黄土丘陵区植被与地形特征对土壤和土壤微生物生物量生态化学计量特征的影响

研究黄土丘陵区植被与地形特征对土壤和土壤微生物生物量生态化学计量特征影响有助于深入理解黄土丘陵区不同植被带下土壤和土壤微生物相互作用及养分循环规律.选择黄土丘陵区延河流域3个植被区(森林区、森林草原区、草原区)和5种地形部位(阴/阳沟坡、阴/阳梁峁坡、峁顶)的土壤作为研究对象,利用生态化学计量学理论研究植被和地形对土壤和土壤微生物生物量生态化学计量特征的影响.结果表明: 土壤及土壤微生物生物量碳、氮、磷含量在不同地形之间的差别主要表现在沟坡位置和阴坡高于其他坡位和阳坡.植被类型的变化对两个土层(0～10、10～20 cm)土壤和土壤微生物生物量碳、氮、磷的影响均达到显著水平,坡向对表层(0～10 cm)土壤和土壤微生物生物量碳、氮、磷的影响强于坡位,而在10～20 cm土层,坡位对土壤和土壤微生物生物量碳、氮、磷影响更显著.植被类型显著影响土壤C∶N、C∶P、N∶P和土壤微生物生物量C∶N、C∶P,坡向和坡位仅影响土壤C∶P和N∶P,植被类型的变化是影响土壤C∶N的主要因素.同时,植被类型对土壤养分和微生物生物量碳、氮、磷含量及其生态化学计量特征的影响大于地形因子.标准化主轴分析结果表明,黄土丘陵区不同植被带土壤微生物具有内稳性,特别在草原带,土壤微生物生物量生态化学计量学特征具有更加严格的约束比例.在黄土丘陵区,土壤微生物生物量N∶P或许可以作为判断养分限制的另一个有力工具,若将土壤微生物生物量N∶P与植物叶片N∶P配合使用可能有助于我们更加精确地判断黄土丘陵区的土壤养分限制情况.     

黄土丘陵区刺槐与油松人工林生态系统生态化学计量特征

为阐明不同人工林生态系统间生态化学计量特征的差异,采用野外采样与室内分析相结合的方式分析了陕北黄土丘陵区落叶阔叶树种刺槐和常绿针叶树种油松人工林乔木、灌草、枯落物和土壤(土层深度0—100cm)C、N、P化学计量特征。结果表明:1)刺槐乔木各器官(叶、枝、干、皮、根)C含量显著低于油松,但N和P含量显著高于油松。因此,油松的C∶N和C∶P显著大于刺槐,而N∶P小于刺槐。2)刺槐林下枯落物N和P含量显著高于油松,但C含量显著小于油松。此外,油松林下枯落物C∶N(70.21)大于刺槐林下枯落物C∶N(19.71),说明油松林下枯落物分解较慢,有利于养分的存储。3)刺槐和油松人工林土壤C、N含量均随土壤深度增加而减少,P含量则基本保持不变。刺槐人工林土壤中C含量低于油松,N、P含量在两者之间无显著差异。4)刺槐人工林内乔灌草叶、枯落物与土壤C、N、P及其计量比的相关性多集中在10—20、20—30cm土层,而油松林中各组分与土壤营养元素的相关性相对较小,其中20—30cm土层中无显著相关性,说明相比刺槐人工林而言,油松人工林内土壤层N、P供应量对植物叶片N、P含量影响不显著。本研究为深入了解黄土丘陵区生态系统养分耦合循环机制奠定了基础,同时也为黄土丘陵区的植被恢复工作提供了一定的指导意义。     

黄土高原典型植被恢复过程土壤与叶片生态化学计量特征

为揭示黄土高原典型人工植被恢复过程中植物叶片与土壤碳(C)、氮(N)、磷(P)元素变化特征及其交互作用,以延安庙咀沟流域恢复20—40a的刺槐(Robinia pseudoacacia)、柠条(Caragana korshinskii)、草地和坡耕地(对照)为研究对象,分析了各样地植物叶片和土壤C、N、P化学计量的变化特征及相互关系。结果表明:从20a到40a的恢复过程中,3种植被叶片C含量均显著增加,草地叶片P含量显著升高,而刺槐、柠条叶片N和P含量则显著降低。刺槐、柠条及草地土壤C、N、P含量随着恢复年限的延长而增加,比耕地分别增加了70%—349%、27%—202%、13%—62%(P0.05),其中刺槐的增幅最大。从增速来看,刺槐和柠条林土壤表层C、N增速表现为前期(0—20a)大于后期(20—40a),而草地则相反。在20—40a的恢复过程中,刺槐、柠条叶片C∶N、C∶P均显著增加,草地叶片C∶P、N∶P则显著降低。恢复过程中,土壤C∶P在刺槐和草地中显著增加,而土壤N∶P仅在草地中显著增加,土壤C∶N则没有显著变化。相关性分析显示叶片C和土壤C、N、P显著正相关,叶片N、P和土壤N显著正相关,叶片和土壤N∶P显著正相关,叶片P、C∶P与土壤C、N增速显著相关,表明叶片P可以指示土壤C、N增速的变化,而N∶P可以将植物和土壤联系起来。植被恢复过程中,叶片和土壤C、N、P含量及增速均发生显著变化,且存在密切的联系,这种变化的趋势在刺槐、柠条和草地中有所不同。     

喀斯特区不同退化程度植被群落植物-凋落物-土壤-微生物生态化学计量特征

为摸清喀斯特植被退化对群落各组分C、N、P生态化学计量特征及内稳态特征的影响，为喀斯特退化生态系统植被恢复与重建提供科学依据，以桂西北喀斯特地区5种退化程度植被群落为研究对象，测定了不同退化程度植被群落植物叶片、凋落物、土壤和微生物生物量的C、N、P含量，分析其化学计量比特征、相互关系及植物内稳性特征。结果表明：(1)随着退化程度加剧，叶片C、N、P含量、N∶P和凋落物N∶P、微生物量C显著下降，而叶片C∶N、C∶P则显著增加，且植物叶片N∶P<14;随退化程度加剧，凋落物N、P含量、土壤C、N、P含量、微生物量N、P呈先略有增后显著降低的趋势，且不同退化程度群落土壤N∶P和微生物量C∶N无显著差异。(2)叶片N、P含量与土壤N、P含量，叶片C∶P与土壤C∶N、C∶P、N∶P,叶片N∶P与凋落物N、N∶P,叶片C、N、P含量与微生物量C呈显著或极显著正相关关系；叶片C∶N与土壤C、N,叶片C∶P与土壤N、P,叶片N∶P与土壤P呈显著或极显著负相关关系。(3)喀斯特地区植物叶片N、P元素的内稳性指数(H)平均值分别为2.74和2.31,属于弱稳态型，叶片N∶P的H值为5.14,为稳...     

Stoichiometric homeostasis in response to variable water and nutrient supply in a Robinia pseudoacacia plant–soil system

刺槐植物-土壤系统生态化学计量内稳性对水分和养分变异的响应特征 所有生物体都需要一定比例的元素来维持正常的生理代谢过程,它们的可塑性取决于它们利用外部资源的效率。阐明不同资源供应水平下植物、土壤和土壤微生物生物量生态化学计量特征之间的相互作用非常重要。本研究以一年生刺槐(Robinia pseudoacacia)幼苗为研究对象,测定不同水平水分、氮素和磷素处理下刺槐叶片、细根、土壤和微生物生物量C、N、P含量及其化学计量学指标。结果表明,刺槐叶片、细根、土壤和微生物生物量C、N、P含量及其化学计量特征会对其生存环境水分和养分条件的变化表现出一定程度的可塑性;方差分解分析结果表明,细根计量比解释了微生物生物量计量比方差的很大一部分;结构方程模型进一步揭示了细根计量比和叶片计量比是影响土壤微生物生物量C:N和C:P 的两个直接因素,而细根计量比具有较大的直接作用。此外,内稳性特征分析表明土壤微生物生物量C 和C:P对土壤养分变化较为敏感,其他指标均具有内稳性。这些结果明确了土壤微生物生物量化学计量的重要性,提高我们对不同生境水分和养分供应水平下植物-土壤系统养分循环机理的认识。     

黄土丘陵区不同森林类型叶片-凋落物-土壤生态化学计量特征

采用野外调查与室内分析相结合的方法,测定了黄土丘陵区主要人工林(刺槐、小叶杨和油松)和天然次生林(辽东栎、麻栎和白桦)中乔叶、凋落物以及土壤的碳(C)、氮(N)和磷(P)含量,探讨不同森林类型叶片-凋落物-土壤生态化学计量特征差异,旨在进一步了解研究区森林生态系统的养分供求现状。结果表明:1)人工林叶片和凋落物的C含量大于天然次生林,N、P含量均小于天然次生林,叶片和凋落物C∶N和C∶P值均表现为人工林大于天然次生林;2)人工林中土壤的C、N、P含量及化学计量比的显著性差异主要集中在土壤表层(0—10 cm),而天然次生林则集中在10—50 cm的土层,随着土层深度增加,二者的C、N、P含量逐渐减小;3)人工林N含量在叶片与凋落物间为显著正相关,天然次生林N含量在凋落物与土壤间为极显著正相关、C∶P值在叶片与土壤间则为显著负相关,其余各指标无显著相关性。揭示了除刺槐和辽东栎的生长受P限制外,其余各树种均受N限制,人工林凋落物的分解速率较快,且人工林土壤P有效性高于天然次生林,这些研究结果可为我国黄土丘陵区的植被恢复与重建工作提供理论依据。     

黄土高原刺槐叶片-土壤生态化学计量参数对降雨量的响应特征

以黄土高原10个典型样点的20年刺槐人工林为研究对象,测定了样地中刺槐叶片碳（C）、氮（N）、磷（P）含量,以及0-10 cm和10-20 cm土层土壤的主要理化指标,探讨了黄土高原刺槐叶片-土壤生态化学计量特征参数随水分梯度的变化规律。研究结果发现：①随着降水量增加,刺槐叶C、N含量呈增加趋势,叶片P含量无一致性规律。② 0-10 cm土层中土壤SOC、TN、TP、铵态氮含量以及C ∶ P和N ∶ P均随降雨量增加呈逐渐增加趋势,而C ∶ N比变化不明显。③刺槐叶片C、N、P及其化学计量比与土壤化学计量比相关性较弱,但叶片N、P与土壤铵态氮、速效磷等速效养分含量呈显著正相关（P<0.05）。本研究叶片N ∶ P>16,说明黄土高原刺槐生长主要受P元素限制,且随着降水的增加,土壤P养分的限制加强。     

不同植茶年限土壤-微生物生物量碳氮磷化学计量特征

退耕还林(草)等生态建设工程的实施引起土壤碳(C)、氮(N)、磷(P)循环及其化学计量特征发生变化,继而对土壤微生物生物量的化学计量造成潜在影响,然而,土壤-微生物C∶N∶P化学计量的时间动态及协调关系仍不明确。本试验选取三峡库区小流域退耕地——茶园为研究对象,以玉米地为对照,探索土壤-微生物生物量C、N、P随植茶年限(<5 a、5～10 a、10～20 a、20～30 a和>30 a)的变化特征,分析其化学计量比、微生物熵(qMBC、qMBN、qMBP)、化学计量不平衡性(土壤C、N、P计量比与微生物生物量C、N、P计量比的比值)之间的关系。结果表明：随着植茶年限增加,土壤和微生物生物量C、N、P、土壤C∶N和C∶P均显著升高,而土壤N∶P整体下降,微生物生物量C∶P和N∶P呈先升后降的变化趋势,微生物生物量C∶N变化不显著。此外,茶树种植年限对土壤、微生物间的化学计量不平衡性以及微生物熵均存在显著影响,随着植茶年限增加,qMBC先降低后升高,qMBN和qMBP呈波动上升;碳氮化学计量不平衡性(C∶N_imb)和碳磷化学计量不平衡性(C∶P_i...     

控雨对荒漠草原植物、微生物和土壤C、N、P化学计量特征的影响

以宁夏荒漠草原为研究对象,于2014—2015年设置了降雨量变化(减雨50%、减雨30%、自然降雨、增雨30%和增雨50%)的野外模拟试验,测定了植物、微生物和土壤C、N、P含量,同时调查了植物群落组成和土壤含水量等指标,研究了各组分C、N、P化学计量特征对连续两年降雨量变化的响应,分析了土壤C∶N∶P和含水量分别与植物生长、养分利用以及微生物量积累的相关性。结果表明,控雨改变了植物叶片C∶N∶P,且其影响程度随物种不同而异:减雨50%提高了牛枝子(Lespedeza potanimill)绿叶N和P以及猪毛蒿(Artemisia scoparia)绿叶P摄取能力,增雨(30%和50%)降低了猪毛蒿绿叶N摄取能力。增雨提高了猪毛蒿绿叶C∶N,增雨30%提高了苦豆子(Sophora alopecuroides)绿叶C∶N。增雨降低了猪毛蒿绿叶N∶P,增雨30%降低了白草(Pennisetum centrasiaticum)绿叶N∶P。相比之下,控雨条件下枯叶C∶N∶P的变化幅度较小;随降雨量增加微生物量C、N以及C∶N逐渐增加,但增雨50%使微生物量C和C∶N降低;控雨对土壤C∶N∶P的影响较小,但增雨提高了土壤水分有效性,因此促进了植物和微生物生长;试验期内,相对稳定的土壤C∶N∶P不能很好地指示植物和微生物生长发育的养分受限状况;干旱时提高叶片养分摄取、湿润时增强叶片养分回收,可能解释了牛枝子对降雨量变化的弹性适应能力。     

亚热带苔藓结皮对土壤-微生物-胞外酶化学计量特征的影响

生物结皮的形成和发育显著影响土壤碳(C)、氮(N)、磷(P)循环及其化学计量特征,土壤微生物如何适应环境资源的化学计量变化仍不明确。本研究以三峡库区苔藓结皮为对象,分析结皮盖度(0、1%～20%、20%～40%、40%～60%、60%～80%和80%～100%)对土壤理化性质(0～5和5～10 cm土层)、微生物生物量和胞外酶活性[(β-1,4-葡萄糖苷酶(BG)、β-1,4-N-乙酰氨基葡萄糖苷酶(NAG)、酸性磷酸酶(AP)]的影响,探索土壤-微生物-胞外酶C∶N∶P化学计量特征间的协变性。结果表明: 生物结皮发育显著提高了土壤黏粒、水稳性团聚体和土壤C、N、P含量,显著降低了土壤容重和砂粒含量;微生物生物量C、N、P和胞外酶活性均随结皮盖度的增大而显著增加;土层深度对土壤理化性质及C∶N∶P均无显著影响,但显著影响微生物生物量、胞外酶活性及BG∶AP和NAG∶AP。相关分析显示,土壤C、N、P含量与微生物生物量和胞外酶活性呈显著正相关,与BG∶NAG呈显著负相关,与NAG∶AP呈显著正相关,但与微生物生物量C∶N∶P无显著相关性;土壤-微生物、微生物-胞外酶C∶N∶P相关性均不显著,BG∶NAG∶AP随着微生物与土壤间C∶N∶P化学计量不平衡性的增加而逐渐降低。表明微生物养分代谢同时受N和P的限制,且P的限制较强烈,微生物可以通过调整自身生物量以及胞外酶C∶N∶P适应生物结皮发育驱动的土壤化学计量变化,从而维持内稳态。     

Synopsis of the tribe Bocageeae (Annonaceae), with revisions of Cardiopetalum,Froesiodendron, Trigynaea,Bocagea, and Hornschuchia

The tribal name Bocageeae Endlicher is reestablished and the tribe is circumscribed on the basis of solitary internodal ebracteate pedicels that are articulated at the base, and pollen shed in polyads of eight or more grains. Septate anther locules, large pollen size, and seed appendages are prevalent in the tribe. Intectate pollen with free-standing columellae, rare in Annonaceae, occurs not only in the genusTrigynaea but also in some species ofBocagea andHornschuchia. As defined here, the Bocageeae include seven neotropical genera:Cymbopetalum, Porcelia, Bocagea, Cardiopetalum, Froesiodendron, Hornschuchia, andTrigynaea. The latter five genera are revised and the treatments include ten new species:Cardiopetalum plicatum, Froesiodendron urceocalyx, Hornschuchia lianarum, H. santosii, H. leptandra, Trigynaea cinnamomea, T. lanceipetala, T. triplinervis, T. lagaropoda, andT. axilliflora, all from tropical South America. A new combination,Froesiodendron longicuspe, changes the rank of that taxon from subspecies to species.Cardiopetalum surinamense is removed fromFroesiodendron and reassigned toCardiopetalum on the basis of its connate petals, dehiscent monocarps, and seeds with bilobed arils. A cladogram provides an explicit hypothesis of intergeneric relationships in the tribe. The new combinationOnychopetalum periquino, based onTrigynaea periquino, is made.     

Lack of relationship between gliding cyanobacteria and filamentous gliding heterotrophic eubacteria: comparison of 16S rRNA catalogues of Spirulina,Saprospira, Vitreoscilla,Leucothrix, and Herpetosiphon

Spirulina platensis and Spirulina maxima were characterized by 16S ribosomal RNA oligonucleotide cataloguing. Both species are highly related. They fall into the main group of the cyanobacteria, showing a remote relationship to Nostoc, Fischerella, Aphanocapsa, and also to Prochloron. Low similarity coefficients were found between the Spirulina species and certain organotrophic and sulfide oxidizing bacteria, viz. Saprospira grandis, Vitreoscilla stercoraria, Leucothrix mucor, Herpetosiphon aurantiacus, and Beggiatoa leptomitiformis, respectively. This result does not support the classical hypothesis that certain filamentous gliding bacteria are apochlorotic cyanobacteria.     

Location,expression and orientation of the putative chlororespiratory enzymes,Ndh and IMMUTANS,in higher-plant plastids

The chloroplasts of many plants contain not only the photosynthetic electron transport chain, but also two enzymes, Ndh and IMMUTANS, which might participate in a chloroplast respiratory chain. IMMUTANS encodes a protein with strong similarities to the mitochondrial alternative oxidase and hence is likely to be a plastoquinol oxidase. The Ndh complex is a homologue of complex I of mitochondria and eubacteria and is considered to be a plastoquinone reductase. As yet these components have not been purified to homogeneity and their expression and orientation within the thylakoid remain ill-defined. Here we show that the IMMUTANS protein, like the Ndh complex, is a minor component of the thylakoid membrane and is localised to the stromal lamellae. Protease digestion of intact and broken thylakoids indicates that both Ndh and IMMUTANS are orientated towards the stromal phase of the membrane in Spinacia oleracea L. Such an orientation is consistent with a role for the Ndh complex in the energisation of the plastid membrane. In expression studies we show that IMMUTANS and the Ndh complex are present throughout the development of both Pisum sativum L. cv Progress No. 9 and Arabidopsis thaliana (L.) Heynh. leaves, from early expansion to early senescence. Interestingly, both the Ndh complex and the IMMUTANS protein accumulate within etiolated leaf tissue, lacking the photosystem II complex, consistent with roles outside photosynthetic electron transport.Abbreviations PQ plastoquinone - PSI, PSII photosystem I, II     

Novel phenylanthraquinones,isofuranonaphthoquinones, homoisoflavonoids,and biflavonoids from African plants in the genera Bulbine,Scilla, Ledebouria,and Rhus

The sale of non-cultivated plants poses a real threat to the survival of a large number of plant species. On the other hand, the fact that a given plant has become a traded item is an important lead in the effort to investigate plants for the purpose of discovering novel secondary metabolites that may have marketable properties. This report is largely based on the investigations of plants collected from vendors of medicinal plants. The identification is based on botanical specimens derived from the purchased plant parts either by growing them in a garden or through micropropagation using tissue culture techniques. Bulbine species yielded novel phenyl anthraquinones and isofuranonaphthoquinones, which were found to exhibit antiparasitic and antioxidant properties. Scilla nervosa subsp. rigidifolia and Ledebouria graminifolia yielded twenty homoisoflavonoids and two xanthones. Bulbs derived from tissue culture based specimens of L. graminifolia contained all the compounds isolated from the marketed samples. Some of the homoisoflavonoids were found to possess activity against colon and breast cancer cell lines. Rhus pyroides furnished six novel bichalcones some of which exhibited insect antifeedant, cytotoxic and antiproliferative activities. Structures of the compounds described in this report were elucidated by spectroscopic means including HRMS and 1D-, and 2D-NMR experiments. Interconversion of some of the phenyl anthraquinones was accomplished as part of their structure proof. The total synthesis of three of the biflavonoids was achieved using a synthetic route which involves a microwave irradiation promoted Ulmann synthesis. This revised version was published online in June 2006 with corrections to the Cover Date.     

Ribotyping of 16S and 23S rRNA genes and organization of rrn operons in members of the bacterial genera Gemmata,Planctomyces, Thermotoga,Thermus, and Verrucomicrobium

The number of organization of rrn genes of two members of the order Planctomycetales, Planctomyces limnophilus and Gemmata obscuriglobus, as well as three species from other bacterial phyla, namely Thermotoga maritima, Thermus aquaticus and Verrucomicrobium spinosum were examined by Southern blot hybridization analysis of restricted DNA with labeled 16S- and 23S rRNAs. Ribotyping analysis revealed that two species contain unlinked 16S- and 23S rRNA genes. Planctomyces limnophilus possessed two unlinked rrn genes which were separated from each other by at least 4.3 kb, and Thermus aquaticus had to unlinked 16S and 23S rRNA genes, separated from each other by at least 2.5 kb. Gemmata obscuriglobus exhibited five genes for which the organization could as yet not be determined because of the complex hybridization patterns. In the other two species, rrn genes clustered in operons. Thermotoga maritima had a single gene for each rRNA species which were separated by not more than 1.5 kb, while Verrucomicrobium spinosum had four copies of probably linked 16S and 23S rRNA genes with a maximal distance between 16S and 23S rRNA genes of 1.3 kb.     

Isolation,Biological Properties,and Spatial Structure of Antibiotic Loloatin A

A peptide antibiotic with cyanolytic activity was isolated from the IGM52 strain of the Brevibacillus laterosporus Gram-positive spore-forming bacteria. By ¹H NMR spectroscopy, this antibiotic was identified as loloatin A, a cyclic decapeptide cyclo(-Asn-Asp-Tyr-Val-Orn-Leu-D-Tyr-Pro-Phe-D-Phe-). The spatial structure of loloatin A in solution was determined.     

Dispersibility, cost and allometry of tumblers (Combretum, Combretaceae) and parachutes (Leucadendron, Proteaceae) of different size

Fall rates of tumblers ofCombretum and of parachutes ofLeucadendron are shown to be generally high (> 1.8 ms^–1). Tumblers contribute up to 77% of the mass of the dispersal unit and parachutes upto 22%. This implies that they are relatively expensive and inefficient in comparison with other propagule types. Morphometric analyses confirm previous findings that species with large seeds also have the largest dispersal structures, but the lowest dispersibility. Finally, there is a correlation between the sizes of these propagules and their associated leaf-size, confirming other studies that link the relative sizes of various plant attributes.     

Identification of b,c, and d cytochromes in the membrane of Vitreoscilla

Cytochromes b, c, d, and o were identified by spectroscopic analysis of respiratory membrane fragments from Vitreoscilla sp., strain C1. Carbon monoxide difference spectra of the reduced membranes had absorption maxima at 416, 534, and 571 nm (ascribed to cytochrome o) and 632 nm (cytochrome d). Derivative spectra of the pyridine hemochromogen spectra of the membranes identified the presence of b- and c-type cytochromes in Vitreoscilla. The cyanide binding curve of the membranes was biphasic with dissociation constants of 2.14 mM and 10.7 mM which were assigned to cytochrome o and cytochrome d, respectively. Membranes bound carbon monoxide with dissociation constant 3.9 M, which was assigned to cytochrome o. Cytochrome c ₅₅₆ and a NADH-p-iodonitrotetrazolium violet reductase component were partially purified from Vitreoscilla membranes.Abbreviations INT p-iodonitrotetrazolium violet - RMF respiratory membrane fragments - K _d dissociation constant - CHAPS 3-[(3-cholamido propyl) dimethylammonio]-1-propanesulfonate - DOC sodium deoxycholate - PAGE polyacrylamide gel electrophoresis - SDS sodium dodecyl sulfate     

Introduction,distribution, spread,and impacts of exotic freshwater gastropods in Texas

We examined the patterns of distribution, vectors of introduction, and potential ecological impacts of freshwater exotic species in Texas over the last 45 years. Currently, five species of exotic gastropods are established: channeled-type applesnail (Pomacea insularum), red-rim melania (Melanoides tuberculatus), quilted melania (Tarebia granifera), giant rams-horn snail (Marisa cornuarietis), and Chinese mysterysnail (Cipangopaludina chinensis). In contrast to the northern part of the US, where shipping appears to be the most important vector for the introduction of aquatic invasive species, aquarium and ornamental trade dominated among unintentional vectors of introduction of all freshwater exotics in Texas, resulting in different patterns of distribution, spread, and ecological impacts. The rate of spread of exotic gastropods in Texas varied from 39 waterbodies colonized over 18 years for P. insularum to only three waterbodies during last 45 years for C. chinensis. Four of five exotic gastropods were found in highly vulnerable aquifer-fed springs and rivers, which contain numerous endemic and endangered species. The fifth species, Pomacea insularum, is an agricultural pest. Potential negative ecological effects of exotic gastropods include impacts on wetlands and wetland restoration, competitive exclusion of native snails, and the introduction of exotic parasites, trematodes, which could infect fish and waterfowl, including federally protected species. Aquifer springs with stable temperature regimes are refuges for both cold and warm intolerant species. Handling editor: D. Dudgeon     

Synthesis, characterization, and antibacterial activity of N,O-quaternary ammonium chitosan

N,N,N-Trimethyl O-(2-hydroxy-3-trimethylammonium propyl) chitosans (TMHTMAPC) with different degrees of O-substitution were synthesized by reacting O-methyl-free N,N,N-trimethyl chitosan (TMC) with 3-chloro-2-hydroxy-propyl trimethyl ammonium chloride (CHPTMAC). The products were characterized by ¹H NMR, FTIR and TGA, and investigated for antibacterial activity against Staphylococcus aureus and Escherichia coli under weakly acidic (pH 5.5) and weakly basic (pH 7.2) conditions. TMHTMAPC exhibited enhanced antibacterial activity compared with TMC, and the activity of TMHTMAPC increased with an increase in the degree of substitution. Divalent cations (Ba²⁺ and Ca²⁺) strongly reduced the antibacterial activity of chitosan, O-carboxymethyl chitosan and N,N,N-trimethyl-O-carboxymethyl chitosan, but the repression on the antibacterial activity of TMC and TMHTMAPC was weaker. This indicates that the free amino group on chitosan backbone is the main functional group interacting with divalent cations. The existence of 100 mM Na⁺ slightly reduced the antibacterial activity of both chitosan and its derivatives.