Environmental calcium modifies induced defences in snails

Inducible defences are adaptive phenotypes that arise in response to predation threats. Such plasticity incurs costs to individuals, but there has been little interest in how such induced traits in animals may be constrained by environmental factors. Here, we demonstrate that calcium availability interacts with predation cues to modify snail shell growth and form. Small snails increased their growth and were heavier when exposed to fish chemical cues, but this response was calcium limited. There was also an interactive effect of fish cues and calcium on the shell growth of larger snails, but shell strength and aperture narrowness were affected by calcium alone. For small snails, behavioural avoidance was greatest for snails exhibiting least morphological plasticity, suggesting a trade-off. There was no trade-off of somatic growth with plasticity. We suggest that the expression of defensive traits in molluscs can be constrained by calcium availability, which has implications for molluscan ecology and evolution.     

Status of copper in some calcareous and non-calcareous soils of Egypt

Summary To assess the status of copper in Egyptian soils, surface and subsurface soil samples were collected from various geographical regions of Egypt and of various genesis. The samples were analyzed for the total Cu, water-soluble plus exchangeable as well as the acid-extractable and EDTA-extractable Cu. The total Cu varied between 3.5 and 72.3 ppm. Less than 2% of this copper was in the soluble plus exchangeable form. The highest values of total and soluble plus exchangeable copper were found in the alluvial soils while the sandy soils had the least amounts. This was attributed to the high clay and organic matter content of the alluvial soils in addition to the dominance of montmorillonitic clay minerals in their fine fraction. The calcareous soils showed intermediate values of total and soluble plus exchangeable Cu due to their lower organic matter and clay mineral content. The clay mineralogy of these soils revealed the dominance of illitic and kaolinitic minerals which are relatively poor in Cu and other trace elements.Because of their relatively higher organic matter content, the alluvial soils had a large percentage (up to 43%) of their total Cu in the EDTA extractable form. The calcareous soils, on the other hand, had a large percentage (up to 52%) of their Cu in the acid-extractable form. The EDTA extractable Cu was correlated with the organic matter content of the soils. Since the pH of the EDTA extract was found to be dependent on the CaCO₃ content of the soils, it was suggested that the method be modified so that the final pH of the extract is constant for all soils.     

Indices of calcium availability and degree of dispersion in soils

Summary A comparison of the various indices of calcium availability and degree of dispersion of the soils showed that Mehlich's 1.0 me H method may be a good index of calcium availability for a particular group of soils, but the modified calcium-release value of the exchangeable phase was a superior index when a more diversified group of soils was involved. The type of the clay, cation-exchange capacity, organic matter content,etc. were the most likely factors to influence the choice of the method.The indices based on exchangeable ions were superior to those based on ion concentration in solution for predicting the dispersability of the soil.     

Water-borne cues from a shell-crushing predator induce a more massive shell in experimental populations of an intertidal snail

Inducible defenses are important in the life strategies of many taxa. In some species of marine gastropods, water-borne chemical cues from potential predators induce defensive changes in shell form and differences in growth rate. We examined such phenotypic plasticity in the direct-developing snail, Littorina subrotundata (Carpenter, 1864). Among experimental field populations of L. subrotundata exposed to differing intensities of predation by the purple shore crab, Hemigrapsus nudus (Dana, 1851), snails collected from predation-intense environments often had more massive shells than closely related snails from adjacent environments where predation was negligible. Snails collected from both environments were raised in tanks containing cages of H. nudus that were feeding on conspecific snails and compared to a control group raised in the absence of this stimulus. Most snails developed significantly more massive shells in the presence of the crabs suggesting that adaptive phenotypic plasticity may account for some of the variation we observed in the field. In one case, snails from a predation-intense environment did not exhibit a statistically significant amount of plasticity, but instead grew a more massive shell irrespective of the laboratory stimulus. We interpret this as evidence for a genetic difference in the plasticity of shell form among experimental populations, caused by intense selection by H. nudus. There was no statistical difference in the growth rates of snails among treatments.     

Impact of Soil Cadmium on Land Snails: A Two-Stage Exposure Approach under Semi-Field Conditions Using Bioaccumulative and Conchological End-Points of Exposure

Land snails are highly tolerant to cadmium exposure and are able to accumulate soil cadmium independently of food ingestion. However, little information exists on the kinetics of cadmium retention in terrestrial gastropods exposed to an increase in the soil cadmium content, over time. There is also little knowledge about how exposure to cadmium-polluted soils influences shell growth and architecture. In this context, we examined cadmium accumulation in the hepatopancreas and shell of juvenile Cantareus aspersus exposed to elevating high levels of cadmium in soil. Also, the toxicity of cadmium to snails was assessed using a range of conchological endpoints, including shell height, width, volume, allometry and integrity. Test snails, aged three months, were reared under semi-field conditions, fed an uncontaminated diet and exposed first, for a period of 30 days, to a series of soil cadmium concentrations, and then, for a second period of 30 days, to soils with higher cadmium content. Cadmium showed a dose-dependent accumulation in both the hepatopancreas and shell. The kinetics of cadmium retention in the hepatopancreas of snails previously exposed to cadmium-spiked soils was significantly influenced by a new exposure event. The shell was not a relevant bioaccumulator for soil cadmium. Under the present experimental conditions, only high cadmium exposure significantly affected either the shell growth or snail survival. There was no consistent effect on shell allometry, but the shell integrity, especially in rapidly growing parts, appeared to be affected by high cadmium exposure. Our results attest to the value of hepatopancreas for describing cadmium retention in land snails and to the difficulty of using conchological parameters in field surveys for estimating the environmental hazard of soil cadmium.     

The effect of aquatic plant abundance on shell crushing resistance in a freshwater snail

Most of the shell material in snails is composed of calcium carbonate but the organic shell matrix determines the properties of calcium carbonate crystals. It has been shown that the deposition of calcium carbonate is affected by the ingestion of organic compounds. We hypothesize that organic compounds not synthesized by the snails are important for shell strength and must be obtained from the diet. We tested this idea indirectly by evaluating whether the abundance of the organic matter that snails eat is related to the strength of their shells. We measured shell crushing resistance in the snail Mexipyrgus churinceanus and the abundance of the most common aquatic macrophyte, the water lily Nymphaea ampla, in ten bodies of water in the valley of Cuatro Ciénegas, Mexico. We used stable isotopes to test the assumption that these snails feed on water lily organic matter. We also measured other factors that can affect crushing resistance, such as the density of crushing predators, snail density, water pH, and the concentration of calcium and phosphorus in the water. The isotope analysis suggested that snails assimilate water lily organic matter that is metabolized by sediment bacteria. The variable that best explained the variation in crushing resistance found among sites was the local abundance of water lilies. We propose that the local amount of water lily organic matter provides organic compounds important in shell biomineralization, thus determining crushing resistance. Hence, we propose that a third trophic level could be important in the coevolution of snail defensive traits and predatory structures.     

The sources and distribution of carbon (DOC,POC, DIC) in a mangrove dominated estuary (French Guiana,South America)

Improved quantification of the factors controlling soil organic matter (SOM) stabilization at continental to global scales is needed to inform projections of the largest actively cycling terrestrial carbon pool on Earth, and its response to environmental change. Biogeochemical models rely almost exclusively on clay content to modify rates of SOM turnover and fluxes of climate-active CO₂ to the atmosphere. Emerging conceptual understanding, however, suggests other soil physicochemical properties may predict SOM stabilization better than clay content. We addressed this discrepancy by synthesizing data from over 5,500 soil profiles spanning continental scale environmental gradients. Here, we demonstrate that other physicochemical parameters are much stronger predictors of SOM content, with clay content having relatively little explanatory power. We show that exchangeable calcium strongly predicted SOM content in water-limited, alkaline soils, whereas with increasing moisture availability and acidity, iron- and aluminum-oxyhydroxides emerged as better predictors, demonstrating that the relative importance of SOM stabilization mechanisms scales with climate and acidity. These results highlight the urgent need to modify biogeochemical models to better reflect the role of soil physicochemical properties in SOM cycling.     

Prey-induced changes to a predator's behaviour and morphology: Implications for shell-claw covariance in the northwest Atlantic

Whereas many plasticity studies demonstrate the importance of inducible defences among prey, far fewer investigate the potential role of inducible offences among predators. Here we ask if natural differences in a snail's shell hardness can induce developmental changes to a predatory crab's claw size. To do this, we fed Littorina obtusata snails from either thick- or thin-shelled populations to captive European green crabs Carcinus maenas. The crabs' shell-breaking behaviour dominated among those fed thin-shelled snails, whereas crabs fed thick-shelled snails mostly winkled flesh through the shell opening without damaging the shell itself (a.k.a. aperture-probing behaviour). Significantly, the size of crab crusher claws grew in proportion to the frequency of shell-crushing behaviour and, for a same shell-crushing frequency, crabs fed thick-shelled snails grew larger crusher claws than those fed thin-shelled snails after two experimental moults. Diet and behaviour had no effect on the growth of the smaller cutter claws of same individuals, providing good evidence that allometric changes to crusher claws were indeed a result of differential use while feeding. Findings indicate that both predation habits and claw sizes are affected by green crabs' diet, supporting the hypothesis that prey-induced phenotypic plasticity contributes to earlier accounts of shell-claw covariance between this predator and its Littorina prey in the wild.     

湖南烟区土壤交换性钙、镁含量及对烤烟品质的影响

分析了湖南烟区主要土壤类型交换性钙、镁元素含量状况及其对烟叶品质的影响,结果表明:(1)土壤交换性钙、镁含量在不同土壤类型间存在显著性差异,交换性钙含量平均为8.87cmol/kg,以红壤含量最高;交换性镁含量平均为1.16cmol/kg,以黄棕壤含量最高;交换性钙镁比值大小依次为:红壤(11.74)>水稻土(10.25)>黄壤(6.84)>黄棕壤(6.14),在烟叶实际生产中,应重视镁肥在红壤和水稻土中的施用;(2)烟叶钙含量偏高(21.93g/kg±4.37g/kg),烟叶镁含量偏低(2.52g/kg±1.26g/kg),两者均存在广泛的变异性;(3)整体来看,烟叶钙含量随土壤中交换性钙含量的升高和镁含量的降低而显著升高;烟叶镁含量随土壤交换性镁含量的升高而升高,与土壤交换性钙含量的相关性不显著;(4)典型相关分析表明,土壤中交换性镁含量的降低可能引起烟叶钾含量的提高,从而使得烟叶钾素和镁素含量达到较好的平衡;(5)土壤交换性钙、镁含量与烟叶其它化学成分指标的相关分析表明,土壤交换性钙有利于烟株对硼和氯素的吸收,对氮、锌和锰素的吸收则有显著的抑制作用;而土壤交换性镁有利于烟叶总糖、硼素和锰素的积累,对氮、磷、铁和锌素的吸收具有显著的抑制作用。     

Soil Manganese Enrichment from Industrial Inputs: A Gastropod Perspective

Manganese is one of the most abundant metal in natural environments and serves as an essential microelement for all living systems. However, the enrichment of soil with manganese resulting from industrial inputs may threaten terrestrial ecosystems. Several studies have demonstrated harmful effects of manganese exposure by cutaneous contact and/or by soil ingestion to a wide range of soil invertebrates. The link between soil manganese and land snails has never been made although these invertebrates routinely come in contact with the upper soil horizons through cutaneous contact, egg-laying, and feeding activities in soil. Therefore, we have investigated the direct transfer of manganese from soils to snails and assessed its toxicity at background concentrations in the soil. Juvenile Cantareus aspersus snails were caged under semi-field conditions and exposed first, for a period of 30 days, to a series of soil manganese concentrations, and then, for a second period of 30 days, to soils with higher manganese concentrations. Manganese levels were measured in the snail hepatopancreas, foot, and shell. The snail survival and shell growth were used to assess the lethal and sublethal effects of manganese exposure. The transfer of manganese from soil to snails occurred independently of food ingestion, but had no consistent effect on either the snail survival or shell growth. The hepatopancreas was the best biomarker of manganese exposure, whereas the shell did not serve as a long-term sink for this metal. The kinetics of manganese retention in the hepatopancreas of snails previously exposed to manganese-spiked soils was significantly influenced by a new exposure event. The results of this study reveal the importance of land snails for manganese cycling in terrestrial biotopes and suggest that the direct transfer from soils to snails should be considered when precisely assessing the impact of anthropogenic Mn releases on soil ecosystems.     

Phenotypic clines, plasticity, and morphological trade-offs in an intertidal snail

Understanding the genetic and environmental bases of phenotypic variation and how they covary on local and broad geographic scales is an important goal of evolutionary ecology. Such information can shed light on how organisms adapt to different and changing environments and how life-history trade-offs arise. Surveys of phenotypic variation in 25 Littorina obtusata populations across an approximately 400-km latitudinal gradient in the Gulf of Maine revealed pronounced clines. The shells of snails from northern habitats weighed less and were thinner and weaker in compression than those of conspecifics from southern habitats. In contrast, body size (as measured by soft tissue mass) followed an opposite pattern; northern snails weighed more than southern snails. A reciprocal transplant between a northern and southern habitat revealed substantial plasticity in shell form and body mass and their respective measures of growth. Southern snails transplanted to the northern habitat produced lighter, thinner shells and more body mass than controls raised in their native habitat. In contrast, northern snails transplanted to the southern site produced heavier, thicker shells and less body mass than controls raised in their native habitat. Patterns of final phenotypic variation for all traits were consistent with cogradient variation (i.e., a positive covariance between genetic and environmental influences). However, growth in shell traits followed a countergradient pattern (i.e., a negative covariance between genetic and environmental influences). Interestingly, body growth followed a cogradient pattern, which may reflect constraints imposed by cogradient variation in final shell size and thickness. This result suggests the existence of potential life-history trade-offs associated with increased shell production. Differences in L. obtusata shell form, body mass, and their respective measures of growth are likely induced by geographic differences in both water temperature and the abundance of an invading crab predator (Carcinus maenas). Water temperatures averaged 6.8 degrees C warmer during the transplant experiment and C. maenas abundance is greater in the southern Gulf of Maine. Because both increased water temperature and crab effluent affect shell form in the same way, future experiments are needed to determine the relative importance of each. Nevertheless, it is clear that phenotypic plasticity has an important role in producing geographic variation in L. obtusata shell form. Moreover, the evolution of phenotypic plasticity in L. obtusata and other marine gastropods may be driven by architectural constraints imposed by shell form on body mass and growth.     

Cue reliability, risk sensitivity and inducible morphological defense in a marine snail

Reliable cues that communicate current or future environmental conditions are a requirement for the evolution of adaptive phenotypic plasticity, yet we often do not know which cues are responsible for the induction of particular plastic phenotypes. I examined the single and combined effects of cues from damaged prey and predator cues on the induction of plastic shell defenses and somatic growth in the marine snail Nucella lamellosa. Snails were exposed to chemical risk cues from a factorial combination of damaged prey presented in isolation or consumed by predatory crabs (Cancer productus). Water-borne cues from damaged conspecific and heterospecific snails did not affect plastic shell defenses (shell mass, shell thickness and apertural teeth) or somatic growth in N. lamellosa. Cues released by feeding crabs, independent of prey cue, had significant effects on shell mass and somatic growth, but only crabs consuming conspecific snails induced the full suite of plastic shell defenses in N. lamellosa and induced the greatest response in all shell traits and somatic growth. Thus the relationship between risk cue and inducible morphological defense is dependent on which cues and which morphological traits are examined. Results indicate that cues from damaged conspecifics alone do not trigger a response, but, in combination with predator cues, act to signal predation risk and trigger inducible defenses in this species. This ability to “label” predators as dangerous may decrease predator avoidance costs and highlights the importance of the feeding habits of predators on the expression of inducible defenses.     

Studies on the prolonged storage of Metarhizium anisopliae conidia: effect of temperature and relative humidity on conidial viability and virulence against mosquitoes

The shell lengths, dried shell weights, soft tissue wet weights, and soft tissue dry weights were ascertained for noninfected Ilyanassa obsoleta and for the same snail species naturally infected with the following digeneans: Himasthla quissetensis, Zoogonus lasius, Cercaria dipterocerca, Lepocreadium setiferoides, Microphalloides nassicola, Stephanostomum tenue, and Microbilharzia variglandis. Analyses of the data obtained indicate that infection with all of the trematodes listed above causes neither enhanced growth of soft tissues nor accelerated lengthening of the shell. However, snails infected with sporocysts of Z. lasius have significantly heavier shells than do noninfected snails. These data suggest that infection with Z. lasius may have caused enhanced growth in the form of increased calcium deposition in the shell of I. obsoleta, possibly as a result of parasitic castration. In addition, the mean dry weight of the soft tissues of snails infected with M. variglandis, when normalized for shell length, is significantly lower than that of noninfected snails, possibly as a result of parasite-induced pathology. Finally, comparisons among infected snails indicate a trend toward increased soft tissue dry weight in snails infected with L. setiferoides, although the mean dry weight of these snails does not differ significantly from the mean dry weight of noninfected snails.     

Nitrogen mineralization and its controlling factors in various kinds of temperate humid-zone soils

The N mineralization capacity of 41 temperate humid-zone soils of NW Spain was measured by aerobic incubation for 15 days at 28°C and 75% of field capacity. The main soil factors affecting organic N dynamics were identified by principal components analysis. Ammonification predominated over nitrification in almost all soils. The mean net N mineralization rate was 1.63% of the organic N content, and varied according to soil parent materials as follows: soils on basic and ultrabasic rocks < soils over acid metamorphic rocks < soils developed over sediments < soils over acid igneous rocks < soils on limestone. The N mineralization capacity was lower in natural soils than in cropped soils or pastures. The accumulation of organic matter (C and N) seems to be due to poor mineralization which was caused, in decreasing order of importance, by high exchangeable H-ion levels, high Al and Fe gel contents and, to a lesser extent (though more markedly in cropped soils), by silty clay texture and exchangeable Al ions.     

The heterogeneous nature of microbial products as shown by solid-state13C CP/MAS NMR spectroscopy

Homoionic Na-, Ca-, and Al-clays were prepared from the <2 m fractions of Georgia kaolinite and Wyoming bentonite and mixed with sand to give artificial soils with 5, and 25% clay. The artificial soils were inoculated with microbes from a natural soil before incubation. Unlabelled and uniformly¹³C-labelled (99.9% atom) glucose were incorporated into the artificial soils to study the effects of clay types, exchangeable cations and clay contents on the mineralization of glucose-carbon and glucose-derived organic materials. Chemical transformation of glucose-carbon upon incorporation into microbial products and metabolites, was followed using solid-state¹³C CP/MAS NMR spectroscopy.There was a significant influence of exchangeable cations on the mineralization of glucose-carbon over a period of 33 days. At 25% clay content, mineralization of glucose-carbon was highest in Ca-soils and lowest in Al-soils. The influence of exchangeable cations on mineralization of glucose-carbon was more pronounced in soils with bentonite clay than those with kaolinite clay. Statistical analysis of data showed no overall effect of clay type on mineralization of glucose-carbon. However, the interactions of clay type with clay content and clay type with clay content and exchangeable cations were highly significant. At 25% clay content, the mineralization of glucose-carbon was significantly lower in Na- and Al-soils with Wyoming bentonite compared with Na- and Al-soils with Georgia kaolinite. For Ca-soils this difference was not significant. Due to the increased osmotic tension induced by the added glucose, mineralization of glucose-carbon was slower in soils with 5% clay than soils with 25% clay.Despite the differences in the chemical and physical characteristics of soils with Ca-, Na- and Al-clays, the chemical composition of organic materials synthesised in these soils were similar in nature. Assuming CP/MAS is quantitative, incorporation of uniformly¹³C-labelled glucose (99.9% atom) in these soils resulted in distribution of carbon in alkyl (24–25%), O-alkyl (56–63%), carbonyl (11–15%) and small amounts of aromatic and olefinic carbon (2–4%). However, as decomposition proceeded, the chemistry of synthesised material showed some changes with time. In the Ca- and Na-soils, the proportions of alkyl and carbonyl carbon decreased and that of O-alkyl carbon increased with time of incubation. However, the opposite trend was found for the Al-soil.Proton-spin relaxation editing (PSRE) subspectra clearly showed heterogeneity within the microbial products. Subspectra of the slowly-relaxing (long T₁(H)) domains were dominated by alkyl carbon in long- and short-chain structures. The signals due to N-alkyl (55 ppm) and carbonyl carbon were also strong in these subspectra. These subspectra were very similar to those obtained for microbial and fungal materials and were probably microbial tissues attached to clay surfaces by polysaccharide extracellular mucilage. Subspectra of fast-relaxing (short T₁(H)) domains comprised mostly O-alkyl and carbonyl carbon and were probably microbial metabolites released as neutral and acidic sugars into the extracellular environment, and strongly sorbed by clay surfaces.     

不同壳色福寿螺形态性状与体质量的关系

【背景】福寿螺因其食性杂、抗逆性和繁殖力强以及自然天敌少等不断扩散,侵害农作物,被列为我国首批外来入侵物种。国内外学者一直致力于研究对其的防治与监控。自然界中福寿螺存在2种壳色——黄色和黑色,壳色受遗传因素和环境因素的双重影响。广东省福寿螺多以黑色为主,福寿螺倾向于与不同壳色的螺交配。壳色在一定程度上影响其交配的选择性,但2种壳色的福寿螺繁殖力指标差异不显著。而关于这2种壳色的螺在形态学上的差异鲜有报道。【方法】利用生物统计软件和分析方法进行相关性分析、通径分析及多元回归分析,计算相关系数、通径系数和决定系数,研究2种壳色福寿螺形态性状与体质量的关系。【结果】2种壳色福寿螺的体质量、层高的变异系数较大,且黄色比黑色变异系数大。对黄色福寿螺体质量影响较大的依次为壳高、口宽;对黑色福寿螺体质量影响较大的依次为口宽、层高。【结论与意义】2种壳色福寿螺在形态性状方面差异显著,可以将壳色作为特征标记,为福寿螺的监测与灾害评估提供参考。     

Adaptive plasticity in predator-induced defenses in a common freshwater snail: altered selection and mode of predation due to prey phenotype

Studies of putatively adaptive plasticity, such as inducible defenses, frequently explore the fitness consequences of expressing alternative phenotypes in alternative environments, but few studies examine how and why the pattern of selection changes in relation to trait induction. We induced snails in the presence/absence of nonlethal predatory crayfish, exposed both phenotypes (alone and combined) to selection by lethal crayfish, and quantified linear and nonlinear selection differentials. Crayfish induced an increase in mass, shell thickness, and absolute (but not relative) shell dimensions. Crayfish predation on uninduced snails was rapid, accomplished via shell-crushing and revealed strong selection for increased size (i.e., mass and shell dimensions). Conversely, crayfish predation on predator-induced snails was slower, often accomplished using an alternative mode of predation (shell-crushing 70% of the time, but shell-extraction 30% of the time), and revealed selection for wide apertures and thick shells. Crayfish selection on uninduced snails in the presence of predator-induced snails was stronger than predation on uninduced snails alone demonstrating that selection can be frequency dependent. Therefore, predator-induced changes in size and shell thickness appear to be adaptive and, along with reciprocal adjustments in the mode of predation, result in altered patterns of selection.     

Biomineralization plasticity and environmental heterogeneity predict geographical resilience patterns of foundation species to future change

Although geographical patterns of species' sensitivity to environmental changes are defined by interacting multiple stressors, little is known about compensatory processes shaping regional differences in organismal vulnerability. Here, we examine large‐scale spatial variations in biomineralization under heterogeneous environmental gradients of temperature, salinity and food availability across a 30° latitudinal range (3,334 km), to test whether plasticity in calcareous shell production and composition, from juveniles to large adults, mediates geographical patterns of resilience to climate change in critical foundation species, the mussels Mytilus edulis and M. trossulus. We find shell calcification decreased towards high latitude, with mussels producing thinner shells with a higher organic content in polar than temperate regions. Salinity was the best predictor of within‐region differences in mussel shell deposition, mineral and organic composition. In polar, subpolar, and Baltic low‐salinity environments, mussels produced thin shells with a thicker external organic layer (periostracum), and an increased proportion of calcite (prismatic layer, as opposed to aragonite) and organic matrix, providing potentially higher resistance against dissolution in more corrosive waters. Conversely, in temperate, higher salinity regimes, thicker, more calcified shells with a higher aragonite (nacreous layer) proportion were deposited, which suggests enhanced protection under increased predation pressure. Interacting effects of salinity and food availability on mussel shell composition predict the deposition of a thicker periostracum and organic‐enriched prismatic layer under forecasted future environmental conditions, suggesting a capacity for increased protection of high‐latitude populations from ocean acidification. These findings support biomineralization plasticity as a potentially advantageous compensatory mechanism conferring Mytilus species a protective capacity for quantitative and qualitative trade‐offs in shell deposition as a response to regional alterations of abiotic and biotic conditions in future environments. Our work illustrates that compensatory mechanisms, driving plastic responses to the spatial structure of multiple stressors, can define geographical patterns of unanticipated species resilience to global environmental change.     

Influence of Edaphic,Climatic, and Agronomic Factors on the Composition and Abundance of Nitrifying Microorganisms in the Rhizosphere of Commercial Olive Crops

The microbial ecology of the nitrogen cycle in agricultural soils is an issue of major interest. We hypothesized a major effect by farm management systems (mineral versus organic fertilizers) and a minor influence of soil texture and plant variety on the composition and abundance of microbial nitrifiers. We explored changes in composition (16S rRNA gene) of ammonia-oxidizing archaea (AOA), bacteria (AOB), and nitrite-oxidizing bacteria (NOB), and in abundance of AOA and AOB (qPCR of amoA genes) in the rhizosphere of 96 olive orchards differing in climatic conditions, agricultural practices, soil properties, and olive variety. Majority of archaea were 1.1b thaumarchaeota (soil crenarchaeotic group, SCG) closely related to the AOA genus Nitrososphaera. Most AOB (97%) were identical to Nitrosospira tenuis and most NOB (76%) were closely related to Nitrospira sp. Common factors shaping nitrifiers assemblage composition were pH, soil texture, and olive variety. AOB abundance was positively correlated with altitude, pH, and clay content, whereas AOA abundances showed significant relationships with organic nitrogen content and exchangeable K. The abundances of AOA differed significantly among soil textures and olive varieties, and those of AOB among soil management systems and olive varieties. Overall, we observed minor effects by orchard management system, soil cover crop practices, plantation age, or soil organic matter content, and major influence of soil texture, pH, and olive tree variety.     

Factors affecting metal concentrations in reed plants (Phragmites australis) of intertidal marshes in the Scheldt estuary

We aimed to identify the environmental factors which significantly affect metal uptake by reed plants in the intertidal marshes along the river Scheldt. Transfer coefficients, defined as the ratio of metal concentrations in reed stems to the metal contents in specific sediment fractions (i.e. the exchangeable Cd and Zn fraction and total Cr, Cu, Ni and Pb content), were calculated for each sampling site. They were inversely related to the sediment clay and/or organic matter content. Metal mobility and thus plant availability is higher in sediments with a lower clay or organic matter content. Moreover, the plants might actively accumulate in particular essential elements when concentrations in the sediments are rather low, which is the case in sediments low in clay and organic matter contents. Finally, more sandy sediments are expected to be susceptible to occasional oxidation of sulphides, which leads to an increased metal availability. A higher salinity promoted the uptake of Cu, Cr and Zn.