The extracellular nuclease of Serratia marcescens: studies on the activity in vitro and effect on transforming DNA in a groundwater aquifer microcosm

A quantitative endonuclease assay, which relies on the introduction of single and double strand breaks into supercoiled plasmid DNA, was used to study the activity of the extracellular nuclease of Serratia marcescens SM6 in buffer and in groundwater. The parallel enzyme concentration-dependent production of relaxed and linear plasmid molecules suggests that the nuclease produces single and double strand breaks in duplex DNA. Bovine serum albumin stimulated the nuclease activity towards DNA and RNA and increased the stability of the enzyme against thermal inactivation. The DNase activity at 4 °C and 50 °C was almost half of that at the optimum temperature (37 °C). The nuclease was active in groundwater, although the specific activity was lower than in buffer. In a groundwater aquifer microcosm, mineral-adsorbed transforming DNA was substantially less accessible to the nuclease than was dissolved DNA. The data suggest that the extracellular nuclease of Serratia marcescens may contribute to DNA turnover in the environment and that adsorption of DNA to minerals provides protection against the nuclease.Abbreviations GW groundwater GWA groundwater aquifer     

The natural course of chronic hepatitis C

Abstract: In 1988, investigators from the Chiron Company (USA) detected the non-A, non-B agent and named it hepatitis C virus (HCV). An anti-HCV antibody assay (ELISA) and subsequently confirmation tests (immunoblot and polymerase chain reaction) were developed. HCV exposure results in a chronic infection in a majority of cases. This chronic infection is associated with slowly progressive chronic liver disease. Chronic HCV infection is, like HBV, also associated with the development of hepatocellular carcinoma. Most HCV carriers are infected by parenteral routes. Intravenous drug users have the highest risk of becoming infected. Intrafamiliar spread is seen in certain parts of the world but sexual and perinatal transmission does not play an important role in spreading the infection. Antiviral therapy (alpha-interferon) in patients with chronic hepatitis C will normalize liver function tests in about 25% of the cases.     

The biogeochemistry of a north-temperate grassland with native ungulates: Nitrogen dynamics in Yellowstone National Park

Nutrient dynamics of large grassland ecosystems possessing abundant migratory grazers are poorly understood. We examined N cycling on the northern winter range of Yellowstone National Park, home for large herds of free-roaming elk (Cervus elaphus) and bison (Bison bison). Plant and soil N, net N mineralization, and the deposition of ungulate fecal-N were measured at five sites, a ridgetop, mid-slope bench, steep slope, valley-bottom bench, and riparian area, within a watershed from May, 1991 to April, 1992.Results indicated similarities between biogeochemical properties of Yellowstone grassland and other grassland ecosystems: (1) landscape position and soil water affected nutrient dynamics, (2) annual mineralization was positively related to soil N content, and (3) the proportion of soil N mineralized during the year was negatively related to soil C/N.Grazers were a particularly important component of the N budget of this grassland. Estimated rates of N flow from ungulates to the soil ranged from 8.1 to 45.6 kg/ha/yr at the sites (average = 27.0 kg/ha/yr), approximately 4.5 times the amount of N in senescent plants. Rates of nitrogen mineralization for Yellowstone northern range grassland were higher than those measured in other temperate grassland ecosystems, possibly due to grazers promoting N cycling in Yellowstone.     

Nutrient fluxes in the shifting cultivation system of south-west Côte d'Ivoire

At two sites, one with a 4-year-old (4-Y) secondary vegetation and the other with a 20-year-old (20-Y) vegetation, the influence of burning slashed vegetation on crop performance was studied during three seasons. In the first season after clearing, also the influence on weed growth was studied. At both sites, burning significantly decreased the number of weed seedlings. The lowest number of seedlings was found on the burnt plots of the 20-Y site. Burning increased yield and nutrient uptake significantly in the first and second season after clearing. In the third season after burning, only at the 4-Y site a significantly higher yield and nutrient uptake were found. At the 20-Y site the effect had disappeared. Calculations of efficiency of utilization of absorbed N, P and K indicated that P was the least available nutrient, also after burning. At both sites three consecutive crops absorbed approximately 40% of P applied in ash, while the cumulative recovery of K was at least 36% at the 4-Y site and at least 59% at the 20-Y site. On non-burnt plots, yields were not lower in the third season than in the first season after clearing, thus indicating that the inherent soil fertility did not decrease. Hence, yield decline on the burnt plots could be ascribed to ash depletion. It was concluded that in the local shifting cultivation system, the combination of ash depletion and infestation of weeds are the main reasons for abandoning the fields.     

The fate of nutrients during fires in a tropical savanna

Abstract The nutrient loads contained in the grassy fuel before fires, and of ash subsequently, were compared to determine the fluxes of macronutrients, copper and zinc during fires at Kapalga in Kakadu National Park. The fluxes were estimated in three vegetation types: forest, woodland and open woodland. The magnitudes of the fluxes were greatest in the forest community where grassy fuel loads were highest at about 6.3 t ha^?1. In these sites, 54–94% of all measured nutrients in the fuel were transferred to the atmosphere during the fires. For each nutrient, the proportion transferred to the atmosphere as entrained ash was calculated by assuming that calcium was not volatilized during the fires. If the transfer of entrained ash represents local redistribution only, then rainfall accession and the deposition of these particu-lates should replace most of the losses of all nutrients except nitrogen (N). Estimated rates of biological fixation of N appear to be insufficient to replace the annual losses of N. It is therefore concluded that a regime of annual fires that completely burn the available grassy fuel would deplete N reserves in these savannas, unless there are other sources of biologically fixed N, which are unknown at present.     

水稻自然生长趋势的数学模型和动态分析

本研究以随机过程理论为基础,应用数理方法,在分期播种的基础上,提出了一个受遗传特性控制的水稻各生物学参量自然生长趋势的数学模型。并进行了重复验证,得到了不同生育时段各生物学参量的生长速度（一阶导数）和生长速度的变化率（二阶导数）,揭示了水稻生长的基本规律。并结合实测资料,通过相关分析,得到影响各生物学参量的关键气象因子,将水稻生长的研究建立在定量的基础上,有利于更加合理地采用各项高产栽培和农田管理措施。     

天然林内红松种群年龄更替数学模型及研究

本文给出天然林内红松种群年令更替数学模型（1）。通过对（1）进行定性分析,得到主要结论是：系统（1）在第一象限内存在唯一稳定周期解的充要条件是bk-bc-2d＞0其生态意义是天然林内红松幼树与母树随时间变化会产生一个有规律、互为消涨的变化特征。     

自然免耕下的稻田生态系统

自然免耕是一种新型的稻田耕作法,通过人为改变地表微地形,建立了一个适合水旱作物复种轮作,鱼、萍、鸭周年共生的稻田生态系统．自然免耕促进了土壤内、外环境的物质、能量和信息交换,水、热、气、肥谐调．并通过环境网络效应,强化了环境与生物区系间的缓冲－调节力．提高了土壤自调能力和土壤有序度．     

促进兴安落叶松天然更新的出苗条件研究

根据４０多块标准地调查,促进更新措施后兴安落叶松１年生幼苗出现的频度及数量与种子年关系很大,幼苗更新效果最好的是种子年当年,更新频度可达６０％以上,幼苗平均为１．０—２．０×１０３株·ｈａ－１,其他年份促进的效果较差．同时,也与迹地类型、种源状况和整地质量关系较大．一般在山坡中、下部,土壤湿润的杜香落叶松和藓类越桔落叶松林迹地促进效果较好,其更新频度为６０—７０％．绝大部分更新幼苗出现在表土裸露、无杂草灌木、土壤湿润的地方．更新频度与植被盖度呈明显的衰减指数相关．促进地块距下种林墙最好不超过６０ｍ范围．     

白水江自然保护区糙花箭竹天然林生物量与生产力的初步研究

 本文根据样方上测定糙花箭竹的地径、茎高、密度、茎重、枝条重、叶重和鞭系重资料,研究竹林生物量和生产力,并建立回归数学模型为：茎重Ws=160.942+0.227(D2NH) p<0.001 枝条重WB=76.008+0.039(D2NH) p<0.001 叶重WL＝80.643+0.051(D2NH) p<0.001 鞭系重WR=285.554+0.074(D2NH) p<0.001由方程估测出天然糙花箭竹林的各器官干重分别为茎7.558,枝条1.782,叶2.143,鞭系4.795t／ha。总生物量为16.278t／ha,平均净生产量为3.256t／ha。