Distribution and dynamics of bacterioplankton production in a polymictic tropical lake (Lago Xolotlán,Nicaragua)

From 1988 to 1993 we assessed the variability of bacterioplankton production and biomass in Lake Xolotlán (L. Managua), Nicaragua via [3H]thymidine incorporation into DNA and cell counting. Bacterial production ranged from 3 to 8 μg C l-1 h-1, and since production was equal throughout the water column, areal production was high (≈ 600–1200 mg C m-2 d-1). Bacterial abundance in Lake Managua was extremely high, 7–30 × 109 cells l-1. Thus, specific rates of bacterial production were low. There was a strong correlation between production and number and the specific rate of bacterial production was constant. Comparable measurements of production via [3H]leucine incorporation into proteins indicated that bacteria were experiencing ‘balanced growth’. We conclude that bacterioplankton in Lake Xolotlán had reached its carrying capacity and a significant correlation between bacterial production and concentration of phaeophytin implied that dead or dying algae was the limiting substrate for bacterioplankton. The majority of bacterial number and most of bacterial production (up to 75%) were associated with particles in the >3-μm fraction, probably lysing algal cells to which bacterioplankton were ‘attached’. Grazing on bacterioplankton must be low and bacteria should be a ‘sink’ for organic matter in Lake Xolotlán. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of winter conditions on distributions of anoxic phototrophic bacteria in two meromictic lakes in Siberia,Russia

The year-to-year variations of vertical distribution and biomass of anoxic phototrophic bacteria were studied during ice periods 2003–2005 and 2007–2008 in meromictic lakes Shira and Shunet (Southern Siberia, Russian Federation). The bacterial layers in chemocline of both lakes were sampled with a thin-layer hydraulic multi-syringe sampler. In winter, biomass of purple sulphur bacteria varied considerably depending on the amount of light penetrating into the chemocline through the ice and snow cover. In relatively weakly stratified, brackish Shira Lake, the depth of chemocline varied between winters, so that light intensity for purple sulphur bacteria inhabiting this zone differed. In Shira Lake, increased transparency of mixolimnion in winter, high chemocline position and absence of snow resulted in light intensity and biomass of purple sulphur bacteria exceeding the summer values in the chemocline of the lake. We could monitor snow cover at the lake surface using remote sensing and therefore estimate dynamics and amount of light under ice and its availability for phototrophic organisms. In Shunet Lake, the light intensities in the chemocline and biomasses of purple sulphur bacteria were always lower in winter than in summer, but the biomasses of green sulphur bacteria were similar.     

Genetic Diversity of Eukaryotic Plankton Assemblages in Eastern Tibetan Lakes Differing by their Salinity and Altitude

Eukaryotic plankton assemblages in 11 high-mountain lakes located at altitudes of 2,817 to 5,134 m and over a total area of ca. one million square kilometers on the Eastern Tibet Plateau, spanning a salinity gradient from 0.2 (freshwater) to 187.1 g l⁻¹ (hypersaline), were investigated by cultivation independent methods. Two 18S rRNA gene-based fingerprint approaches, i.e., the terminal restriction fragment length polymorphism and denaturing gradient gel electrophoresis (DGGE) with subsequent band sequencing were applied. Samples of the same lake type (e.g., freshwater) generally shared more of the same bands or T-RFs than samples of different types (e.g., freshwater versus saline). However, a certain number of bands or T-RFs among the samples within each lake were distinct, indicating the potential presence of significant genetic diversity within each lake. PCA indicated that the most significant environmental gradient among the investigated lakes was salinity. The observed molecular profiles could be further explained (17–24%) by ion percentage of chloride, carbonate and bicarbonate, and sulfate, which were also covaried with change of altitude and latitude. Sequence analysis of selected major DGGE bands revealed many sequences (largely protist) that are not related to any known cultures but to uncultured eukaryotic picoplankton and unidentified eukaryotes. One fourth of the retrieved sequences showed ≤97% similarity to the closest sequences in the GenBank. Sequences related to well-known heterotrophic nanoflagellates were not retrieved from the DGGE gels. Several groups of eukaryotic plankton, which were found worldwide and detected in low land lakes, were also detected in habitats located above 4,400 m, suggesting a cosmopolitan distribution of these phylotypes. Collectively, our study suggests that there was a high beta-diversity of eukaryotic plankton assemblages in the investigated Tibetan lakes shaped by multiple geographic and environmental factors.     

Changes in Pelagic Bacteria Communities Due to Leaf Litter Addition

In many limnetic systems, the input of allochthonous organic matter, e.g., leaf litter, is a substantial source of dissolved organic carbon (DOC) for pelagic bacteria, especially in fall and winter when autochthonous DOC production is low. However, relatively little is known about community changes of pelagic lake bacteria due to leaf litter input which includes both the release of leaf leachates and microorganisms from the leaf litter into the surrounding water. Therefore, we have experimentally studied the effects of different types of leaf litter (Betula pendula, Fagus silvatica, and Pinus silvestris) on the pelagic bacterial community composition by adding leaves to different treatments of epilimnic water samples (unfiltered, 0.2 μm and 5.0 μm-pre-filtered) from humic Lake Grosse Fuchskuhle (Northeastern Germany). The addition of leaf litter led to a significant increase in DOC concentration in lake water, and each leaf litter type produced significantly different amounts of DOC (p = <0.001) as well as of specific DOC fractions (p = <0.001), except of polysaccharides. DGGE banding patterns varied over time, between types of leaf litter, and among treatments. Bacteria belonging to known bacterial phylotypes in the southwest basin of Lake Grosse Fuchskuhle were frequently found and even persisted after leaf litter additions. Upon leaf litter addition, α-proteobacteria (Azospirillum, Novosphingobium, and Sphingopyxis) as well as β-proteobacteria (Curvibacter and Polynucleobacter) were enriched. Our results indicate that supply of leaf litter DOM shifted the bacterial community in the surrounding water towards specific phylotypes including species capable of assimilating the more recalcitrant DOC pools. Statistical analyses, however, show that DGGE banding patterns are not only affected by DOC pools but also by treatment. This indicates that biological factors such as source community and grazing may be also important for shifts in bacterial community structure following leaf litter input into different lakes.     

Shoreline vegetation of Lake Nubia, Sudan

The paper reports on the flora along the banks of Lake Nubia and the long-term changes that have taken place since the formation of the Aswan High Dam Lake 30 years ago. The study also aims to determine the main factors that govern plant distribution in this area. Shoreline vegetation was studied from Debeira at the Sudanese–Egyptian border to the ‘Dal’ Cataract at the south end of the lake, in Sudan. The shoreline was classified into four moisture gradient zones depending on the period of inundation, namely: wet zone (frequently inundated and recently exposed); moist zone (periodically inundated); semi-dry zone (rarely inundated); and dry zone (never inundated). Plants characterising each zone were identified. Soil texture and thickness of the deposits were measured for each zone at each site. According to Grime’s strategy, the study revealed that species of the wet and the moist zones are mainly disturbance-tolerant; species observed in the ‘semi-dry’ zone are competitive-stress-tolerant; and those of the ‘dry’ zone are stress-tolerant. The present study indicates that elevation above mean sea level, and period of inundation and texture of deposits play an important role in governing the distribution of the shoreline vegetation.     

Dominant bacterioplankton populations in the meromictic Lake Suigetsu as determined by denaturing gradient gel electrophoresis of 16S rRNA gene fragments

Lake Suigetsu is a typical meromictic lake in Japan characterized by a permanent chemocline at a depth of between 3 and 8 m separating the oxic freshwater mixolimnion from anoxic saline sulfidogenic monimolimnion. Dominant bacterioplankton populations in Lake Suigetsu were investigated using PCR-denaturing gradient gel electrophoresis (DGGE) of 16S rRNA gene fragments. The bacterial population was vertically stratified, and temporal shifts in the microbial communities were observed in both the oxic and anoxic layers of Lake Suigetsu during the sampling period. Several dominant DGGE bands were excised and sequenced. In the chemocline, green sulfur bacteria phylogenetically related to the genera Prosthecochloris, Pelodyctyon, and Chlorobium within the phylum Chlorobi were dominant; the colorless sulfur bacteria closely related to the genus Thiomicrospira were detected. These sulfur bacterial groups appear to be important in the biogeochemical cycling of sulfur and/or carbon in Lake Suigetsu. Bacterial sequences affiliated with the Bacteroidetes phylum were frequent among the dominant fragments in the DGGE profiles throughout the water column. Populations possessing a fermentative metabolism exist in Bacteroidetes, suggesting they may contribute to the degradation of organic matter in the anoxic environment of Lake Suigetsu.     

Phylogenetic relationships ofThiomicrospira species and their identification in deep-sea hydrothermal vent samples by denaturing gradient gel electrophoresis of 16S rDNA fragments

Denaturing gradient gel electrophoresis (DGGE) of PCR-amplified 16S rDNA fragments was used to explore the genetic diversity of hydrothermal vent microbial communities, specifically to determine the importance of sulfur-oxidizing bacteria therein. DGGE analysis of two different hydrothermal vent samples revealed one PCR band for one sample and three PCR bands for the other sample, which probably correspond to the dominant bacterial populations in these communities. Three of the four 16S rDNA fragments were sequenced. By comparison with 16S rRNA sequences of the Ribosomal Database Project, two of the DGGE-separated fragments were assigned to the genusThiomicrospira. To identify these ‘phylotypes’ in more detail, a phylogenetic framework was created by determining the nearly complete 16S rRNA gene sequence (approx. 1500 nucleotides) from three describedThiomicrospira species, viz.,Tms. crunogena, Tms. pelophila, Tms. denitrificans, and from a new isolate,Thiomicrospira sp. strain MA2-6. AllThiomicrospira species exceptTms. denitrificans formed a monophyletic group within the gamma subdivision of the Proteobacteria.Tms. denitrificans was assigned as a member of the epsilon subdivision and was distantly affiliated withThiovulum, another sulfur-oxidizing bacterium. Sequences of two dominant 16S rDNA fragments obtained by DGGE analysis fell into the gamma subdivisionThiomicrospira. The sequence of one fragment was in all comparable positions identical to the 16S rRNA sequence ofTms. crunogena. Identifying a dominant molecular isolate asTms. crunogena indicates that this species is a dominant community member of hydrothermal vent sites. Another ‘phylotype’ represented a newThiomicrospira species, phylogenetically in an intermediate position betweenTms. crunogena andTms. pelophila. The third ‘phylotype’ was identified as aDesulfovibrio, indicating that sulfate-reducing bacteria, as sources of sulfide, may complement sulfur- and sulfide-oxidizing bacteria ecologically in these sulfide-producing hydrothermal vents.     

Microbial diversity in lake sediments detected by PCR-DGGE

In this study, PCR-denaturing gradient gel electrophoresis (DGGE) was applied to analyze the microbial communities in lake sediments from Lake Xuanwu, Lake Mochou in Nanjing and Lake Taihu in Wuxi. Sediment samples from seven locations in three lakes were collected and their genomic DNAs were extracted. The DNA yields of the sediments of Lake Xuanwu and Lake Mochou were high (10 μg/g), while that of sediments in Lake Taihu was relatively low. After DNA purification, the 16S rDNA genes (V3 to V5 region) were amplified and the amplified DNA fragments were separated by parallel DGGE. The DGGE profiles showed that there were five common bands in all the lake sediment samples indicating that there were similarities among the populations of microorganisms in all the lake sediments. The DGGE profiles of Lake Xuanwu and Lake Mochou were similar and about 20 types of microorganisms were identified in the sediment samples of both lakes. These results suggest that the sediment samples of these two city lakes (Xuanwu, Mochou) have similar microbial communities. However, the DGGE profiles of sediment samples in Lake Taihu were significantly different from these two lakes. Furthermore, the DGGE profiles of sediment samples in different locations in Lake Taihu were also different, suggesting that the microbial communities in Lake Taihu are more diversified than those in Lake Xuanwu and Lake Mochou. The differences in microbial diversity may be caused by the different environmental conditions, such as redox potential, pH, and the concentrations of organic matters. Seven major bands of 16S rDNA genes fragments from the DGGE profiles of sediment samples were further re-amplified and sequenced. The results of sequencing analysis indicate that five sequences shared 99%–100% homology with known sequences (Bacillus and Brevibacillus, uncultured bacteria), while the other two sequences shared 93%–96% homology with known sequences (Acinetobacter, and Bacillus). The study shows that the PCR-DGGE technique combined with sequence analysis is a feasible and efficient method for the determination of microbial communities in sediment samples. __________ Translated from Acta Ecologica Sinica, 2006, 26(11): 3610–3616 [译自: 生态学报]     

Specific detection of different phylogenetic groups of chemocline bacteria based on PCR and denaturing gradient gel electrophoresis of 16S rRNA gene fragments

Specific amplification of 16S rRNA gene fragments in combination with denaturing gradient gel electrophoresis (DGGE) was used to generate fingerprints of Chromatiaceae, green sulfur bacteria, Desulfovibrionaceae, and β-Proteobacteria. Sequencing of the gene fragments confirmed that each primer pair was highly specific for the respective phylogenetic group. Applying the new primer sets, the bacterial diversity in the chemoclines of a eutrophic freshwater lake, a saline meromictic lake, and a laminated marine sediment was investigated. Compared to a conventional bacterial primer pair, a higher number of discrete DGGE bands was generated using our specific primer pairs. With one exception, all 15 bands tested yielded reliable 16S rRNA gene sequences. The highest diversity was found within the chemocline microbial community of the eutrophic freshwater lake. Sequence comparison revealed that the six sequences of Chromatiaceae and green sulfur bacteria detected in this habitat all represent distinct and previously unknown phylotypes. The lowest diversity of phylotypes was detected in the chemocline of the meromictic saline lake, which yielded only one sequence each of the Chromatiaceae, β-2-Proteobacteria, and Desulfovibrionaceae, and no sequences of green sulfur bacteria. The newly developed primer sets are useful for the detection of previously unknown phylotypes, for the comparison of the microbial diversity between different natural habitats, and especially for the rapid monitoring of enrichments of unknown bacterial species. Received: 22 January 1999 / Accepted: 28 April 1999     

Vertical stratification of physical,chemical and biological components in two saline lakes Shira and Shunet (South Siberia,Russia)

A feature of meromictic lakes is that several physicochemical and biological gradients affect the vertical distribution of different organisms. The vertical stratification of physical, chemical and biological components in saline, fishless meromictic lakes Shira and Shunet (Siberia, Russia) is quite different mainly because both mean depth and maximum depth of lakes differ as well as their salinity levels differ. The chemocline of the Lake Shira, as in many meromictic lakes, is inhabited by bacterial community consisting of purple sulphur and heterotrophic bacteria. As the depth of the chemocline is variable, the bacterial community does not attain high densities. The mixolimnion in Lake Shira, which is thermally stratified in summer, also creates different habitat for various species. The distribution of phytoplankton is non-uniform with its biomass peak in the metalimnion. The distribution of zooplankton is also heterogeneous with rotifers and juvenile copepods inhabiting the warmer epilimnion and older copepods found in the cold but oxic hypolimnion. The amphipod Gammarus lacustris which can be assigned to the higher trophic link in the fishless lake’s ecosystem, such as Lake Shira, is also distributed non-uniformly, with its peak density generally observed in the thermocline region. The chemocline in Lake Shunet is located at the depth of 5 m, and unlike in Lake Shira, due to a sharp salinity gradient between the mixolimnion and monimolimnion, this depth is very stable. The mixolimnion in Lake Shunet is relatively shallow and the chemocline is inhabited by (1) an extremely dense bacterial community; (2) a population of Cryptomonas sp.; and (3) ciliate community comprising several species. As the mixolimnion of Lake Shunet is not thermally stratified for long period, the phytoplankton and zooplankton populations are not vertically stratified. The gammarids, however, tend to concentrate in a narrow layer located 1–2 m above the chemocline. We believe that in addition to vertical inhomogeneities of both physicochemical parameters, biological and physical factors also play a role in maintaining these inhomogeneities. We conclude that the stratified distributions of the major food web components will have several implications for ecosystem structure and dynamics. Trophic interactions as well as mass and energy flows can be significantly impacted by such heterogeneous distributions. Species spatially separated even by relatively short distances, say a few centimetres will not directly compete. Importantly, we demonstrate that not only bacteria, phytoflagellates and ciliate tend to concentrate in thin layers but also larger-sized species such Gammarus (amphipods) can also under certain environmental conditions have stratified distribution with maxima in relatively thin layer. As the vertical structure of the lake ecosystem is rather complex in such stratified lakes as ours, the strategy of research, including sampling techniques, should consider potentially variable and non-homogeneous distributions.     

Non-destructive,stereological estimation of plant root lengths,branching pattern and diameter distribution

The total length of a linear structure contained in an unbounded, transparent reference space can be estimated from ‘total vertical projections’ obtained by rotating the linear structure about an arbitrary ‘vertical’ axis, and projecting the linear structure onto a plane parallel to the axis of rotation. The total number of intersections between cycloid arcs with their minor axis perpendicular to the axis of rotation and the projected linear structure then provides an unbiased estimator of the total length of the structure. In this study, a stereological procedure based on the method of ‘total vertical projections’ was used to non-destructively estimate total root length, number of branches, diameter distribution and mean root diameter of crested wheatgrass plants (Agropyron cristatum L.) growing in a transparent medium. Root lengths, diameters, and number of branching points of various orders were determined at 3-day intervals over a 4-week growing period. The length estimator was very robust and efficient with sampling coefficients of error usually less than 5% for a total of 50–150 grid intersection counts over two projection directions per plant. Biological coefficients of variance for total length were between 30–70%, and were largely related to variation in the extent of branching. This revised version was published online in June 2006 with corrections to the Cover Date.     

Spatial Heterogeneity of Bacterial Populations in Monomictic Lake Estanya (Huesca,Spain)

Bacterial population changes were investigated in the monomictic Lake Estanya by combining microscopic analysis and two molecular methods involving the amplification of 16S rDNA genes using primers for the domain Bacteria and subsequent restriction fragment length polymorphism (PCR–RFLP) and denaturing gradient gel electrophoresis (PCR–DGGE). Both approaches revealed the vertical distribution of predominant microbial morphotypes and phylotypes in both holomictic and stratified periods, respectively, and showed that variations in structure and composition of bacterial populations are occurring in this lake as a function of depth and time. Through principal component analysis (PCA), these shifts could be related to different physicochemical parameters with temperature, oxygen concentration, and the incident light being of paramount importance as structuring variables. Comparison of RFLP and DGGE profiles by scoring similarities using the Jaccard coefficient and then building a multidimensional scaling map (MDS) showed equivalent results. Both techniques revealed that bacterial populations, present in the whole water column in the holomictic period, showed a high similarity with those located in the deeper part of the lake in the stratified period, evidencing that other factors, both biotic and abiotic, should also be considered as a force driving change in the composition of the bacterial community. Furthermore, DGGE analysis showed that sequences from prominent bands were affiliated to members of four major phyla of the domain Bacteria: Cyanobacteria, Bacteroidetes, Proteobacteria, and Actinobacteria, most of which corresponded to heterotrophic bacterial populations involved in carbon, sulfide, and nitrogen biogeochemical cycles, which were indistinguishable under the light microscope.     

Community Dynamics of Free-living and Particle-associated Bacterial Assemblages during a Freshwater Phytoplankton Bloom

Bacterial community dynamics were followed in a 19-day period during an induced diatom bloom in two freshwater mesocosms. The main goal was to compare diversity and succession among free-living (<10 MM) AND PARTICLE-ASSOCIATED (>10 mm) bacteria. Denaturing gradient gel electrophoresis (DGGE) of PCR amplified 16S rDNA showed the highest number of bands among free-living bacteria, but with a significant phylogenetic overlap in the two size fractions indicating that free-living bacteria were also important members of the particle-associated bacterial assemblage. Whereas the number of bands in the free-living fraction decreased during the course of the bloom, several phylotypes unique to particles appeared towards the end of the experiment. Besides the primer set targeting Bacteria, a primer set targeting most members of the Cytophaga-Flavobacterium (CF)-cluster of the Cytophaga-Flavobacterium-Bacteroides group and a primer set mainly targeting a-Proteobacteria were applied. PCR-DGGE analyses revealed that a number of phylotypes targeted by those primer sets were found solely on particles. Almost all sequenced bands from the bacterial DGGE gel were related to phylogenetic groups commonly found in freshwater: a-Proteobacteria, CF, and Firmicutes. Despite the use of primers intended to be specific mainly for a-Proteobacteria most bands sequenced from the a-proteobacterial DGGE gel formed a cluster within the Verrucomicrobiales subdivision of the Verrucomicrobia division and were not related to a-Proteobacteria. Bands sequenced from the CF DGGE gel were related to members of the CF cluster. From the present study, we suggest that free-living and particle-associated bacterial communities should not be perceived as separate entities, but rather as interacting assemblages, where the extent of phylogenetic overlap is dependent on the nature of the particulate matter.     

Small scale gradient effects on isopods (Crustacea: Oniscidea) in karstic sinkholes

We studied abundance and diversity patterns of terrestrial isopod assemblages along a ‘micro-scale’ vertical gradient in sinkholes in the Aggtelek National Park, Hungary. Time restricted manual sampling yielded ten native species, including endemic and rare ones. Along the gradient we found no major differences in species richness and -composition, and abundance decreased from the bottoms to the upper zones of the sinkholes. Species specific habitat preference on a vertical gradient showed two distinct groups by indicator species analysis: occurrence of habitat “generalists” was irrespective of vertical zones while “specialists” were restricted to the bottoms of the dolines. The latter group is formed mainly by rare species. We found that both diversity and evenness of isopod assemblages were highest in the bottom zone. Our results draw the attention to the significance of such common, yet undiscovered surficial depressions that can provide shelters for rare and specialist species and can provide shelter for survival of populations under changing climatic conditions.     

Bacterial and archaeal assemblages in sediments of a large shallow freshwater lake, Lake Taihu, as revealed by denaturing gradient gel electrophoresis

Aims:  To explore the association of microbial community structure with the development of eutrophication in a large shallow freshwater lake, Lake Taihu.
Methods and Results:  The bacterial and archaeal assemblages in sediments of different lake areas were analysed using denaturing gradient gel electrophoresis (DGGE) of amplified 16S rDNA fragments. The bacterial DGGE profiles showed that eutrophied sites, grass-bottom areas and relatively clean sites with a eutrophic (albeit dredged) site are three respective clusters. Fifty-one dominant bacterial DGGE bands were detected and 92 corresponding clones were sequenced, most of which were affiliated with bacterial phylotypes commonly found in freshwater ecosystems. Actinobacteria were detected in the centre of the lake and not at eutrophied sites whereas the opposite was found with respect to Verrucomicrobiales . Twenty-five dominant archaeal DGGE bands were detected and 31 corresponding clones were sequenced, most of which were affiliated with freshwater archaeal phylotypes.
Conclusions:  The bacterial community structures in the sediments of different areas with similar water quality and situation tend to be similar in Taihu Lake.
Significance and Impact of the Study:  This study may expand our knowledge on the relationship between the overall microbial assemblages and the development of eutrophication in the shallow freshwater lake.     

Effects of trace element concentrations on culturing thermophiles

The majority of microorganisms in natural environments resist laboratory cultivation. Sometimes referred to as ‘unculturable’, many phylogenetic groups are known only by fragments of recovered DNA. As a result, the ecological significance of whole branches of the ‘tree of life’ remains a mystery; this is particularly true when regarding genetic material retrieved from extreme environments. Geochemically relevant media have been used to improve the success of culturing Archaea and Bacteria, but these efforts have focused primarily on optimizing pH, alkalinity, major ions, carbon sources, and electron acceptor–donor pairs. Here, we cultured thermophilic microorganisms from ‘Sylvan Spring’ (Yellowstone National Park, USA) on media employing different trace element solutions, including one that mimicked the source fluid of the inocula. The growth medium that best simulated trace elements found in ‘Sylvan Spring’ produced a more diverse and faster growing mixed culture than media containing highly elevated trace element concentrations. The elevated trace element medium produced fewer phylotypes and inhibited growth. Trace element concentrations appear to influence growth conditions in extreme environments. Incorporating geochemical data into cultivation attempts may improve culturing success.     

Effect of humic material on the bacterioplankton community composition in boreal lakes and mesocosms

The bacterioplankton community composition in two Finnish forest lakes with different content of humic substances was studied by denaturing gradient gel electrophoresis (DGGE) and sequencing of the major bands. The same dominant bacterial phylotypes were detected in the bacterioplankton communities of clear-water Lake Ahvenlammi and humic Lake Sammalisto. For 4 years, in every water layer, Actinobacteria was the dominant and Verrucomicrobia the second most common phylum. In the hypolimnion, other dominant phyla were also found. We set up a mesocosm experiment to assess the effect of a sudden load of allochthonous humus extract to the bacterioplankton community composition. Changes in the bacterial communities were followed in four control and four humus extract-added mesocosms for 50 days. In the humic mesocosms the phylotypes of allochthonous Proteobacteria arriving with the humus extract were initially prevalent but disappeared during the first weeks. After this the Actinobacteria-dominated communities resembled the bacterioplankton communities of the control mesocosms and Lake Ahvenlammi. Towards the end of the experiment the community patterns in all the mesocosms started to change slightly because of erratic occurrence of new proteobacterial phylotypes. Thus the effects of a sudden load of allochthonous humic material and bacteria to the bacterioplankton community composition were transient.     

Modelling hydrological characteristics of Mediterranean Temporary Ponds and potential impacts from climate change

‘Mediterranean Temporary Ponds’ (MTP) constitutes a priority, substantially vulnerable and unstable habitat (Natura code: 3170*). In this article, the influences of climate change on the hydroperiod of two MTPs in Crete, have been quantitatively explored by using: (i) a physically based, semi-distributed lake basin model of Lake Kourna, where the hydrology of the lake is directly related to that of the adjacent MTP and (ii) a conceptual/mathematical model of an MTP in Omalos plateau. A water balance model was also set up to estimate net groundwater inflows for Lake Kourna and the basin. The water balance estimates and GIS tools were then used to set up the physically based model which was calibrated for the hydrological year 2005–2006 and validated for two periods: April–September 2005 and the hydrological year 2006–2007 (two split-sample tests). Calibration of the mathematical model was very good, while for the physically based model calibration was satisfactory. The two models were then setup and simulated for two future Intergovernmental Panel for Climate Change (IPCC) scenarios: A2 (pessimistic) and B2 (more optimistic). The responses of Lake Kourna and Omalos MTP water levels and their hydroperiods were then predicted. Results for IPCC B2 and A2 climate scenarios show longer hydroperiod and smaller decreases in the future for Omalos MTP than in Lake Kourna MTP. Results for Lake Kourna MTP demonstrated a hydroperiod decrease of more than 52 days after the application of the IPCC scenarios. Scenario A2 does not present a significantly different higher impact on the MTPs’ hydroperiod. Guest editors: B. Oertli, R. Cereghino, A. Hull & R. Miracle Pond Conservation: From Science to Practice. 3rd Conference of the European Pond Conservation Network, Valencia, Spain, 14–16 May 2008.     

Factors influencing induction and maintenance of <Emphasis Type="Italic">Vitis rotundifolia</Emphasis> Michx. embryogenic cultures

Unopened leaves, petioles and fully opened leaves from micropropagation cultures of five Vitis rotundifolia Michx. varieties were cultured on induction medium to study their embryogenic response. Among the various explants tested, the maximum number of varieties produced embryogenic cultures from unopened leaves followed by fully opened leaves and petioles. Based on morphological differences, two types of embryogenic cultures were identified. Friable cultures typically arose as proembryonic masses (PEM) on induction medium, whereas somatic embryo production without an intervening PEM stage was observed in compact cultures. Of the five varieties tested, the highest frequency of embryogenic response was observed from fully opened leaves of ‘Supreme’ and unopened leaves and petioles of ‘Delicious’. Attempts to initiate suspension cultures from varieties resulted in proliferation and maintenance of ‘Alachua’ and ‘Carlos’ cultures in liquid medium for 16 weeks. Embryogenic potential of varieties was studied on cultures growing on embryo development medium. The maximum number of cotyledonary stage somatic embryos from 0.2 g proembryonic masses were observed in ‘Carlos’ (379.3) followed by ‘Alachua’ (350.0) and ‘Delicious’ (305.0). Cotyledonary stage somatic embryos germinated when cultured on Murashige and Skoog medium containing 1 μM Benzyladenine (BA). Although high embryo germination rates (80–100%) were observed in the varieties tested, plant recovery from germinated somatic embryos ranged from 6–47%. Embryogenic cultures could be maintained on X6 medium and used in genetic engineering studies.     

Observations on the use of membrane filtration and liquid impingement to collect airborne microorganisms in various atmospheric environments

The influence of sample-collection-time on the recovery of culturable airborne microorganisms using a low-flow-rate membrane-filtration unit and a high-flow-rate liquid impinger were investigated. Differences in recoveries were investigated in four different atmospheric environments, one mid-oceanic at an altitude of ~10.0 m, one on a mountain top at an altitude of ~3,000.0 m, one at ~1.0 m altitude in Tallahassee, Florida, and one at ~1.0 m above ground in a subterranean-cave. Regarding use of membrane filtration, a common trend was observed: the shorter the collection period, the higher the recovery of culturable bacteria and fungi. These data also demonstrated that lower culturable counts were common in the more remote mid-oceanic and mountain-top atmospheric environments with bacteria, fungi, and total numbers averaging (by sample time or method categories) <3.0 colony-forming units (CFU) m⁻³. At the Florida and subterranean sites, the lowest average count noted was 3.5 bacteria CFU m⁻³, and the highest averaged 140.4 total CFU m⁻³. When atmospheric temperature allowed use, the high-volume liquid impinger utilized in this study resulted in much higher recoveries, as much as 10× greater in a number of the categories (bacterial, fungal, and total CFU). Together, these data illustrated that (1) the high-volume liquid impinger is clearly superior to membrane filtration for aeromicrobiology studies if start-up costs are not an issue and temperature permits use; (2) although membrane filtration is more cost friendly and has a ‘typically’ wider operational range, its limits include loss of cell viability with increased sample time and issues with effectively extracting nucleic acids for community-based analyses; (3) the ability to recover culturable microorganisms is limited in ‘extreme’ atmospheric environments and thus the use of a ‘limited’ methodology in these environments must be taken into account; and (4) the atmosphere culls, i.e., everything is not everywhere.