection for this capacity to increase rates of adaptive evolution can maintain bacteria-encoded mechanisms of recombination and prevent invasion of non-recombining populations, even when recombination engenders a modest fitness cost; and (3) because of the density- and frequency-dependent nature of HGR in bacteria, this capacity to increase rates of adaptive evolution is not sufficient as a selective force to provide a recombining population a selective advantage when it is rare. Under realistic conditions, homologous gene recombination will increase the rate of adaptive evolution in bacterial populations and, once established, selection for higher rates of evolution will promote the maintenance of bacteria-encoded mechanisms for HGR. On the other hand, increasing rates of adaptive evolution by HGR is unlikely to be the sole or even a dominant selective pressure responsible for the original evolution of transformation.     

福建地区橘小实蝇田间种群动态监测研究

利用Steiner诱捕器内置诱捕剂Me或Cue与0.2%马拉硫磷混合物制成棉球诱芯,在福建省各个地区共设置50个监测点,诱捕监测橘小实蝇（Bactrocera dorsalis Hendel）种群动态。从2002～2005年,连续4年的诱捕结果表明,橘小实蝇在福建省的发生呈单峰特点,7～9月为发生盛期,高峰出现在8月份。瓜菜园生境、石榴芒果园生境和龙眼香蕉园生境中,石榴芒果园生境橘小实蝇种群密度最高。运用地理信息系统（MapInfo）对该虫监测数据进行地图分析表明,橘小实蝇种群密度的地域特征明显,橘小实蝇有不断向外扩散的趋势。     

Population Dynamics and Diversity of Viruses,Bacteria and Phytoplankton in a Shallow Eutrophic Lake

We have studied the temporal variation in viral abundances and community assemblage in the eutrophic Lake Loosdrecht through epifluorescence microscopy and pulsed field gel electrophoresis (PFGE). The virioplankton community was a dynamic component of the aquatic community, with abundances ranging between 5.5 x 10(7) and 1.3 x 10(8) virus-like particles ml(-1) and viral genome sizes ranging between 30 and 200 kb. Both viral abundances and community composition followed a distinct seasonal cycle, with high viral abundances observed during spring and summer. Due to the selective and parasitic nature of viral infection, it was expected that viral and host community dynamics would covary both in abundances and community composition. The temporal dynamics of the bacterial and cyanobacterial communities, as potential viral hosts, were studied in addition to a range of environmental parameters to relate these to viral community dynamics. Cyanobacterial and bacterial communities were studied applying epifluorescence microscopy, flow cytometry, and denaturing gradient gel electrophoresis (DGGE). Both bacterial and cyanobacterial communities followed a clear seasonal cycle. Contrary to expectations, viral abundances were neither correlated to abundances of the most dominant plankton groups in Lake Loosdrecht, the bacteria and the filamentous cyanobacteria, nor could we detect a correlation between the assemblage of viral and bacterial or cyanobacterial communities during the overall period. Only during short periods of strong fluctuations in microbial communities could we detect viral community assemblages to covary with cyanobacterial and bacterial communities. Methods with a higher specificity and resolution are probably needed to detect the more subtle virus-host interactions. Viral abundances did however relate to cyanobacterial community assemblage and showed a significant positive correlation to Chl-a as well as prochlorophytes, suggesting that a significant proportion of the viruses in Lake Loosdrecht may be phytoplankton and more specific cyanobacterial viruses. Temporal changes in bacterial abundances were significantly related to viral community assemblage, and vice versa, suggesting an interaction between viral and bacterial communities in Lake Loosdrecht.     

Population Characteristics and Dosage Trajectory Analysis for Mesocriconema xenoplax in California Prunus Orchards

The Mesocriconema xenoplax population increased exponentially in a newly planted peach orchard. The rate of increase was greater on Nemaguard than on Lovell rootstock and was reduced by postplant nematicides. Population levels were more stable in an established almond orchard on Nemaguard rootstock. All life stages of the nematode were present year round; lower ratios of juveniles to adults in summer suggested adverse effects of temperature and dry soil. Also in summer, there was a smaller proportion of the population in the upper 30 cm of soil than at greater depths. Nematode dosage, average nematode density multiplied by accumulated degree-days (physiological time) of the sampling interval, was useful in quantifying nematode stress on trees and as an indicator of the nematode management effectiveness. The annual trajectory of the nematode dosage could be determined by two samplings, one in spring and one in fall. A nematode predator, the parasitic fungus Hirsutella rhossiliensis, did not regulate ring nematode populations in the newly planted orchard; a recovery period was necessary for increase in the prevalence of parasitism.     

Fate of Ice Nucleation-Active Pseudomonas syringae Strains in Alpine Soils and Waters and in Synthetic Snow Samples

The stability of the ice nucleation activity (INA) and viability of INA Pseudomonas syringae 31a, used as an ice nucleator in the manufacture of synthetic snow, was determined in snow. The viability of P. syringae 1-2b, a rifampin-resistant mutant selected from strain 31a to improve recovery from test samples, was determined in laboratory tests of three alpine soil and water samples from three different sources. Snow samples were exposed to environmental conditions or held in darkness at −20°C. Samples of soil and water were maintained in darkness at 0, 7.5, or 15°C. Parent strain 31a INA decreased significantly (>99.0%) in snow exposed to sunlight and freeze-thaw, while the INA of the cell population in snow held in darkness at −20°C remained essentially unchanged. No viable strain 31a was detected in snow exposed to the environment after 7 days, while the viability of strain 31a in snow held in darkness at −20°C decreased to <3% of the original inoculation at the test conclusion. Mutant strain 1-2b viability was undetectable or had decreased significantly 19 days postinoculation in soil samples held at 0 or 15°C. In contrast, 1-2b viability remained detectable at low levels for the duration of the test in soils held at 7.5°C. The 1-2b population demonstrated a significantly longer half-life in peatlike soil than in the loam soils tested. The rate of decrease in 1-2b viability was essentially the same in the three alpine water samples tested with respect to water temperature and sample location.     

The Presence of Complete but Masked Freezing Nuclei in Various Artificially Constructed Ice Nucleation-Active Proteobacteria

Disparate gamma-subdivision proteobacteria artifically endowed with the same ice gene of enteric origin acquired water-freezing potential at −12°C, but expressed it to varying extents under identical conditions of culture as well as after being subjected to certain post-culture treatments. Varying rates of cell-bound ice nucleus synthesis were probably not the root cause of these observed interspecies differences in nucleation-active cell frequency because potentially functional but masked ice-forming templates were found in the outer cell envelope of even initially inactive individuals taken from physiologically uniform populations of virtually all tested species. We therefore propose that the extent of bacterial ice nucleation generally reflects species-specified extent of ice nucleus sequestration. Received: 4 November 1996 / Accepted: 6 November 1996     

Long-Term Population Dynamics of Phototrophic Sulfur Bacteria in the Chemocline of Lake Cadagno, Switzerland

Population analyses in water samples obtained from the chemocline of crenogenic, meromictic Lake Cadagno, Switzerland, in October for the years 1994 to 2003 were studied using in situ hybridization with specific probes. During this 10-year period, large shifts in abundance between purple and green sulfur bacteria and among different populations were obtained. Purple sulfur bacteria were the numerically most prominent phototrophic sulfur bacteria in samples obtained from 1994 to 2001, when they represented between 70 and 95% of the phototrophic sulfur bacteria. All populations of purple sulfur bacteria showed large fluctuations in time with populations belonging to the genus Lamprocystis being numerically much more important than those of the genera Chromatium and Thiocystis. Green sulfur bacteria were initially represented by Chlorobium phaeobacteroides but were replaced by Chlorobium clathratiforme by the end of the study. C. clathratiforme was the only green sulfur bacterium detected during the last 2 years of the analysis, when a shift in dominance from purple sulfur bacteria to green sulfur bacteria was observed in the chemocline. At this time, numbers of purple sulfur bacteria had decreased and those of green sulfur bacteria increased by about 1 order of magnitude and C. clathratiforme represented about 95% of the phototrophic sulfur bacteria. This major change in community structure in the chemocline was accompanied by changes in profiles of turbidity and photosynthetically available radiation, as well as for sulfide concentrations and light intensity. Overall, these findings suggest that a disruption of the chemocline in 2000 may have altered environmental niches and populations in subsequent years.     

Asymptotic Distribution of Density-Dependent Stage-Grouped Population Dynamics Models

Matrix models are widely used in biology to predict the temporal evolution of stage-structured populations. One issue related to matrix models that is often disregarded is the sampling variability. As the sample used to estimate the vital rates of the models are of finite size, a sampling error is attached to parameter estimation, which has in turn repercussions on all the predictions of the model. In this study, we address the question of building confidence bounds around the predictions of matrix models due to sampling variability. We focus on a density-dependent Usher model, the maximum likelihood estimator of parameters, and the predicted stationary stage vector. The asymptotic distribution of the stationary stage vector is specified, assuming that the parameters of the model remain in a set of the parameter space where the model admits one unique equilibrium point. Tests for density-dependence are also incidentally provided. The model is applied to a tropical rain forest in French Guiana.     

Changes in Leaf Phenology are Dependent on Tree Height inAcer mono,a Deciduous Broad-leaved Tree

Ontogenetic changes in leaf phenology of a hardwood tree,Acer mono, were investigated in individuals in different size classes in a temperate forest. Leaf emergence was earliest in current-year seedlings, and was increasingly delayed with increasing height of the individual. The shorter the tree, the longer the duration of leaf emergence. Timing of leaf emergence of the dominant heterospecific canopy trees was almost identical to that of conspecific adults; understorey light then gradually decreased with expansion of canopy leaves. These traits indicate that smaller individuals that receive the least light in summer can acquire favourable light for a longer period in spring than taller plants even in a forest understorey, but the advantage decreases with increasing plant height. Changes in the duration of leaf emergence and leaf longevity in response to environmental light regime [sun (forest edge)vs. shade (forest understorey)], were greatest for current year seedlings but decreased with increasing plant height. These results suggest that the plastic response of leaf phenology in juvenile stages may reduce the risk of losing an entire cohort in spatially heterogenous environments in the understorey of temperate forests.