Application of CO2 carbon stable isotope analysis to ant trophic ecology

Stable isotope analysis of animal tissues is commonly used to infer diet and trophic position. However, it requires destructive sampling. The analysis of carbon isotopes from exhaled CO₂ is non-invasive and can provide useful ecological information because isotopic CO₂ signatures can reflect the diet and metabolism of an animal. However, this methodology has rarely been used on invertebrates and never on social insects. Here, we first tested whether this method reflects differences in δ¹³C-CO₂ between workers of the Mediterranean ant Crematogaster scutellaris (Olivier) (Hymenoptera: Formicidae, Crematogastrini) fed with sugar from beet (C3; Beta vulgaris L., Amaranthaceae) or cane (C4; Saccharum officinarum L., Poaceae). We found that a significant difference can be obtained after 24 h. Consequently, we used this technique on wild co-occurring ant species with different feeding preferences to assess their reliance on C3 or C4 sources. For this purpose, we sampled workers of C. scutellaris, the invasive garden ant Lasius neglectus (van Loon et al.) (Lasiini), and the harvester ant Messor capitatus (Latreille) (Stenammini). No significant differences in their carbon isotopic signatures were recorded, suggesting that in our study site no niche partitioning occurs based on the carbon pathway, with all species sharing similar resources. However, further analysis revealed that M. capitatus, a seed-eating ant, can be regarded as a C3 specialist, whereas L. neglectus and C. scutellaris are generalists that rely on both C3 and C4 pathways, though with a preference for the former. Our results show that this methodology can be applied even to small animals such as ants and can provide useful information on the diets of generalist omnivores.     

First estimates of metabolic rate in Atlantic bluefin tuna larvae

Atlantic bluefin tuna is an iconic scombrid species with a high commercial and ecological value. Despite their importance, many physiological aspects, especially during the larval stages, are still unknown. Metabolic rates are one of the understudied aspects in scombrid larvae, likely due to challenges associated to larval handling before and during respirometry trials. Gaining reliable estimates of metabolic rates is essential to understand how larvae balance their high growth needs and activity and other physiological functions, which can be very useful for fisheries ecology and aquaculture. This is the first study to (a) estimate the relationship between routine metabolic rate (RMR) and larval dry weight (DW) (mass scaling exponent) at a constant temperature of 26°C, (b) measure the RMR under light and darkness and (c) test whether the interindividual differences in the RMR are related to larval nutritional status (RNA/DNA and DNA/DW). The RMR scaled nearly isometrically with body size (b = 0.99, 0.60–31.56 mg DW) in contrast to the allometric relationship observed in most fish larvae (average b = 0.87). The results show no significant differences in larval RMR under light and darkness, suggesting similar larval activity levels in both conditions. The size explained most of the variability in RMR (97%), and nutritional condition was unrelated to the interindividual differences in routine metabolism. This is the first study to report the metabolic rates of Atlantic bluefin tuna larvae and discuss the challenges of performing bioenergetic studies with early life stages of scombrids.     

鲍曼不动杆菌体外诱导耐药及其诱导前后交叉耐药和呼吸耗氧率分析

【背景】鲍曼不动杆菌是院内感染的重要病原菌,因其耐药率高、治疗难度大而备受关注。然而,对于该菌的交叉耐药及耐药相关因素尚未完全阐明。【目的】通过体外诱导分别获得耐美罗培南或耐替加环素的鲍曼不动杆菌菌株,并研究其诱导前后的交叉耐药性和细菌呼吸耗氧率差异。【方法】采用多步法对鲍曼不动杆菌ATCC19606进行体外诱导耐药,PCR扩增诱导前后菌株的16S rRNA基因并测序鉴定,微量肉汤稀释法检测诱导前后鲍曼不动杆菌对美罗培南、亚胺培南、替加环素、阿米卡星、头孢吡肟及左氧氟沙星等抗菌药物的最低抑菌浓度变化,Seahorse XF~e96细胞能量代谢实时测定仪对诱导前后菌株的耗氧率进行分析。【结果】通过88d的体外诱导实验,分别获得耐美罗培南或耐替加环素的鲍曼不动杆菌ATCC19606菌株。耐美罗培南鲍曼不动杆菌ATCC19606对替加环素、亚胺培南、阿米卡星、左氧氟沙星仍处于敏感状态,但是对头孢吡肟交叉耐药;耐替加环素鲍曼不动杆菌ATCC19606对美罗培南、亚胺培南、阿米卡星、左氧氟沙星及头孢吡肟仍处于敏感状态。鲍曼不动杆菌ATCC19606被美罗培南或替加环素诱导耐药之后的耗氧率均下降,差异均具有统计学意义。【结论】美罗培南的使用不仅可能诱导鲍曼不动杆菌ATCC19606对美罗培南耐药,也可能会导致该菌对其它一种或几种抗菌药物产生交叉耐药。鲍曼不动杆菌ATCC19606对美罗培南或替加环素耐药后其耗氧率下降,从而说明呼吸耗氧率下降可能是该菌耐药的因素之一。     

Small tropical forest trees have a greater capacity to adjust carbon metabolism to long-term drought than large canopy trees

The response of small understory trees to long-term drought is vital in determining the future composition, carbon stocks and dynamics of tropical forests. Long-term drought is, however, also likely to expose understory trees to increased light availability driven by drought-induced mortality. Relatively little is known about the potential for understory trees to adjust their physiology to both decreasing water and increasing light availability. We analysed data on maximum photosynthetic capacity (J_max, V_cmax), leaf respiration (R_leaf), leaf mass per area (LMA), leaf thickness and leaf nitrogen and phosphorus concentrations from 66 small trees across 12 common genera at the world's longest running tropical rainfall exclusion experiment and compared responses to those from 61 surviving canopy trees. Small trees increased J_max, V_cmax, R_leaf and LMA (71, 29, 32, 15% respectively) in response to the drought treatment, but leaf thickness and leaf nutrient concentrations did not change. Small trees were significantly more responsive than large canopy trees to the drought treatment, suggesting greater phenotypic plasticity and resilience to prolonged drought, although differences among taxa were observed. Our results highlight that small tropical trees have greater capacity to respond to ecosystem level changes and have the potential to regenerate resilient forests following future droughts.     

Novel technique for component monitoring of CO2 exchange in plants

A novel technique designed for component monitoring of CO₂ exchange in plants is described. The system is based on application of self-clamping leaf chambers connected to an open gas-exchange measuring system and on automatic recording of CO₂ concentration. This technique was implemented in a commercially available instrument, PTM-48A Photosynthesis Monitor, which provides for long-term measurements of gas exchange and for discrimination of its separate components. Furthermore, many other plant functions can be monitored during plant growth and development under laboratory, greenhouse, and field conditions.     

The epigenetic calnexin-independent state is induced in response to environmental changes

Yeasts have evolved numerous responsive pathways to survive in fluctuating and stressful environments. The endoplasmic reticulum (ER) is sensitive to adverse conditions, which are detected by response pathways to ensure correct protein folding. Calnexin is an ER transmembrane chaperone acting in both quality control of folding and response to persistent stress. Calnexin is a key protein required for viability in certain organisms such as mammals and the fission yeast Schizosaccharomyces pombe . Nevertheless, S. pombe calnexin-independent (Cin) cells were obtained after transient expression of a particular calnexin mutant. The Cin state is dominant, is stably propagated by an epigenetic mechanism and segregates in a non-Mendelian fashion to the meiotic progeny. The nucleolar protein Cif1p was identified as an inducer of the Cin state in a previous genetic screen. Here, we report the identification of novel inducers isolated in an overexpression genetic screen: pyruvate kinase (Pyk1p) and phosphoglycerate kinase (Pgk1p). Addition of pyruvate, the end product of pyruvate kinase and glycolysis, also induced calnexin independence in a dose-dependent manner. Remarkably, growth in respiration media or cold temperatures induced the appearance of Cin cells at high frequencies. Taken together, our results indicate that the Cin state can be triggered by extracellular changes, suggesting that this state represents an epigenetic adaptative response to environmental modifications.     

Water availability controls microbial temperature responses in frozen soil CO2 production

Soil processes in high-latitude regions during winter are important contributors to global carbon circulation, but our understanding of the mechanisms controlling these processes is poor and observed temperature response coefficients of CO₂ production in frozen soils deviate markedly from thermodynamically predicted responses (sometimes by several orders of magnitude). We investigated the temperature response of CO₂ production in 23 unfrozen and frozen surface soil samples from various types of boreal forests and peatland ecosystems and also measured changes in water content in them after freezing. We demonstrate that deviations in temperature responses at subzero temperatures primarily emanates from water deficiency caused by freezing of the soil water, and that the amount of unfrozen water is mainly determined by the quality of the soil organic matter, which is linked to the vegetation cover. Factoring out the contribution of water limitation to the CO₂ temperature responses yields response coefficients that agree well with expectations based on thermodynamic theory concerning biochemical temperature responses. This partitioning between a pure temperature response and the effect of water availability on the response of soil CO₂ production at low temperatures is crucial for a thorough understanding of low-temperature soil processes and for accurate predictions of C-balances in northern terrestrial ecosystems.     

Effects of soil frost on soil respiration and its radiocarbon signature in a Norway spruce forest soil

Apart from a general increase of mean annual air temperature, climate models predict a regional increase of the frequency and intensity of soil frost with possibly strong effects on C cycling of soils. In this study, we induced mild soil frost (up to −5 °C in a depth of 5 cm below surface) in a Norway spruce forest soil by removing the natural snow cover in the winter of 2005/2006. Soil frost lasted from January to April 2006 and was detected down to 15 cm depth. Soil frost effectively reduced soil respiration in the snow removal plots in comparison to undisturbed control plots. On an annual basis 6.2 t C ha⁻¹ a⁻¹ were emitted in the control plots compared with 5.1 t C ha⁻¹ a⁻¹ in the snow removal plots. Only 14% of this difference was attributed to reduced soil respiration during the soil frost period itself, whereas 63% of this difference originated from differences during the summer of 2006. Radiocarbon (Δ¹⁴C) signature of CO₂ revealed a considerable reduction of heterotrophic respiration on the snow removal plots, only partly compensated for by a slight increase of rhizosphere respiration. Similar CO₂ concentrations in the uppermost mineral horizons of both treatments indicate that differences between the treatments originated from the organic horizons. Extremely low water contents between June and October of 2006 may have inhibited the recovery of the heterotrophic organisms from the frost period, thereby enhancing the differences between the control and snow removal plots. We conclude that soil frost triggered a change in the composition of the microbial community, leading to an increased sensitivity of heterotrophic respiration to summer drought. A CO₂ pulse during thawing, such as described for arable soils several times throughout the literature, with the potential to partly compensate for reduced soil respiration during soil frost, appears to be lacking for this soil. Our results from this experiment indicate that soil frost reduces C emission from forest soils, whereas mild winters may enhance C losses from forest soils.     

Responses of dryland soil respiration and soil carbon pool size to abrupt vs. gradual and individual vs. combined changes in soil temperature, precipitation, and atmospheric [CO2]: a simulation analysis

With the large extent and great amount of soil carbon (C) storage, drylands play an important role in terrestrial C balance and feedbacks to climate change. Yet, how dryland soils respond to gradual and concomitant changes in multiple global change drivers [e.g., temperature (T_s), precipitation (Ppt), and atmospheric [CO₂] (CO₂)] has rarely been studied. We used a process‐based ecosystem model patch arid land simulator to simulate dryland soil respiration (R_s) and C pool size (C_s) changes to abrupt vs. gradual and single vs. combined alterations in T_s, Ppt and CO₂ at multiple treatment levels. Results showed that abrupt perturbations generally resulted in larger R_s and had longer differentiated impacts than did gradual perturbations. R_s was stimulated by increases in T_s, Ppt, and CO₂ in a nonlinear fashion (e.g., parabolically or asymptotically) but suppressed by Ppt reduction. Warming mainly stimulated heterotrophic R_s (i.e., R_h) whereas Ppt and CO₂ influenced autotrophic R_s (i.e., R_a). The combined effects of warming, Ppt, and CO₂ were nonadditive of primary single‐factor effects as a result of substantial interactions among these factors. Warming amplified the effects of both Ppt addition and CO₂ elevation whereas Ppt addition and CO₂ elevation counteracted with each other. Precipitation reduction either magnified or suppressed warming and CO₂ effects, depending on the magnitude of factor's alteration and the components of R_s (R_a or R_h) being examined. Overall, Ppt had dominant influence on dryland R_s and C_s over T_s and CO₂. Increasing Ppt individually or in combination with T_s and CO₂ benefited soil C sequestration. We therefore suggested that global change experimental studies for dryland ecosystems should focus more on the effects of precipitation regime changes and the combined effects of Ppt with other global change factors (e.g., T_s, CO₂, and N deposition).