流溪河水库颗粒有机物及浮游动物碳、氮稳定同位素特征

为了解影响流溪河水库颗粒有机物(POM)碳和氮稳定同位素(δ¹³C和δ¹⁵N)变化的主要因素,及其与浮游动物δ¹³C和δ¹⁵N之间的关系,于2008年5月至12月份对POM及浮游动物的δ¹³C和δ¹⁵N进行了研究。颗粒有机物碳稳定同位素(δ¹³C_POM)和氮稳定同位素(δ¹⁵N_POM)的季节性变化幅度分别为5.1‰和2.2‰,5月和7月份δ¹³C_POM较高,而在10月和12月份降低,这主要与降雨将大量外源有机物带入水库而引起的外源及内源有机物在POM组成上发生变化有关。δ¹⁵N_POM总体呈上升趋势,可能是由降雨引起的外源负荷、初级生产力、生物固氮等因素共同作用的结果。浮游动物的δ¹³C及δ¹⁵N总的变化趋势与POM的相似,也具有明显的季节性变化,食物来源的季节变化可能是造成其变化的主要原因。在5月份,浮游动物的食物来源为POM中δ¹³C较高的部分,也就是外源有机物,而在10月及12月份,其食物则可能主要为浮游植物。     

红池坝草地常见物种叶片性状沿海拔梯度的响应特征

叶片性状是决定植物光合能力和羧化能力的关键因素,研究叶片性状在海拔梯度上的变化特征是解释植物对于环境变化的适应策略的重要手段。本文以分布于红池坝(10958′E, 3130′ N)草地的5个常见物种红三叶(Trifolium pratense)、老鹳草(Geranium wilfordii)、紫菀(Aster tataricus)、火绒草(Leontopodium leontopodioides)和绣线菊(Spiraea prunifolia)为研究对象,分析了所有物种(n=56)和不同物种的叶片比叶重(LMA)、叶氮含量(单位面积氮含量Narea、单位重量氮含量Nmass)以及叶片δ13C含量沿海拔梯度(815-2545m)的变化趋势及叶片性状之间的关系。研究结果表明：所有物种样品(n=56)的比叶重(LMA)、Narea和δ13C含量沿海拔梯度的增加呈显著增加趋势;Nmass沿海拔梯度的变化趋势不明显;δ13C含量与LMA、Narea呈现极显著正相关关系;不同物种的叶片性状沿着海拔梯度的响应特征有所不同,绣线菊(S. prunifolia)和老鹳草(G. wilfordii)的叶片性状沿海拔梯度的分布规律与所有物种(n=56)样品分布规律一致,红三叶(T. pratense)、紫菀(A. tataricus)、火绒草(L. leontopodioides)的各叶片性状沿海拔梯度的分布特征有所不同。     

不同供水下白羊草(Bothriochloa ischaemum)离体根呼吸特征——基于稳定碳同位素示踪技术

植物根呼吸是土壤呼吸的主要组成,研究根呼吸对生态系统碳收支及碳平衡有重要意义。采用~(13)C脉冲标记技术,在3种供水条件下,对比不同根离体时间(标记后0,6,24,48,216,360 h)的白羊草离体根呼吸速率和根呼吸释放的δ~(13)C同位素比值变化,分析根参数与离体根呼吸相关性。结果表明:1)不同离体时间的离体根呼吸速率变化趋势一致,3种供水条件下无显著差异,均在0—20 min急剧下降,下降范围为32%—39%。2)测定离体根呼吸释放的δ~(13)C在不同离体时间的变化,为实时监测转移到白羊草根系的~(13)CO_2在根部释放的过程提供了新思路;不同离体时间,3种供水条件下根呼吸释放的δ~(13)C在2 h内均值大小呈:供水充分轻度胁迫重度胁迫。随离体时间(0—360 h)推移根呼吸释放的δ~(13)C均值先增大后减小,在216 h达到峰值31.46‰;3)离体根呼吸速率和根呼吸释放的δ~(13)C受根系根面积、比根面积、N含量、C/N及根组织δ~(13)C的影响显著。4)轻度水分胁迫可促使根系生长(C固定)和根呼吸(C代谢)同时增加。     

不同生态烟区烤烟δ~(13)C值与生理及品质特征的比较研究

以烤烟品种K326为试验材料,在云南、福建和河南三个生态烟区大田种植,自烟叶生理成熟期起至工艺成熟期,分4次采集中部(第11叶位)烟样,对烟叶的δ13 C值、总碳、全氮、光合色素等进行测定,比较不同生态烟区烤烟δ13 C值的分布、生理生态适应性及品质特征。结果表明:(1)三个生态烟区烟叶δ13 C值、总碳、碳氮比、比叶重、叶绿素a、总叶绿素含量均表现为云南福建河南,全氮含量则为河南云南福建,叶绿素b含量为云南河南福建,类胡萝卜素含量为河南福建云南,其中的δ13 C值、总碳、碳氮比、类胡萝卜素含量在福建和云南烟区间较为接近。(2)烟叶δ13 C值与总碳含量在云南呈正相关,在福建、河南呈负相关关系;三个生态烟区烟叶δ13 C值与全氮含量均呈负相关关系;δ13 C值与光合色素含量在云南、河南烟区均呈正相关关系,在福建烟区均呈负相关关系;δ13 C值与烟碱、氮、钾、氯呈负相关关系,与总糖、还原糖呈正相关关系。(3)云南烤烟香韵丰富,刺激性中等,化学成分协调性最好;河南烤烟香气量较高,刺激性较大;福建烤烟在香气量和化学成分协调性方面表现较差。研究发现,烟叶δ13 C值与烟叶的生理特征、品质特征存在紧密联系,用烟叶的δ13 C值、生理指标、化学品质可区分不同生态烟区烤烟香气风格和品质特征。     

模拟增温对高寒草甸植物叶片碳氮及其同位素δ~(13)C和δ~(15)N含量的影响

基于国际冻原计划(ITEX)模拟增温效应对植物影响的研究方法,以高寒矮嵩草草甸4种植物(矮嵩草、垂穗披碱草、棘豆、麻花艽)为实验材料,设置大(OTC1)和小(OTC2)两类增温小室,测定了其叶片碳氮及其稳定性碳同位素(δ~(13)C、δ~(15)N)等指标在增温3年后的变化。结果表明:除矮嵩草在较小增温小室(OTC2)中C/N比值比对照降低了14.1%,其它物种C/N在两个增温处理下都有所增加,但差异均不显著(P0.05)。4种植物叶片δ~(13)C值在-24.12‰~-28.34‰之间,矮嵩草叶片δ~(13)C值随增温而升高,棘豆、麻花艽随增温而降低,且矮嵩草在OTC2的δ~(13)C值变化达到显著水平(P0.05)。矮嵩草和麻花艽的叶片δ~(15)N值在OTC1和OTC2中均比对照增加,且麻花艽增加较显著(P0.05)。垂穗披碱草在OTC1和OTC2的叶片δ~(15)N值比对照分别减少18.7%和26.9%,差异都不显著(P0.05);棘豆叶片δ~(15)N值在OTC2内比对照低11.0%(P0.05),在OTC1的内比对照高2.8%(P0.05)。可见,高寒矮嵩草草甸不同功能群植物物种碳氮含量及稳定性碳氮同位素含量对短期增温有不同的响应模式和规律。     

基于稳定同位素技术的黑河下游不同林龄胡杨的吸水深度研究

水是影响陆地生态系统植被分布最重要的因素之一,特别是在干旱地区,水资源将是限制植物生长的关键因素。不同植物具有不同的吸水深度,植物的吸水深度可影响它的分布区域,因此,精确测量植物的吸水深度对预测全球气候变化下植被的时空分布具有重要意义。稳定同位素技术逐渐被公认为是一种精确测量植物吸水深度的有效方法。利用稳定同位素技术分析了不同林龄胡杨的吸水深度和不同林龄胡杨的水分利用效率,对确定胡杨生存的合理地下水位、制定胡杨保育恢复策略具有重要意义。通过对黑河下游不同林龄胡杨木质部水及其不同潜在水源δD、δ~(18)O的测定分析,并运用深度模型计算了不同林龄胡杨的吸水深度,探讨了不同林龄胡杨的水分利用效率。结果发现:1)不同林龄胡杨木质部δ~(18)O差异显著,胡杨幼苗、成熟木、过熟木的δ~(18)O分别为-5.37‰、-6.03‰、-6.92‰;2)不同林龄胡杨的平均吸水深度不同,且具有随林龄变老而选择利用更深的土壤水分的特点:胡杨幼苗的平均吸水深度为37 cm,胡杨成熟木的平均吸水深度为145 cm,胡杨过熟木的平均吸水深度为149.5 cm。3)不同林龄胡杨水分利用效率不同,随林龄的变老水分利用效率具有降低的趋势。     

黔西北次生林优势树种叶片-凋落物-土壤连续体有机质碳稳定同位素特征

稳定性碳同位素自然丰度(δ13C)能够揭示生态系统长时间尺度的有机碳动态变化,阐明生态系统功能的变化特征。以黔西北次生林14个优势树种为研究对象,测定叶片、凋落物以及根区土壤有机碳含量和δ13C值,分析不同层次碳含量和δ13C丰度之间的相关性。结果表明:14个优势树种叶片碳含量为404.67—487.14 g/kg,总体为针叶树种较高、常绿灌木较低;δ13C值为-31.2‰—-27.1‰,随生活型的变化规律不明显。凋落物碳含量为414.62—561.31 g/kg,与叶片碳含量的变化规律较为一致;δ13C值为-31.5‰—-27.3‰,随树种生活型的变化特征也不明显。根区土壤碳含量为10.02—91.59 g/kg,δ13C值为-26.8‰—-22.5‰,碳含量以光皮桦、银白杨等落叶乔木较高。叶片、凋落物和根区土壤3个层次的碳含量与δ13C丰度之间均呈不显著相关,不同层次的δ13C丰度之间和碳含量之间均为正相关。研究结果有助于反映森林生态系统碳循环过程的关键信息,为森林植被恢复提供理论依据。     

会仙喀斯特湿地三种典型植物叶片碳同位素(δ13C)特征及其指示意义

为探讨会仙喀斯特湿地不同生长环境下植物叶片碳(C)、氮(N)、磷(P)、稳定碳同位素(δ~(13)C)特征及其生态学指示意义,该文以挺水植物芦苇、浮水植物水葫芦和沉水植物金鱼藻为研究对象,分析了三种典型不同生活型植物叶片的δ~(13)C特征及种间和微生境的差异,并基于植物碳同位素与碳酸酐酶显著正相关的二端元模型,估算了植物利用光合作用固定的碳酸氢根离子(HCO3-)的碳量。结果表明:(1)三种植物叶片δ~(13)C的变化范围为-28.47‰～-21.69‰,平均值为-24.83‰,不同生活型植物间δ~(13)C存在差异,金鱼藻水葫芦芦苇。(2)植物δ~(13)C值与叶片C、N和P元素含量间呈显著正相关,与C/N、C/P和N/P呈显著负相关关系,与底泥的有机质、速效氮、总氮、速效磷和总磷含量呈显著正相关。(3)会仙喀斯特湿地三种不同生活型植物叶片N/P平均值为10.34,表现出植物受N、P共同影响的特征。(4)δ~(13)C的变化特征,揭示了三种水生植物可能通过增加磷利用效率来促进低水分利用率环境下的碳的合成,通过提高植物水分利用效率的策略来代偿较低的氮素利用效率。(5)芦苇光合作用固定HCO3-碳量为159.60 t·a~(-1)·km~(-2),水葫芦为10.80 t·a~(-1)·km~(-2),金鱼藻为9.24 t·a~(-1)·km~(-2),平均值为59.88 t·a~(-1)·km~(-2)。会仙喀斯特湿地植物的不同生活型、光合作用途径和生长微环境,是影响叶片δ~(13)C变化的主要因素。     

北京市四环路及路旁绿地CO2变化特征及来源分析

城市大气中CO_2的变化特征及来源解析是制定节能减排措施的重要依据,对比非采暖季与采暖季北京市四环路(阜通东大街-京密路)路旁及距离道路100 m绿地中不同高度大气中CO_2浓度,并利用Keeling plot方程结合Iso Source软件进行分析,以期获得不同季节CO_2变化特征及定量估算其来源。结果表明,不同来源的CO_2中具有差异显著的δ~(13)C值,其中:土壤呼吸(-18.92‰)植物呼吸(-23.40‰)燃煤废气(-24.10‰)机动车尾气(-28.14‰)天然气废气(-33.34‰)。路旁和绿地的大气CO_2浓度在采暖季中分别比非采暖季中高26.2%和41.2%,路旁与绿地的大气CO_2浓度在非采暖季中差异显著而采暖季中无明显差异。在非采暖季中,CO_2浓度在6:00和20:00时较高,路旁大气CO_2随高度升高而降低,绿地大气CO_2浓度在8 m处最高,日变化明显。在采暖季中,CO_2浓度与车流量有相似的日变化趋势,在8:00和19:00时较高,路旁和绿地处大气CO_2浓度都随高度的升高而降低。路旁和绿地的大气CO_2来源差别明显,非采暖季中路旁大气CO_2主要来自于机动车尾气而绿地中大气CO_2主要来自于土壤和植物呼吸,在采暖季中路旁及绿地中大气CO_2的来源差别较小,主要来自于燃煤废气和机动车尾气。     

柴达木盆地白刺叶片δ13C与叶片和土壤养分指标的关系

在青海省柴达木盆地选择6个重要的唐古特白刺分布区域,通过测定不同白刺居群的叶片营养元素、土壤养分含量和叶片碳同位素组成δ¹³C,对比不同白刺居群间的叶片δ¹³C特征差异,并分析白刺叶片δ¹³C与地理位置信息及叶片营养和土壤养分含量指标的关系。结果显示:（1）白刺居群间叶片δ¹³C存在显著空间变异性,其叶片δ¹³C变化范围在-25.47‰~-27.66‰之间,平均为-26.66‰。(2）纬度、经度和海拔是白刺叶片δ¹³C重要影响因子,白刺叶片δ¹³C与纬度、经度和海拔分别呈极显著负相关,正相关和正相关关系（n=10）。（3）白刺叶片δ¹³C与叶片有机C含量、N、P、K含量总体均呈负相关关系,但仅叶片有机C含量达到极显著水平（n=30）,其余均未达到显著性水平（n=30）。（4）叶片δ¹³C与表层土壤（0~15 cm）有机质、全N、速效K含量呈极显著负相关关系（n=30）,与全P、碱解N和有效P含量呈显著负相关关系（n=30）,并与15~30 cm土壤有机质和全N含量显著负相关（n=30）,但叶片δ¹³C与30~45 cm土壤各化学指标相关性均不显著（n=30）。研究表明,环境因子纬度、经度、海拔,叶片有机C含量,以及立地土壤全N、全P、碱解N、有效P、速效K含量的变化均显著影响柴达木盆地唐古特白刺居群叶片δ¹³C值。     

Stable carbon isotope composition of bone hydroxylapatite: significance in paleodietary analysis

The stable carbon isotope composition of the structural carbonate derived from animal bone hydroxylapatite (δ¹³C_B-HA) could record an animal’s diet. These records provide critical evidence for different paleontological disciplines, e.g., paleodiet analyses, and paleoclimate reconstructions. Compared to those of other body tissues, such as bone collagen or teeth enamel hydroxylapatite, δ¹³C_B-HA values record information on the whole diet of an animal in its last years. δ¹³C_B-HA can be applied to fossil animals of various body sizes. The δ¹³C analytical instruments available only require that prepared bone samples be approximately 2–5 mg for precise measurement, allowing δ¹³C_B-HA analysis to be feasible on most vertebrate fossils without destructive sampling, especially on small mammals or birds whose teeth are not large enough for sampling or are lost. Moreover, δ¹³C_B-HA can be used from different times or under less than ideal burial environments. For fossils dating back to Devonian or buried in hot and humid regions, dietary information has been completely lost in bone collagen during post-depositional processes but still remained in the δ¹³C_B-HA values because hydroxylapatite is less influenced by diagenetic effects after deposition. In addition, systematic methods such as X-ray diffraction and Fourier transform infrared spectroscopy have been developed to qualitatively or semiquantitatively assess the influence of diagenesis on bone hydroxylapatite to ensure the credibility of the δ¹³C_B-HA values. With the above merits, δ¹³C_B-HA analysis is therefore becoming an increasingly important method in paleodiet-related research. Currently, applications of the δ¹³C_B-HA method on fossil animals are primarily focused on two aspects, namely, paleodietary reconstruction of fossil animals with uncertain diets and paleoenvironmental reconstruction based on the δ¹³C_B-HA values of fossil herbivores. The published researches, combined with our new results from early birds, demonstrate the considerable significance of the δ¹³C_B-HA method in paleontological and paleoenvironmental research. Notably, the δ¹³C_B-HA-based paleodietary analysis of early vertebrates, especially the large number of small birds or mammals discovered in the past decades would be an important work in the near future.     

Stable carbon isotope variability associated with taxonomic composition of lotic benthic algae

We examined the relationship between the δ¹³C and taxonomic composition of benthic algae collected from a riffle (fast current habitat) of a non‐shaded mountain stream, which is a tributary of the Kiso River, Japan. The benthic algal δ¹³C ranged from ?20.6 to ?14.2‰ and tended to be ¹³C‐depleted with increasing relative abundance of upright filamentous cyanobacteria and ¹³C‐enriched with increasing relative abundance of prostrate filamentous cyanobacteria. Using isotopic mass balance equations, the relative abundance of the dominant taxa, upright and prostrate filamentous cyanobacteria, small diatoms and others, explained 74% of δ¹³C variability. This study shows a case where the difference in taxonomic composition is a possible source of the isotopic variability of benthic algae, which is a mixture of taxa with distinct isotopic signatures.     

Stable isotope composition of water in desert plants

A survey of the stable isotope content of tissue waters of plants from the Negev desert was conducted. Large differences were observed in the extent of enrichment of the heavy isotopes in leaf water relative to local precipitation among different plants. This is apparently caused by the species-dependent stratagems adopted by the plants to cope with water stress, primarily by differences in the depth of water uptake in the soil and through the timing of stomatal openings during the daily cycle. Salt stressed plants showed extreme variability in the isotopic composition of leaf–water. The results show that plants with adaptation to arid conditions can avoid the transpiration regime, which would lead to the strong isotopic enrichment in their leaf water expected under arid conditions. This has implications for the use of stable isotopes in plants as indicators of either plant ecophysiology or paleoclimate. Responsible Editor: Hans Lambers. G. Goodfriend is deceased.     

Carbon isotope composition in landscape components and its changes under different ecological conditions

The composition of stable carbon isotopes in plants, plant litter, leaf litter, and soil organic matter was studied experimentally in the western part of the northern foothills of the Caucasus and mountainsides. It was found that the changes in carbon isotope composition depending on the vertical zonation do not exceed 8‰ and depend on the type of C3 plant communities, its presence in biogeocenosis components (living matter, plant litter, soil organic matter), and the degree of moistening of the plot studied.     

Stable isotope composition of organic compounds transported in the phloem of European beech--evaluation of different methods of phloem sap collection and assessment of gradients in carbon isotope composition during leaf-to-stem transport

The analysis of stable isotope composition (delta13C, delta15N, delta18O) of phloem-transported organic matter is a useful tool for assessing short-term carbon and water balance of trees. A major constraint of the general application of this method to trees at natural field sites is that the collection of phloem sap with the "phloem bleeding" technique is restricted to particular species and plant parts. To overcome this restriction, we compared the contents (amino compounds and sugars) and isotope signatures (delta13C, delta15N, delta18O) of phloem sap directly obtained from incisions in the bark (bleeding technique) with phloem exudates where bark pieces were incubated in aqueous solutions (phloem exudation technique with and without chelating agents [EDTA, polyphosphate] in the initial sampling solution, which prevent blocking of sieve tubes). A comparable spectrum of amino compounds and sugars was detected using the different techniques. O, C, or N compounds in the initial sampling solution originating from the chelating agents always decreased precision of determination of the respective isotopic signatures, as indicated by higher standard deviation, and/or led to a significant difference of mean delta as compared to the phloem bleeding technique. Hence, depending on the element from which the ratio of heavy to light isotope is determined, compounds lacking C, N, and/or O should be used as chelating agents in the exudation solution. In applying the different techniques, delta13C of organic compounds transported in the phloem of the twig (exudation technique with polyphosphate as chelating agent) were compared with those in the phloem of the main stem (phloem bleeding technique) in order to assess possible differences in carbon isotope composition of phloem carbohydrates along the tree axis. In July, organic compounds in the stem phloem were significantly enriched in 13C by > 1.3 per thousand as compared to the twig phloem, whereas this effect was not observed in September. Correlation analysis between delta13C and stomatal conductance (Gs) revealed the gradient from the twigs to the stem observed in July may be attributed to temporal differences rather than to spatial differences in carbon isotope composition of sugars. As various authors have produced conflicting results regarding the enrichment/depletion of 13C in organic compounds in the leaf-to-stem transition, the different techniques presented in this paper can be used to provide further insight into fractionation processes associated with transport of C compounds from leaves to branches and down the main stem.     

CAM and carbon isotope composition: Origin of the constitutive carbon in young leaves of Bryophyllum daigremontianum Berger

Abstract. The export of assimilates from mature leaves towards the young leaves was investigated: 100% and 65% of constitutive matter of leaves of rank 0 (the youngest leaves at the top of the plant) and of rank 1, respectively, originated from other parts of the plant. Photosynthesis of a particular leaf covers the total carbon requirement of that leaf only when it reaches about two-thirds of its mature size. When pairs of mature leaves were excised, the young leaves increased their own autotrophic growth while the level of assimilates exported by the remaining leaves remained unchanged. The existence of permanent pools in the leaves that export the assimilates was demonstrated; about 50% of the carbon, both in the soluble and insoluble fractions of mature and senescent leaves (ranks 5 to 8 from the apex), was not renewed by turnover. It is shown that the ¹³C-enrichment of the components of the starch-malate sequence in young leaves results, at least in part, from the incorporation of imported carbon chains. The significance of the δ¹³C diagnosis in CAM determination is discussed in relation to the origin of the constitutive carbon of the leaves.     

Stable isotope methods in biological and ecological studies of arthropods

This is an eclectic review and analysis of contemporary and promising stable isotope methodologies to study the biology and ecology of arthropods. It is augmented with literature from other disciplines, indicative of the potential for knowledge transfer. It is demonstrated that stable isotopes can be used to understand fundamental processes in the biology and ecology of arthropods, which range from nutrition and resource allocation to dispersal, food‐web structure, predation, etc. It is concluded that falling costs and reduced complexity of isotope analysis, besides the emergence of new analytical methods, are likely to improve access to isotope technology for arthropod studies still further. Stable isotopes pose no environmental threat and do not change the chemistry or biology of the target organism or system. These therefore represent ideal tracers for field and ecophysiological studies, thereby avoiding reductionist experimentation and encouraging more holistic approaches. Considering (i) the ease with which insects and other arthropods can be marked, (ii) minimal impact of the label on their behaviour, physiology, and ecology, and (iii) environmental safety, we advocate more widespread application of stable isotope technology in arthropod studies and present a variety of potential uses.     

不同生态区烟草叶片蛋白质组学的比较

为探讨不同生态区烟叶香气风格形成的机理,应用蛋白质双向电泳联用质谱技术,对河南平顶山（浓香型烟叶的典型生态区）和福建龙岩（清香型烟叶的典型生态区）的烟草（Nicotina tobaccum L．cv．K326）叶片蛋白质组成进行了比较研究。结果发现,51个蛋白质在两个生态区发生了差异表达,其中在河南表达量上升的有15个,在福建表达量上升的有25个。另外,还分别有2个和9个蛋白点为在河南和在福建样品中特异表达。采用MALDI-TOF/MS进行肽质量指纹图谱分析,经MSDB、NCBInr和SwissPort数据库查询,共鉴定出25种蛋白质,其中参与叶绿体发育、色素代谢、光合作用相关的蛋白在福建烟区高表达,与糖酵解途径相关的蛋白质在河南烟区中高表达。另外,在两生态区烟叶中都发现有相当数量的特异表达的抗逆和防御蛋白。首次在蛋白质组学水平对不同香气风格烟叶的形成机理进行了探讨。     

Stable isotope composition of fatty acids in organisms of different trophic levels in the Yenisei River

We studied four-link food chain, periphytic microalgae and water moss (producers), trichopteran larvae (consumers I), gammarids (omnivorous--consumers II) and Siberian grayling (consumers III) at a littoral site of the Yenisei River on the basis of three years monthly sampling. Analysis of bulk carbon stable isotopes and compound specific isotope analysis of fatty acids (FA) were done. As found, there was a gradual depletion in (13)C contents of fatty acids, including essential FA upward the food chain. In all the trophic levels a parabolic dependence of δ(13)C values of fatty acids on their degree of unsaturation/chain length occurred, with 18:2n-6 and 18:3n-3 in its lowest point. The pattern in the δ(13)C differences between individual fatty acids was quite similar to that reported in literature for marine pelagic food webs. Hypotheses on isotope fractionation were suggested to explain the findings.     

Stable carbon isotope fractionation by sulfate-reducing bacteria

Biogeochemical transformations occurring in the anoxic zones of stratified sedimentary microbial communities can profoundly influence the isotopic and organic signatures preserved in the fossil record. Accordingly, we have determined carbon isotope discrimination that is associated with both heterotrophic and lithotrophic growth of pure cultures of sulfate-reducing bacteria (SRB). For heterotrophic-growth experiments, substrate consumption was monitored to completion. Sealed vessels containing SRB cultures were harvested at different time intervals, and delta(13)C values were determined for gaseous CO(2), organic substrates, and products such as biomass. For three of the four SRB, carbon isotope effects between the substrates, acetate or lactate and CO(2), and the cell biomass were small, ranging from 0 to 2 per thousand. However, for Desulfotomaculum acetoxidans, the carbon incorporated into biomass was isotopically heavier than the available substrates by 8 to 9 per thousand. SRB grown lithoautotrophically consumed less than 3% of the available CO(2) and exhibited substantial discrimination (calculated as isotope fractionation factors [alpha]), as follows: for Desulfobacterium autotrophicum, alpha values ranged from 1.0100 to 1.0123; for Desulfobacter hydrogenophilus, the alpha value was 0.0138, and for Desulfotomaculum acetoxidans, the alpha value was 1.0310. Mixotrophic growth of Desulfovibrio desulfuricans on acetate and CO(2) resulted in biomass with a delta(13)C composition intermediate to that of the substrates. The extent of fractionation depended on which enzymatic pathways were used, the direction in which the pathways operated, and the growth rate, but fractionation was not dependent on the growth phase. To the extent that environmental conditions affect the availability of organic substrates (e.g., acetate) and reducing power (e.g., H(2)), ecological forces can also influence carbon isotope discrimination by SRB.