Rapid Environmental Fluctuations Research Articles

Impact of 17-β estradiol on growth and metabolism of marine diatom Thalassiosira weissflogii

• Cultivation of T.weissflogii on synthetically designed media of seawater and estradiol. • Biomass productivity and cell number declined with increase of E2 concentration in samples. • Enhanced production of protein and carbohydrate content mainly in samples T4 and T5. • The level of oxidative stress induced was assessed through MDA and other enzymatic reactions. The inefficiency of existing effluent treatment systems to eliminate pharmaceutical chemicals like endocrine disrupting estrogenic compounds from contaminated waters has raised global environmental and ecological concerns. Diatoms are unicellular, eukaryotic phytoplankton that inhabits ubiquitously and possess adaptive plasticity which enables them to withstand rapid environmental fluctuations. Thus, diatoms can be employed as indicators organisms and bioremediation agents of even trace amounts of Pharmaceutical Contaminants (PCs) like the estrogenic compounds. In this study, the influence of simulated wastewater containing pharmaceutical effluent 17-β Estradiol (E2) on the growth, cellular metabolites and oxidative stress response of the diatom, Thalassiosira weissflogii was assessed. Though an increasing concentration of the chemical displayed a negative correlation with growth, number of cells and biomass productivity; the photosynthetic profile of the diatom showed varied responses with significant enhancement in chlorophyll a and carotenoid content. The test concentrations T1 and T3 showed elevated carbohydrate and protein levels (0.82 ± 0.003 g L -1 and 2.04 ± 0.020 g L -1 ) establishing that E2 stress can be optimised to enhance primary metabolite secretion in diatoms, thereby valorizing its potential as suitable aqua feed. Besides this, an elevation in the antioxidative response against the reactive oxygen species generated was also distinctly recorded with respect to the increased release of catalase (152.70 ± 0.05–265.52 ± 0.06 k mg -1 ) and ascorbate peroxidases (0.959 ± 0.008–1.92 ± 0.021 U mg -1 ) as well as a conspicuous increment in the lipid peroxidation and hydrogen peroxide content was also there. Thus, this novel strategy of employing diatom for multitude of biorefinery aspects besides reacclimating contaminated water reveals the perspective application of this approach. .

Bacterial quorum sensing and phenotypic heterogeneity: how the collective shapes the individual.

Bacteria communicate with each other through a plethora of small, diffusible organic molecules called autoinducers. This cell-density-dependent regulatory principle is termed quorum sensing, and in many cases the process indeed coordinates group behavior of bacterial populations. Yet, even clonal bacterial populations are not uniform entities; rather, they adopt phenotypic heterogeneity to cope with consecutive, rapid, and frequent environmental fluctuations (bet-hedging) or to concurrently interact with each other by exerting different, often complementary, functions (division of labor). Quorum sensing is mainly regarded as a coordinator of bacterial collective behavior. However, it can also be a driver or a target of individual phenotypic heterogeneity. Hence, quorum sensing increases the overall fitness of a bacterial community by orchestrating group behavior as well as individual traits.

Population genetic analysis of two Polylepis microphylla (Wedd.) Bitter (Rosaceae) forests in Ecuador

ABSTRACT Habitat fragmentation is a severe problem that negatively affects forests around the world. This phenomenon has consequences at a genetic level in populations such as genetic diversity and gene flow reduction. As a result, population fitness decreases, and the possibility of extinction increases. Polylepis (Rosaceae) forests are endemic to the Andes and their distribution reflects hundreds of years of human activities. Polylepis microphylla is the most endangered species of the genus in Ecuador and is represented by only two known forests in Chimborazo province, separated from each other by 1,6 km. The present study focused on i) exploring the genetic structure and connectivity, ii) analyzing the genetic diversity patterns in these two forests of P. microphylla. Parameters such as Fixation index, molecular variance (AMOVA) and genetic diversity estimators were calculated from the haplotypic sequences of one chloroplastic DNA region (trnG intron), one nuclear DNA region (ITS), and the concatenation of both. We found that DNA regions used were highly polymorphic with a total of nine, four and thirteen segregation sites, respectively, and nine, six and fifteen haplotypes, respectively. Also, we detected that both forest work as a single population unit due to low fixation index values (Fst = 0,03822, −0,03883 and −0,00469, respectively), which indicate high gene flow between these two forests. In addition, we detected high genetic diversity reflected by haplotype diversity in both forest (hd = 0,54204, 0,63664 and 0,81682, respectively). Likewise, haplotype relationships were visualized in networks, which demonstrated limited spatial structuring between haplotypes, corroborating the low genetic differentiation among these forests. In conclusion, our results suggest there is a single population unit of P. microphylla in Ecuador. Habitat fragmentation does not seem to have negative effects on the genetic diversity of this species. Nevertheless, the species may be vulnerable to rapid environmental fluctuations despite having relatively high genetic diversity. Its long generational time causes increased susceptibility of P. microphylla to environmental changes and may eventually lead to the extinction of the species in Ecuador.

High-resolution taphonomic and palaeoecological analyses of the Jurassic Yanliao Biota of the Daohugou area, northeastern China

The Yanliao Biota contains numerous exceptionally preserved fossils of evolutionary importance. However, the palaeoenvironment of this biota has not been fully studied. Here we present the first taphonomic and palaeoecological analysis of fossil abundance data from two newly excavated sites in the Daohugou area. In our study we have investigated the taxonomic composition, specimen abundance and size distribution patterns of invertebrate palaeocommunities, and corresponding taphonomic features, such as completeness, articulation and fossil orientation. Our results show that abundant aquatic and terrestrial organisms from various ecological niches are preserved in very thinly planar-laminated mudstone. Aquatic organisms of the two excavations are represented by low-diversity but high-density assemblages containing four species: the clam shrimp Triglypta haifanggouensis, the mayfly nymphs Fuyous gregarious and Shantous lacustris, and the water boatman Daohugocorixa vulcanica. Triglypta haifanggouensis dominates the fauna with carapace lengths ranging from 1.34 mm to 5.62 mm. Size distributions of different bedding planes vary between normal, bimodal and polymodal, implying multiple causes of death: reproductive senescence, and mass mortality events related to active volcanism or to rapid environmental fluctuations within the lake. Generally calm depositional conditions were repeatedly interrupted by weak currents, indicated by preferred plan-view orientations of aquatic insects on four out of nine investigated bedding planes. Changes in the community composition of aquatic arthropods relate to varying lake depth through time, or to different dispersal strategies after community breakdown. Terrestrial insects are highly diverse but less abundant. As a result of rapid burial, 49.8% of which are almost complete and articulated. The flora surrounding the comparatively shallow lake was rich, picturing a thriving forest-like ecosystem with moisture relation in a warm-temperate climate.

New Insights into the LGM and LG in Southern France (Vaucluse): The Mustelids, Micromammals and Horses from Coulet des Roches

Coulet des Roches is a natural karst trap in Southern France. Its infilling dates back to the end of the Pleniglacial (Last Glacial Maximum, LGM) and the end of the Tardiglacial (Last Glacial, LG). Three mustelid species have been identified in this infilling: the common polecat (Mustela putorius, minimum number of individuals (MNI) = 4), the stoat (Mustela erminea, MNI = 14) and the weasel (Mustela nivalis, MNI = 48). The common polecat remains are metrically and morphologically indistinguishable from recent European specimens. The smallest mustelids are mainly represented by average-sized specimens, which are slightly smaller than extant species. A partial weasel skeleton of an extremely small pygmy weasel, regarded as a typical glacial element, was also discovered. Sexual dimorphism is strongly pronounced. Seventeen horses have been identified, corresponding to the chrono subspecies Equus ferus gallicus. The analysis of the muzzles and metapodials shows overall adaptation to cool and dry weather conditions. The ibexes are typical of Capra ibex (MNI = 15). The p3 morphology is similar to the LGM populations located on the southern side of the Durance River, with an important dilation of the metaconid, except for the oldest LGM specimen. This dilation is older on the southern side of the Durance River, as it occurs at the end of MIS 3. This difference could reflect the barrier role of the Durance River. Micromammals are abundant (mostly related to rodents and shrews; 18 genera/species; MNI = 470). The paleoecological study highlighted important and rapid climatic and environmental fluctuations throughout the sequence. As a result of climatic fluctuations, the plains constituted a corridor for the migration of temperate species to Provence during cold periods (“southern refuge zone”) and their re-immigration to Western Europe during temperate episodes. In a related and complementary way, the highland areas not only played a natural role as a geographical barrier, but also constituted a refuge zone during interglacial episodes for some micromammal species originating from northern and eastern parts of Europe (“cryptic southern refugia”).

Transcriptomics and Fitness Data Reveal Adaptive Plasticity of Thermal Tolerance in Oysters Inhabiting Different Tidal Zones.

Fine-scale adaptive evolution is always constrained by strong gene flow at vertical level in marine organisms. Rapid environmental fluctuations and phenotypic plasticity through optimization of fitness-related traits in organisms play important roles in shaping intraspecific divergence. The coastal systems experience strong variations in multiple abiotic environmental factors, especially the temperature. We used a typical intertidal species, Pacific oyster (Crassostrea gigas), to investigate the interaction between plasticity and adaptive evolution. We collected intertidal and subtidal oysters from two ecological niches and carried out common garden experiments for one generation. We identified fine-scale vertical adaptive divergence between intertidal and subtidal F1 progeny at both sites, based on different hierarchical phenotypes, including morphological, physiological, and molecular traits. We further quantified the global plasticity to thermal stress through transcriptomic analysis. The intertidal oysters exhibited slow growth rate. However, they showed high survival and metabolic rates under heat stress, indicating vertically fine-scale phenotypic adaptive mechanisms and evolutionary trade-offs between growth and thermal tolerance. Transcriptomic analysis confirmed that the intertidal oysters have evolved high plasticity. The genes were classified into three types: evolutionarily divergent, concordantly plastic, and adaptive plastic genes. The evolved divergence between intertidal and subtidal oysters for these gene sets showed a significant positive correlation with plastic changes of subtidal populations in response to high temperature. Furthermore, the intertidal oysters exhibited delayed large-scale increase in expressional plasticity than that in subtidal counterparts. The same direction between plasticity and selection suggests that the oysters have evolved adaptive plasticity. This implies that adaptive plasticity facilitates the oyster to adapt to severe intertidal zones. The oysters exposed to strong environmental variability are thermal tolerant and have high adaptive potential to face the current global warming. Our findings will not only provide new insights into the significant role of plasticity in adaptive evolution that can be extended to other marine invertebrates, but also provide basic information for oyster resources conservation and reef reestablishment.

Replication of Simulated Prebiotic Amphiphilic Vesicles in a Finite Environment Exhibits Complex Behavior That Includes High Progeny Variability and Competition.

We studied the simulated replication and growth of prebiotic vesicles composed of 140 phospholipids and cholesterol using our R-GARD (Real Graded Autocatalysis Replication Domain) formalism that utilizes currently extant lipids that have known rate constants of lipid-vesicle interactions from published experimental data. R-GARD normally modifies kinetic parameters of lipid-vesicle interactions based on vesicle composition and properties. Our original R-GARD model tracked the growth and division of one vesicle at a time in an environment with unlimited lipids at a constant concentration. We explore here a modified model where vesicles compete for a finite supply of lipids. We observed that vesicles exhibit complex behavior including initial fast unrestricted growth, followed by intervesicle competition for diminishing resources, then a second growth burst driven by better-adapted vesicles, and ending with a final steady state. Furthermore, in simulations without kinetic parameter modifications (“invariant kinetics”), the initial replication was an order of magnitude slower, and vesicles' composition variability at the final steady state was much lower. The complex kinetic behavior was not observed either in the previously published R-GARD simulations or in additional simulations presented here with only one lipid component. This demonstrates that both a finite environment (inducing selection) and multiple components (providing variation for selection to act upon) are crucial for portraying evolution-like behavior. Such properties can improve survival in a changing environment by increasing the ability of early protocellular entities to respond to rapid environmental fluctuations likely present during abiogenesis both on Earth and possibly on other planets. This in silico simulation predicts that a relatively simple in vitro chemical system containing only lipid molecules might exhibit properties that are relevant to prebiotic processes. Key Words: Phospholipid vesicles—Prebiotic compartments—Prebiotic vesicle competition—Prebiotic vesicle variability. Astrobiology 18, 419–430.

Adaptation to fluctuations in temperature by nine species of bacteria.

Rapid environmental fluctuations are ubiquitous in the wild, yet majority of experimental studies mostly consider effects of slow fluctuations on organism. To test the evolutionary consequences of fast fluctuations, we conducted nine independent experimental evolution experiments with bacteria. Experimental conditions were same for all species, and we allowed them to evolve either in fluctuating temperature alternating rapidly between 20°C and 40°C or at constant 30°C temperature. After experimental evolution, we tested the performance of the clones in both rapid fluctuation and in constant environments (20°C, 30°C and 40°C). Results from experiments on these nine species were combined meta‐analytically. We found that overall the clones evolved in the fluctuating environment had evolved better efficiency in tolerating fluctuations (i.e., they had higher yield in fluctuating conditions) than the clones evolved in the constant environment. However, we did not find any evidence that fluctuation‐adapted clones would have evolved better tolerance to any measured constant environments (20°C, 30°C, and 40°C). Our results back up recent empirical findings reporting that it is hard to predict adaptations to fast fluctuations using tolerance curves.

Diatom responses to Holocene environmental changes in a karstic Lake Vrana in Dalmatia (Croatia)

The 11 m sediment core of karstic Lake Vrana located at the Dalmatian coast of Eastern Adriatic (Croatia) was investigated through the analyses of diatoms. The diatoms exhibited six marked changes in Diatom Zones (DZ) with included water/sediment interface, as the record of environmental changes detected by marine and other terrestrial palaeoclimate records. The diatoms recorded at the deepest sediments were interpreted to represent Pleistocene/Holocene transition. The dated Holocene history is top 810 cm core section, ca. 8.1 cal kyr BP to the present. The first zone record (1101-1050 cm) was interpreted as alkaline, oligo-mesotrophic, oxic lentic ecosystem of the karstic flooding plain based on the dominance of Epithemia adnata. Enhanced bioproductivity of the shallow and unstable environment was noticed in the second diatom zone (1050-815 cm), which was interpreted as the beginning of the Holocene and it was divided into 4 subzones because of rapid environmental fluctuations. A warmer period of temperate climate with a pluvial phase initiated development of the oligo-β mesosaprobic shallow lake in the early Holocene. The expansion of the macrophytes and higher cyanobacteria production that caused acidification and oxygen depletion at the lake bottom was determined in the mid-Holocene (810-713 cm; 8.1–7.1 cal kyr BP). Mid-Holocene (8.1–4.7 cal kyr BP) is marked by oligohalobic freshwater Epithemia argus characteristic for alkaline water. High cyanobacteria production was determined as a consequence of algal production that induced anoxic conditions in the lake bottom sediments from the warmer period till today. The initial marine influx through the permeable karst underground was observed in the sediment record from ca. 5 cal kyr BP. The discovered marine influence on the lake indicating brackish water conditions related to the Holocene sea-level high stand. The sea intrusion had different intensities through time and is still observed today in the seasonal fluctuation of lake water salinity. The Late Holocene (2.7 cal kyr BP to the present) sediment record is characterized with the strongest marine influxes and highest biodiversity of the β mesosaprobic lake. Major changes in the water/sediment interface of very shallow karstic lake were caused by anthropogenic agricultural activity in Vrana. Diatom compositions of Lake Vrana indicate that they are sensitive to both paleoclimatic changes, freshwater influx and sea level rise (marine intrusion through permeable karst).

Optimized extraction of coherent arrivals from ambient noise correlations in a rapidly fluctuating medium.

Ambient noise correlations can be used to estimate Green's functions for passive monitoring purposes. However, this method traditionally relies on sufficient time-averaging of the noise-correlations to extract coherent arrivals (i.e., Green's function estimates), and is thus limited by rapid environmental fluctuations occurring on short time scales while the averaging takes place. This letter demonstrates with simulation and data that the use of a stochastic search algorithm to correct and track these rapid environmental fluctuations can significantly reduce the required averaging time to extract coherent arrivals from noise correlations in a fluctuating medium.

Optimized extraction of coherent arrivals from ambient noise correlations in a rapidly fluctuating medium, with an application to passive acoustic tomography

Ambient noise correlations can be used to estimate Green’s functions for passive monitoring purposes. However, this method traditionally relies on sufficient time-averaging of the noise-correlations to extract coherent arrivals (i.e., Green’s function estimates), and is thus limited by rapid environmental fluctuations occurring on short time scales while the averaging takes place. For instance, based on extrapolating results from a previous study [Woolfe et al., 2015], passive ocean monitoring across basin scales (i.e., between hydrophones separated by ∼1000 km) may require at least 10 weeks of averaging time to extract coherent arrivals; but such an averaging time would be too long to capture some aspects of the mesoscale variability of the ocean. To address this limitation, we will demonstrate with simulation and data that the use of a stochastic search algorithm to correct and track these rapid environmental fluctuations can reduce the required averaging time to extract coherent arrivals from noise correlations in a fluctuating medium. The algorithm optimizes the output of an objective function based on a matched filter that uses a known reference waveform to track a set of weak coherent arrivals buried in noise.

Rapidly fluctuating environments constrain coevolutionary arms races by impeding selective sweeps

Although pervasive, the impact of temporal environmental heterogeneity on coevolutionary processes is poorly understood. Productivity is a key temporally heterogeneous variable, and increasing productivity has been shown to increase rates of antagonistic arms race coevolution, and lead to the evolution of more broadly resistant hosts and more broadly infectious parasites. We investigated the effects of the grain of environmental heterogeneity, in terms of fluctuations in productivity, on bacteria–phage coevolution. Our findings demonstrate that environmental heterogeneity could constrain antagonistic coevolution, but that its effect was dependent upon the grain of heterogeneity, such that both the rate and extent of coevolution were most strongly limited in fine-grained, rapidly fluctuating heterogeneous environments. We further demonstrate that rapid environmental fluctuations were likely to have impeded selective sweeps of resistance alleles, which occurred over longer durations than the fastest, but not the slowest, frequency of fluctuations used. Taken together our results suggest that fine-grained environmental heterogeneity constrained the coevolutionary arms race by impeding selective sweeps.

El Niño Impact on Mollusk Biomineralization–Implications for Trace Element Proxy Reconstructions and the Paleo-Archeological Record

Marine macroinvertebrates are ideal sentinel organisms to monitor rapid environmental changes associated with climatic phenomena. These organisms build up protective exoskeletons incrementally by biologically-controlled mineralization, which is deeply rooted in long-term evolutionary processes. Recent studies relating potential rapid environmental fluctuations to climate change, such as ocean acidification, suggest modifications on carbonate biominerals of marine invertebrates. However, the influence of known, and recurrent, climatic events on these biological processes during active mineralization is still insufficiently understood. Analysis of Peruvian cockles from the 1982–83 large magnitude El Niño event shows significant alterations of the chemico-structure of carbonate biominerals. Here, we show that bivalves modify the main biomineralization mechanism during the event to continue shell secretion. As a result, magnesium content increases to stabilize amorphous calcium carbonate (ACC), inducing a rise in Mg/Ca unrelated to the associated increase in sea-surface temperature. Analysis of variations in Sr/Ca also suggests that this proxy should not be used in these bivalves to detect the temperature anomaly, while Ba/Ca peaks are recorded in shells in response to an increase in productivity, or dissolved barium in seawater, after the event. Presented data contribute to a better understanding of the effects of abrupt climate change on shell biomineralization, while also offering an alternative view of bivalve elemental proxy reconstructions. Furthermore, biomineralization changes in mollusk shells can be used as a novel potential proxy to provide a more nuanced historical record of El Niño and similar rapid environmental change events.

Stomatal control and leaf thermal and hydraulic capacitances under rapid environmental fluctuations.

Leaves within a canopy may experience rapid and extreme fluctuations in ambient conditions. A shaded leaf, for example, may become exposed to an order of magnitude increase in solar radiation within a few seconds, due to sunflecks or canopy motions. Considering typical time scales for stomatal adjustments, (2 to 60 minutes), the gap between these two time scales raised the question whether leaves rely on their hydraulic and thermal capacitances for passive protection from hydraulic failure or over-heating until stomata have adjusted. We employed a physically based model to systematically study effects of short-term fluctuations in irradiance on leaf temperatures and transpiration rates. Considering typical amplitudes and time scales of such fluctuations, the importance of leaf heat and water capacities for avoiding damaging leaf temperatures and hydraulic failure were investigated. The results suggest that common leaf heat capacities are not sufficient to protect a non-transpiring leaf from over-heating during sunflecks of several minutes duration whereas transpirative cooling provides effective protection. A comparison of the simulated time scales for heat damage in the absence of evaporative cooling with observed stomatal response times suggested that stomata must be already open before arrival of a sunfleck to avoid over-heating to critical leaf temperatures. This is consistent with measured stomatal conductances in shaded leaves and has implications for water use efficiency of deep canopy leaves and vulnerability to heat damage during drought. Our results also suggest that typical leaf water contents could sustain several minutes of evaporative cooling during a sunfleck without increasing the xylem water supply and thus risking embolism. We thus submit that shaded leaves rely on hydraulic capacitance and evaporative cooling to avoid over-heating and hydraulic failure during exposure to typical sunflecks, whereas thermal capacitance provides limited protection for very short sunflecks (tens of seconds).

A Dynamic Model Utilizing Linear Operators for the Characterization of Stochastic Gene Expression

Gene expression events happen stochastically because of the low number of DNA, mRNA and protein in single cells. A theoretical model for central dogma is needed to explain the variation of molecular number among cell population. Most of the analytical models in literature assume that this stochastic gene expression occurs in a stable system. However, especially in single cell organisms, gene expressions are often accompanied with rapid cell growth, division and environmental fluctuations, which are more dynamic compared to biomacromolecules synthesis and degradation. Based on this, we develop a linear operator model that takes into account other types of stochastic events besides gene expression. This model provides a theoretical explanation for how cells maintain molecular concentration homeostasis during volume expansion and molecule redistribution into daughter cells in the process of cell division. Additionally, using this approach, we are able to distinguish protein concentration reductions caused by degradation, dilution and cell division, as well as compare the effects from various types of extrinsic noise including difference between individual cells and fluctuations in intracellular substrate level and extracellular environment. This dynamic model is confirmed by numerical simulation and experimental observations.

Evidence for evolutionary convergence at MHC in two broadly distributed mesocarnivores

Variation within major histocompatibility complex (MHC) genes is important in recognizing pathogens and initiating an immune response. These genes are relevant in enhancing our understanding of how species cope with rapid environmental changes and concomitant fluctuations in selective pressures such as invasive, infectious diseases. Disease-based models suggest that diversity at MHC is maintained through balancing selection arising from the coevolution of hosts and pathogens. Despite intensive balancing selection, sequence motifs or even identical MHC alleles can be shared across multiple species; three potential mechanisms have been put forth to explain this phenomenon: common ancestry, convergent evolution, and random chance. To understand the processes that maintain MHC similarity across divergent species, we examined the variation at two orthologous MHC-DRB genes in widespread North American Musteloid species, striped skunks (Mephitis mephitis), and raccoons (Procyon lotor). These species are often sympatric and exposed to a similar suite of diseases (e.g., rabies, canine distemper, and parvovirus). Given their exposure to similar selective pressures from pathogens, we postulated that similar DRB alleles may be present in both species. Our results indicated that similar motifs are present within both species, at functionally relevant polymorphic sites. However, based on phylogenetic analyses that included previously published MHC sequences of several closely related carnivores, the respective MHC-DRB alleles do not appear to have been maintained through common ancestry and unlikely through random chance. Instead, the similarities observed between the two mesocarnivore species may rather be due to evolutionary convergence.

Role of VltAB, an ABC Transporter Complex, in Viologen Tolerance in Streptococcus mutans

Streptococcus mutans, a Gram-positive organism, is the primary causative agent in the formation of dental caries in humans. To persist in the oral cavity, S. mutans must be able to tolerate rapid environmental fluctuations and exposure to various toxic chemicals. However, the mechanisms underlying the ability of this cariogenic pathogen to survive and proliferate under harsh environmental conditions remain largely unknown. Here, we wanted to understand the mechanisms by which S. mutans withstands exposure to methyl viologen (MV), a quaternary ammonium compound (QAC) that generates superoxide radicals in the cell. To elucidate the essential genes for MV tolerance, screening of ∼3,500 mutants generated by ISS1 mutagenesis, revealed 15 MV-sensitive mutants. Among them, five and four independent insertions had occurred in SMU.905 and SMU.906 genes, respectively. These two genes are appeared to be organized in an operon and encode a putative ABC transporter complex; we designated the genes as vltA and vltB, for viologen transporter. To verify our results, vltA was deleted by using an antibiotic resistance marker; the mutant was just as sensitive to MV as the ISS1 insertion mutants. Furthermore, vltA and vltB mutants were also sensitive to other viologen compounds such as benzyl and ethyl viologens. Complementation assays were also carried out to confirm the role of VltA and VltB in viologen tolerance. Sensitivity to various drugs, including a wide range of QACs, was evaluated. It appears that a functional VltA is also required for full resistance toward acriflavin, ethidium bromide, and safranin; all are well-known QACs. These results indicate that VltA/B constitute a heterodimeric multidrug efflux pump of the ABC family. BLAST-P analysis suggests that homologs of VltA/B are widely present in streptococci, enterococci, and other important Gram-positive pathogens.

Groundwater discharge to a Gobi desert lake during Mid and Late Holocene dry periods

High levels of Lake Eastern Juyanze and a wetter climate in the Gobi desert (NW China) were recorded between 5100 and 4100 cal. year BP from ostracod distribution, shell geochemistry, sedimentology and palynological data. After about 4100 cal. year BP lake levels decreased and groundwater discharge became dominant source of water during short-term regressive events at about 4000, 3800 and 3500 cal. year BP as suggested by a significant difference in modern river water and groundwater chemistry and the geochemical signal of fossil ostracod shells. The overall trend towards lower lake levels culminated in repeated episodes of desiccation of Lake Eastern Juyanze between 3200 and 2900 cal. year BP. Individual regressive events as well as the rapid environmental fluctuations of Lake Eastern Juyanze at about 3000 cal. year BP were likely caused by Mid to Late Holocene temperature fluctuations, recorded by the Dunde ice core [Yao, T., Thompson, L.G., 1992. Trends and features of climatic changes in the past 5000 years recorded by the Dunde ice core. Annals of Glaciology 16, 21–24] and shifts of the southeastern Asian monsoon.

Biodiversity in model ecosystems, II: species assembly and food web structure

This is the second of two papers dedicated to the relationship between population models of competition and biodiversity. Here, we consider species assembly models where the population dynamics is kept far from fixed points through the continuous introduction of new species, and generalize to such models the coexistence condition derived for systems at the fixed point. The ecological overlap between species and shared preys, that we define here, provides a quantitative measure of the effective interspecies competition and of the trophic network topology. We obtain distributions of the overlap from simulations of a new model based both on immigration and speciation, and show that they are in good agreement with those measured for three large natural food webs. As discussed in the first paper, rapid environmental fluctuations, interacting with the condition for coexistence of competing species, limit the maximal biodiversity that a trophic level can host. This horizontal limitation to biodiversity is here combined with either dissipation of energy or growth of fluctuations, which in our model limit the length of food webs in the vertical direction. These ingredients yield an effective model of food webs that produce a biodiversity profile with a maximum at an intermediate trophic level, in agreement with field studies.

Responses of Cariogenic Streptococci to Environmental Stresses

To persist in the oral cavity, bacteria must be able to tolerate rapid and substantial environmental fluctuations, particularly in pH and nutrient source and availability. Various species of Streptococcus, one of the most abundant genera in the mouth, are associated with oral health, as well as with dental caries. Cariogenic streptococci depend on a biofilm lifestyle for survival and persistence in the oral cavity and have developed sophisticated mechanisms to cope with environmental stresses. Here, we analyze the primary factors that allow these bacteria to emerge as significant members of tooth biofilms during adverse conditions. Our focus is on the molecular mechanisms of biofilm formation, stress tolerance and sugar metabolism by pathogenic oral streptococci, mainly Streptococcus mutans. Overlaps in the roles and regulation of these virulence attributes are highlighted and areas of research that deserve further investigation are proposed.