Climate change and freshwater ecosystems: impacts across multiple levels of organization

Fresh waters are particularly vulnerable to climate change because (i) many species within these fragmented habitats have limited abilities to disperse as the environment changes; (ii) water temperature and availability are climate-dependent; and (iii) many systems are already exposed to numerous anthropogenic stressors. Most climate change studies to date have focused on individuals or species populations, rather than the higher levels of organization (i.e. communities, food webs, ecosystems). We propose that an understanding of the connections between these different levels, which are all ultimately based on individuals, can help to develop a more coherent theoretical framework based on metabolic scaling, foraging theory and ecological stoichiometry, to predict the ecological consequences of climate change. For instance, individual basal metabolic rate scales with body size (which also constrains food web structure and dynamics) and temperature (which determines many ecosystem processes and key aspects of foraging behaviour). In addition, increasing atmospheric CO₂ is predicted to alter molar CNP ratios of detrital inputs, which could lead to profound shifts in the stoichiometry of elemental fluxes between consumers and resources at the base of the food web. The different components of climate change (e.g. temperature, hydrology and atmospheric composition) not only affect multiple levels of biological organization, but they may also interact with the many other stressors to which fresh waters are exposed, and future research needs to address these potentially important synergies.     

模拟增温和不同凋落物基质质量对凋落物分解速率的影响

采用凋落物分解袋法,研究了在土壤、水分相当的条件下模拟增温对红松(Pinus koraiensis)、蒙古栎(Quercus mongolica)及其混合凋落物分解的影响,以及在不同温度水平下,不同凋落物质量(两种单一凋落物和混合凋落物)的分解特性。利用碱式吸收法测量了凋落物分解累积释放CO₂动态。将N浓度和C/N率作为凋落物质量参数,用呼吸产生CO₂的积累值和凋落物质量损失率确定凋落物分解率。结果表明温度升高对单一凋落物和混合凋落物分解均有促进作用,在不同温度水平上,不同质量凋落物的分解特性有所差别,25 ℃和29 ℃条件下混合凋落物分解速率>蒙古栎单一凋落物>红松单一凋落物分解速率。然而,在31 ℃条件下混合凋落物与蒙古栎单一凋落物分解速率相差不大,二者均大于红松单一凋落物分解速率。     

Using the biomass-ratio and idiosyncratic hypotheses to predict mixed-species litter decomposition

Background and Aims

Microsporogenesis leading to monosulcate pollen grains has already been described for a wide range of monocot species. However, a detailed study of additional callose deposition after the completion of the cleavage walls has been neglected so far. The study of additional callose deposition in monosulcate pollen grain has gained importance since a correlation between additional callose deposition and aperture location has recently been revealed.

Methods

Microsporogenesis is described for 30 species belonging to eight families of the monocots: Acoraceae, Amaryllidaceae, Alstroemeriaceae, Asparagaceae, Butomaceae, Commelinaceae, Liliaceae and Xanthorrhoeaceae.

Key Results

Five different microsporogenesis pathways are associated with monosulcate pollen grain. They differ in the type of cytokinesis, tetrad shape, and the presence and shape of additional callose deposition. Four of them present additional callose deposition.

Conclusions

In all these different microsporogenesis pathways, aperture location seems to be linked to the last point of callose deposition.     

闽楠林土壤-凋落物-叶片碳氮磷生态化学计量对人为干扰的响应

干扰是影响森林生态系统稳定性和功能的重要因子,干扰程度直接影响天然林的生长进而影响其生态系统能量流动和养分循环过程,为此开展干扰对天然林生态系统影响研究,对于揭示干扰对天然林生态系统养分平衡特征机制具有重要意义。以福建两种人为干扰模式下(重度干扰和轻度干扰)闽楠林为研究对象,通过分析“土壤-凋落物-叶片”三个组分化学计量特征,结合养分利用效率、养分再吸收效率、内稳态理论解析干扰对闽楠林养分资源利用策略和生态适应。结果表明：(1)两种干扰模式下,叶片C、N、P含量均显著高于其土壤和凋落物,且三个组分中N和P含量均表现出重度干扰显著高于轻度干扰,但三个组分C/N、C/P和N/P呈现轻度干扰显著高于其重度干扰。(2)闽楠林叶片N、P养分利用效率表现出：重度干扰<轻度干扰,但P再吸收效率则是重度干扰高于轻度干扰,且两种干扰模式下P养分利用效率和再吸收效率显著高于N。(3)随干扰强度的增加,闽楠林叶片N呈现内稳态弱,而叶片P的内稳态强以适应低P环境。(4)凋落物与叶片两组分P、C/P、N/P存在显著正相关关系,土壤C/N分别与叶片P、C/P、N/P以及凋落物P、C/N和C/P存在显著相关关...     

陕西黄土高原刺槐枯落叶生态化学计量学特征

生态系统元素平衡是当前全球变化生态学和生物地球化学循环研究的焦点和热点,生态化学计量学结合了生物学、物理学和化学等基本原理,是研究生物系统能量平衡与多重化学元素平衡的科学,为研究元素在生物地球化学循环与生态过程中的规律及其之间的计量关系提供了一种综合的方法。以陕西黄土高原人工刺槐林为研究对象,结合纬度和坡向两个因素,分析了三原、淳化、耀州区、宜君、黄陵、洛川、富县、甘泉、宝塔区、安塞、米脂、神木12个县区的刺槐枯落叶生态化学计量学特征。结果发现,阳坡刺槐枯落叶C、N、P含量的变化范围分别为318.34—428.01 g/kg、13.27—24.07 g/kg、1.66—2.57 g/kg;阴坡刺槐枯落叶C、N、P含量的变化范围分别为306.70—433.68 g/kg、12.55—24.39 g/kg、1.62—2.99 g/kg。阳坡刺槐枯落叶C∶N、C∶P、N∶P的变化范围分别为14.23—24.61、148.67—215.92、7.37—14.47;阴坡刺槐枯落叶C∶N、C∶P、N∶P的变化范围分别为16.87—26.54、130.06—234.41、7.05—13.22。随着纬度的升高,刺槐枯落叶C、N显著下降,刺槐枯落叶P、C∶N、C∶P、N∶P无明显差异。刺槐枯落叶C、N、P之间呈显著正相关。刺槐枯落叶C、N、P、C∶N、C∶P、N∶P在阴坡和阳坡之间无明显差异。研究区,阴坡和阳坡的刺槐枯落叶N∶P均较低,刺槐林土壤的全氮平均含量也低于全国平均水平,推测陕西黄土高原刺槐林的生长可能主要受到氮素的限制。     

全球环境变化对森林凋落物分解的影响

全球环境变化将对森林生态系统凋落物的分解和养分循环产生直接和间接的多重影响.就全球环境变化如全球变暖、大气CO2浓度升高、UV-B辐射增强、氮沉降等对凋落物分解影响的研究进展进行了综合述评.影响凋落物分解的内部因素为凋落物基质质量,外部因素包括生物因素(微生物和动物)和非生物因素(温度、水分和土壤性质等).全球变暖对凋落物分解的非生物作用有正效应,也有负效应.全球变暖对凋落物化学组成虽然只有轻微的影响,但可以通过影响植被的物种组成来间接改变凋落物的产量、化学性质和分解.全球变暖对凋落物分解生物作用的主要影响是增强土壤微生物活性,从而加速凋落物的分解.CO2浓度上升将增加凋落物产量,并通过影响凋落物质量(提高C/N比、木质素/N比等)和生物环境(微生物的数量和活性)而影响分解过程.UV-B辐射和大气N沉降的增加亦对凋落物分解产生直接和间接的影响,但影响效果尚不很清楚,有待进一步的研究.总起来看,全球环境变化将通过影响凋落物的分解速率而对全球碳循环产生重要影响,但由于气候变化和凋落物分解响应的复杂性以及各因子之间的相互作用,气候变化对凋落物分解的总效应尚需更深入的研究来定量化.     

光周期对白头鹎（Pycnonotus sinensis）体重、器官重量和能量代谢的影响

光周期是四季环境变化的最直接表现因素之一,并影响动物的生理变化特征。为探讨光周期驯化对白头鹎(Pycnonotus sinensis)体重、器官重量及能量代谢的影响,以室温28℃、不同光周期 (16L ∶ 8D,LD组和8L ∶ 16D,SD组)对两组白头鹎进行为期4周的光周期驯化,测定其体重、各器官鲜重和干重、基础代谢率(BMR)和食物摄入能、排泄能及同化能并计算同化率。结果发现,SD组个体体重、内部器官(肝、小肠)重量、BMR及同化率相应显著高于LD组个体;短光照刺激白头鹎显著降低摄入能、排泄能及同化能。这些结果表明:光周期对白头鹎的体重、器官重量、BMR及能量收支有着一定影响,并且短光照较长光照更能引起白头鹎体重、器官重量及能量代谢的明显变化,同时验证了"中心限制假说",即白头鹎BMR与中心器官代谢(肝、小肠等)具有相关性,中心器官是改变白头鹎BMR的主要原因之一。     

Effects of three global change drivers on terrestrial C:N:P stoichiometry: a global synthesis

Over the last few decades, there has been an increasing number of controlled‐manipulative experiments to investigate how plants and soils might respond to global change. These experiments typically examined the effects of each of three global change drivers [i.e., nitrogen (N) deposition, warming, and elevated CO₂] on primary productivity and on the biogeochemistry of carbon (C), N, and phosphorus (P) across different terrestrial ecosystems. Here, we capitalize on this large amount of information by performing a comprehensive meta‐analysis (>2000 case studies worldwide) to address how C:N:P stoichiometry of plants, soils, and soil microbial biomass might respond to individual vs. combined effects of the three global change drivers. Our results show that (i) individual effects of N addition and elevated CO₂ on C:N:P stoichiometry are stronger than warming, (ii) combined effects of pairs of global change drivers (e.g., N addition + elevated CO₂, warming + elevated CO₂) on C:N:P stoichiometry were generally weaker than the individual effects of each of these drivers, (iii) additive interactions (i.e., when combined effects are equal to or not significantly different from the sum of individual effects) were more common than synergistic or antagonistic interactions, (iv) C:N:P stoichiometry of soil and soil microbial biomass shows high homeostasis under global change manipulations, and (v) C:N:P responses to global change are strongly affected by ecosystem type, local climate, and experimental conditions. Our study is one of the first to compare individual vs. combined effects of the three global change drivers on terrestrial C:N:P ratios using a large set of data. To further improve our understanding of how ecosystems might respond to future global change, long‐term ecosystem‐scale studies testing multifactor effects on plants and soils are urgently required across different world regions.     

Increasing water temperature and disease risks in aquatic systems: Climate change increases the risk of some, but not all, diseases

Global warming may impose severe risks for aquatic animal health if increasing water temperature leads to an increase in the incidence of parasitic diseases. Essentially, this could take place through a temperature-driven effect on the epidemiology of the disease. For example, higher temperature may boost the rate of disease spread through positive effects on parasite fitness in a weakened host. Increased temperature may also lengthen the transmission season leading to higher total prevalence of infection and more widespread epidemics. However, to date, general understanding of these relationships is limited due to scarcity of long-term empirical data. Here, we present one of the first long-term multi-pathogen data sets on the occurrence of pathogenic bacterial and parasitic infections in relation to increasing temperatures in aquatic systems. We analyse a time-series of disease dynamics on two fish farms in northern Finland from 1986 to 2006. We first demonstrate that the annual mean water temperature increased significantly on both farms over the study period and that the increase was most pronounced in the late summer (July-September). Second, we show that the prevalence of infection (i.e. proportion of fish tanks infected each year) increased with temperature. Interestingly, this pattern was observed in some of the diseases (Ichthyophthirius multifiliis, Flavobacterium columnare), whereas in the other diseases, the pattern was the opposite (Ichthyobodo necator) or absent (Chilodonella spp.). These results demonstrate the effect of increasing water temperature on aquatic disease dynamics, but also emphasise the importance of the biology of each disease, as well as the role of local conditions, in determining the direction and magnitude of these effects.     

An information-theoretic approach to evaluating the size and temperature dependence of metabolic rate

The effects of body mass and temperature on metabolic rate (MR) are among the most widely examined physiological relationships. Recently, these relationships have been incorporated into the metabolic theory of ecology (MTE) that links the ecology of populations, communities and ecosystems to the MR of individual organisms. The fundamental equation of MTE derives the relation between mass and MR using first principles and predicts the temperature dependence of MR based on biochemical kinetics. It is a deliberately simple, zeroth-order approximation that represents a baseline against which variation in real biological systems can be examined. In the present study, we evaluate the fundamental equation of MTE against other more parameter-rich models for MR using an information-theoretic approach to penalize the inclusion of additional parameters. Using a comparative database of MR measurements for 1359 species, from 11 groups ranging from prokaryotes to mammals, and spanning 16 orders of magnitude in mass and a 59°C range in body temperature, we show that differences between taxa in the mass and temperature dependence of MR are sufficiently large as to be retained in the best model for MR despite the requirement for estimation of 22 more parameters than the fundamental equation of MTE.     

全球气候变化对陆地植物碳氮磷生态化学计量学特征的影响

全球气候变化背景下生物地球化学循环的响应规律和陆地植物适应对策已受到广泛关注.本文在分析气候变暖和降水变化对不同生态系统植物C∶N∶P的影响、CO₂浓度升高对不同光合途径物种元素的影响,以及氮沉降对土壤 植物元素影响的短期和长期效应等基础上,从植物生理特性和土壤有效营养元素变化等方面揭示了其可能存在的内在机理,以期为研究C、N、P化学元素在土壤 植物之间传递与调节机制、陆地生态系统结构和功能,以及生物地球化学元素循环对气候变化的响应提供理论依据.最后提出了该领域研究中存在的问题及对今后研究的展望.     

Climate change triggers effects of fungal pathogens and insect herbivores on litter decomposition

Increasing infestation by insect herbivores and pathogenic fungi in response to climate change will inevitably impact the amount and quality of leaf litter inputs into the soil. However, little is known on the interactive effect of infestation severity and climate change on litter decomposition, and no such study has been published for deciduous forests in Central Europe. We assessed changes in initial chemical quality of beech (Fagus sylvatica L.) and maple litter (Acer platanoides L.) in response to infestation by the gall midge Mikiola fagi Hart. and the pathogenic fungus Sawadaea tulasnei Fuckel, respectively, and investigated interactive effects of infestation severity, changes in temperature and soil moisture on carbon mineralization in a short-term laboratory study. We found that infestation by the gall midge M. fagi and the pathogenic fungus S. tulasnei significantly changed the chemical quality of beech and maple litter. Changes in element concentrations were generally positive and more pronounced, and if negative less pronounced for maple than beech litter most likely due to high quality fungal tissue remaining on litter after abscission. More importantly, alterations in litter chemical quality did not translate to distinct patterns of carbon mineralization at ambient conditions, but even low amounts of infested litter accelerated carbon mineralization at moderately increased soil moisture and in particular at higher temperature. Our results indicate that insect herbivores and fungal pathogens can markedly alter initial litter chemical quality, but that afterlife effects on carbon mineralization depend on soil moisture and temperature, suggesting that increased infestation severity under projected climate change potentially increases soil carbon release in deciduous forests in Central Europe.     

Universal temperature and body-mass scaling of feeding rates

Knowledge of feeding rates is the basis to understand interaction strength and subsequently the stability of ecosystems and biodiversity. Feeding rates, as all biological rates, depend on consumer and resource body masses and environmental temperature. Despite five decades of research on functional responses as quantitative models of feeding rates, a unifying framework of how they scale with body masses and temperature is still lacking. This is perplexing, considering that the strength of functional responses (i.e. interaction strengths) is crucially important for the stability of simple consumer–resource systems and the persistence, sustainability and biodiversity of complex communities. Here, we present the largest currently available database on functional response parameters and their scaling with body mass and temperature. Moreover, these data are integrated across ecosystems and metabolic types of species. Surprisingly, we found general temperature dependencies that differed from the Arrhenius terms predicted by metabolic models. Additionally, the body-mass-scaling relationships were more complex than expected and differed across ecosystems and metabolic types. At local scales (taxonomically narrow groups of consumer–resource pairs), we found hump-shaped deviations from the temperature and body-mass-scaling relationships. Despite the complexity of our results, these body-mass- and temperature-scaling models remain useful as a mechanistic basis for predicting the consequences of warming for interaction strengths, population dynamics and network stability across communities differing in their size structure.     

Isopod effects on decomposition of litter produced under elevated CO2, N deposition and different soil types

The performance of Oniscus asellus (Isopoda) and its influence on litter mass loss and mineralization was assessed in a microcosm experiment, using beech (Fagus sylvatica) leaf litter that was produced on different soil types, contrasting atmospheric CO₂ concentrations, and different nitrogen deposition rates. Litter quality was significantly altered by these treatments, and many of the CO₂ and N effects differed between soil types. Litter quality affected subsequent litter mass loss rates, microbial respiration, and leaching of dissolved organic carbon (DOC) and nitrate. These effects were largely independent of the presence of isopods, even though isopods highly accelerated litter mass loss, stimulated microbial respiration by 37%, and increased nitrate leaching by 50%. Isopods did not change their relative rates of litter consumption and growth in response to litter quality. Isopod mortality, however, increased with increasing litter lignin/N ratios, and was significantly different between soil types, which may indicate long‐term effects on litter decomposition through altered isopod densities. Having the choice among the litter of three different species [maple (Acer pseudoplatanus), beech (Fagus sylvatica) and oak (Quercus robur)] grown at either ambient or elevated CO₂, isopods preferred maple to beech when all the litter was produced under elevated CO₂. This suggests that beyond changes in consumption, an altered food selection by isopods in a CO₂‐enriched atmosphere may affect the temporal and spatial composition of the litter layer in temperate forests. In contrast to previous findings, we conclude that isopods do not always increase their consumption rates, and hence do not differentially affect microbial decomposition in litter of poorer quality. Nevertheless changes in animal densities and/or shifts in their food preferences, could result in altered decomposition and carbon and nutrient turnover rates.     

Tree litter and forest understorey vegetation: a conceptual framework to understand the effects of tree litter on a perennial geophyte, Anemone nemorosa

Background and Aims

Litter is a key factor in structuring plant populations, through positive or negative interactions. The litter layer forms a mechanical barrier that is often strongly selective against individuals lacking hypocotyle plasticity. Litter composition also interacts with plant growth by providing beneficial nutrients or, inversely, by allowing harmful allelopathic leaching. As conspicuous litter fall accumulation is often observed under deciduous forests, interactions between tree litter and understorey plant populations are worthy of study.

Methods

In a 1-year ex-situ experiment, the effects of tree litter on the growth of Anemone nemorosa, a small perennial forest geophyte, were investigated. Three ‘litter quantity’ treatments were defined, representative of forest floor litter (199, 356·5 and 514 g m⁻²), which were crossed with five ‘litter composition’ treatments (Quercus petraea, Fagus sylvatica, Carpinus betulus, Q. petraea + F. sylvatica and Q. petraea + C. betulus), plus a no-litter control. Path analysis was then used to investigate the pathways linking litter characteristics and components of adult plant growth.

Key Results

As expected, the heavier the litter, the longer the petiole; rhizome growth, however, was not depreciated by the litter-induced petiole lengthening. Both rhizome mass increment and number of initiated buds marginally increased with the amount of litter. Rhizome mass increment was in fact determined primarily by leaf area and leaf life span, neither of which was unequivocally correlated with any litter characteristics. However, the presence of litter significantly increased leafing success: following a late frost event, control rhizomes growing in the absence of litter experienced higher leaf mortality before leaf unfolding.

Conclusions

The study questions the role of litter as a physical or chemical barrier to ground vegetation; to better understand this role, there is a need for ex-situ, longer-term experiments coupled with in-situ observations in the forest.     

模拟N沉降下不同林龄马尾松林凋落叶分解-土壤C、N化学计量特征

模拟N沉降对森林生态系统的影响是当今全球变化生态学研究的一个热点问题,土壤碳库对N沉降比较敏感,N沉降增加了凋落叶分解过程中外源N含量,间接影响凋落叶分解的化学过程并改变凋落叶分解速率,因此,研究模拟N沉降下凋落叶分解-土壤C-N关系对预测森林C吸存有重要意义。利用原位分解袋法研究了模拟N沉降下三峡库区不同林龄马尾松林(Pinus massoniana)凋落叶分解过程中凋落叶-土壤C、N化学计量响应及其关系;N沉降水平分对照(CK,0 g m~(-2)a~(-1))、低氮(LN,5 g m~(-2)a~(-1))、中氮(MN,10 g m~(-2)a~(-1))和高氮(HN,15 g m~(-2)a~(-1))。结果表明:分解540 d后,N沉降促进20年生和30年生马尾松林凋落叶分解,46年生马尾松林中仅低氮处理促进凋落叶分解,4种处理均是30年生分解最快,说明同一树种起始N含量低的凋落叶对N沉降呈正响应,N沉降处理促进起始N含量低的凋落叶分解,起始N含量高的凋落叶分解过程中易达到"N饱和"。N沉降抑制20年生和46年生凋落叶C释放(低于对照0.62%—6.69%),促进30年生C释放(高于对照0.28%—5.55%);30年生和46年生林分N固持量均高于对照(高于对照0.15%—21.34%),20年生则低于对照(5.70%—13.87%),说明模拟N沉降处理促进起始C含量低的凋落叶C释放和起始N含量低的凋落叶N固持。N沉降处理下仅30年生马尾松林土壤有机碳较对照增加,且土壤有机质与凋落叶C、N和分解速率呈正相关,与凋落叶C/N比呈显著负相关;土壤总氮与凋落叶分解速率、凋落叶N含量呈正相关,土壤有机碳/总氮比与凋落叶C、N含量呈正相关;对照处理中凋落叶分解指标对土壤养分影响顺序是分解速率凋落物C含量凋落物C/N比凋落物N含量,低、中、高氮处理中则是凋落物C含量分解速率凋落物N含量凋落物C/N比。研究表明低土壤养分含量马尾松林对N沉降呈正响应,N沉降促进低土壤养分马尾松林凋落叶分解并提高土壤肥力;凋落叶质量和土壤养分含量低的生态系统土壤C对N沉降响应更显著。     

Effects of increased water temperature on leaf litter quality and detritivore performance: a whole‐reach manipulative experiment

相似文献   

Metabolic responses of sand fiddler crabs, Uca pugilator, in northwest Florida to seasonal temperature change

Metabolic responses of sand fiddler crab, Uca pugilator, populations in northwest Florida are greatly influenced by seasonal temperature fluctuations. Crabs acclimated at 20 °C and immediately transferred to either 14 or 26 °C produced an acute metabolic response with respective temperature quotient (Q₁₀) values of 3.46 and 3.91. Crabs acclimated at 10 and 20 °C exhibited a Q₁₀ of 2.62 indicating a partial compensation response. A brumation (reverse) response (Q₁₀ value of 20.11) was observed for acclimated crabs between 5 and 10 °C. Brumation is advantageous during winter when food supplies are scarce and crabs must survive extensive periods of inactivity.     

不同地理种源杉木根叶功能性状与碳氮磷化学计量分析

为认识地理环境因子对不同种源杉木的影响及其响应和适应特征,对生长在同质园的7个种源3年生杉木王幼树根、叶功能性状及碳氮磷化学计量与环境因子的关系进行了研究。结果表明:不同地理种源杉木功能性状种源间变异系数为7.3%-18.9%,化学计量种源间变异系数为1.6%-18.9%,均小于20%,叶、根C含量变异系数较小,说明C是植物体最稳定的元素。不同地理种源杉木幼树叶厚、比叶面积、叶组织密度、比叶重、叶干物质含量差异显著,表明杉木叶片形态具有较强的可塑性来适应不同种源地地理环境变化。不同地理种源杉木根N含量、根C∶N和根C∶P差异极显著,反映不同种源地气候与土壤养分对根系化学计量特征的潜在影响。江西文公山种源的杉木比叶面积最大,比根长、根干物质含量最小;安徽休宁种源的杉木叶片C含量、C∶P、根C∶N、N∶P最小,细根N、P含量最高;江西安福山种源的杉木细根C∶N最大,叶片N∶P、细根C、N、P含量、C∶N、C∶P最小。不同种源地经度与杉木叶组织密度、叶C∶P呈极显著正相关。随着降水梯度升高,不同种源杉木叶根N∶P、比叶面积、比叶重升高。随海拔升高和气温的降低,叶片厚度增加,叶组织密度密度减小,叶干物质含量减小,比叶面积减小。对不同地理种源杉木功能性状间的权衡关系分析表明,不同种源杉木根比表面积与比叶面积呈显著负相关且与比叶重呈极显著正相关,叶片及细根N含量与P含量均呈极显著正相关,说明杉木叶、根面积存在关联性,根长与根面积生长趋势相同,根叶对N、P元素含量的响应也相同。     

ORMDL/serine palmitoyltransferase stoichiometry determines effects of ORMDL3 expression on sphingolipid biosynthesis

The ORM1 (Saccharomyces cerevisiae)-like proteins (ORMDLs) and their yeast orthologs, the Orms, are negative homeostatic regulators of the initiating enzyme in sphingolipid biosynthesis, serine palmitoyltransferase (SPT). Genome-wide association studies have established a strong correlation between elevated expression of the endoplasmic reticulum protein ORMDL3 and risk for childhood asthma. Here we test the notion that elevated levels of ORMDL3 decrease sphingolipid biosynthesis. This was tested in cultured human bronchial epithelial cells (HBECs) (an immortalized, but untransformed, airway epithelial cell line) and in HeLa cells (a cervical adenocarcinoma cell line). Surprisingly, elevated ORMDL3 expression did not suppress de novo biosynthesis of sphingolipids. We determined that ORMDL is expressed in functional excess relative to SPT at normal levels of expression. ORMDLs and SPT form stable complexes that are not increased by elevated ORMDL3 expression. Although sphingolipid biosynthesis was not decreased by elevated ORMDL3 expression, the steady state mass levels of all major sphingolipids were marginally decreased by low level ORMDL3 over-expression in HBECs. These data indicate that the contribution of ORMDL3 to asthma risk may involve changes in sphingolipid metabolism, but that the connection is complex.