联合固氮菌的合成生物学研究进展

氮素是作物生长过程中最重要的元素,氮素缺乏将会严重影响作物生长。随着人类对粮食的需求量增加,化学氮肥的施用量越来越多。生物固氮在全球氮素循环中有着重要的作用,60%的氮来源于生物固氮。因此,生物固氮,尤其是能够在作物中定殖的联合固氮菌,最有可能代替氮肥成为粮食作物的主要氮源。长期以来,如何提高生物固氮效率以及在作物中实现生物固氮是生物学家的重要研究方向。合成生物学的出现和发展为能够生物固氮的研究带了新的机遇,有望缓解粮食作物对化学氮肥的大量需求。本文概述了固氮菌的种类、联合固氮菌中固氮基因岛的组成以及转录调控机理,阐述了合成生物学在生物固氮领域中的研究现状,对未来的联合固氮菌合成生物学的发展方向作出了展望。     

基于多出行模式两步移动搜索法的广州多尺度绿地可达性与公平性研究

绿地可达性与公平性是衡量城市居民接触和使用绿地便捷程度与机会的重要指标,也是评价和反映城市绿地资源空间配置科学性与合理性的重要体现,并与城市环境、生态、健康、社会和可持续发展密切相关。已有研究较少从多个尺度、多种出行模式测度绿地可达性与公平性。以广州为研究区域,结合出行O-D点智能查询系统（Travel O-D Point Intelligent Query System,TIQS）构建了多出行模式两步移动搜索法模型（Multi-mode Two-step Floating Catchment Area Method,M2SFCA）和绿地公平性模型,对广州绿地可达性和公平性空间格局与差异进行了研究。研究发现：（1）广州多尺度绿地可达性水平存在明显的"城郊两极化"空间差异,低绿地可达性区域集聚于人口密度较高的中心城区,而人口较少的外围城市区域绿地可达性值普遍较高。（2）广州市域绿地资源空间分布处于严重不公平状态（绿地公平性指数为0.58）,且各行政区之间也存在较大差异,南沙区、天河区和花都区绿地公平性相对较好,其他区域绿地资源空间分配均有待提高与优化。（3）在镇街层面,有73.29%的镇街绿地资源空间分配处于高度平均水平（绿地公平性指数≤0.2）;在社区层面,低可达性-公平类型的社区数量最多（占社区总量的39.85%）,高可达性类型的社区分布最少：有15.91%的社区属于高可达性-公平类型,有12.94%的社区属于高可达性-不公平类型。研究结论将为优化城市区域绿地空间结构、保障绿地空间公平提供科学依据。     

Studies on the relations of selenium and Keshan disease

Keshan disease is an endemic cardiomyopathy of unknown cause in The People’s Republic of China that occurs most frequently in children under 15 years of age and women of child-bearing age. Studies of children 1–9 years old in Mianing County of Sichuan Province have indicated that Keshan disease is a selenium responsive condition. Incidence rates of 9.5–13.5/1000 in 1974–1975 were reduced to 1–2/1000 in children treated with a tablet weekly of 0.5–1 mg sodium selenite. During 1974–1977, only 21 cases of the disease occurred in 36,603 treated children, compared with 106 cases in 9430 untreated children, of whom 53 died and 5 still have insufficient heart function. Occurrence of the disease was invariably associated with a lower selenium content of cereals, and of hair (less than 0.12 ppm Se) in residents from affected, compared with non-affected, areas. The dose relationship between selenium and regional characteristics of Keshan disease suggests that it is probably a biogeochemical disease; other etiological factors have also been considered.