Carbon isotopic fractionation in lipids from methanotrophic bacteria: relevance for interpretation of the geochemical record of biomarkers

Experiments with cultured aerobic methane oxidising bacteria confirm that their biomarker lipids will be significantly depleted in 13C compared to the substrate. The methanotrophic bacteria Methylococcus capsulatus and Methylomonas methanica, grown on methane and using the RuMP cycle for carbon assimilation, show maximum 13C fractionation of approximately 30% in the resultant biomass. In M. capsulatus, the maximum fractionation is observed in the earliest part of the exponential growth stage and decreases to approximately 16% as cells approach stationary phase. This change may be associated with a shift from the particulate form to the soluble form of the methane monooxygenase enzyme. Less than maximum fractionation is observed when cells are grown with reduced methane availability. Biomass of M. capsulatus grown on methanol was depleted by 9% compared to the substrate. Additional strong 13C fractionation takes place during polyisoprenoid biosynthesis in methanotrophs. The delta 13C values of individual hopanoid and steroid biomarkers produced by these organisms were as much as l0% more negative than total biomass. In individual cultures, squalene was 13C-enriched by as much as 14% compared to the triterpane skeleton of bacteriohopaneaminopentol. Much of the isotopic dispersion in lipid metabolites could be attributed to shifts in their relative abundances, combined with an overall reduction in fractionation during the growth cycle. In cells grown on methanol, where there was no apparent effect of growth stage on overall fractionation there were still significant isotopic differences between closely related lipids including a 5.3% difference between the hopane and 3 beta-methylhopane skeletons. Hopane and sterane polyisoprenoids were also 13C-depleted compared to fatty acids. These observations have significant implications for the interpretation of specific compound isotopic signatures now being measured for hydrocarbons and other lipids present in sediments and petroleum. In particular, biomarker lipids produced by a single organism do not necessarily have the same carbon isotopic composition.     

Population pressure and land use change: from tree crops to sawah in Northwestern Kalimantan,Indonesia

The author describes the role of population pressure in the current transition from tree crops to wet rice cultivation (sawah) in Northwestern Kalimantan, Indonesia. He contends that this change is occurring "because the senescent trees no longer produce enough commodities to exchange for rice to feed an enlarged population." The historical background is described, policies of the Indonesian government are discussed, and data are presented on the rate of population growth     

地理问答题的答题方法分析

不少教师和学生在做问答题时肯定遇到过这样的情景：答题时自我感觉良好，可是一对参考答案，却有偏差，心里很不是滋味。细细想来，可能存在着三个问题：一是命题做的答案有误；二是命题对这个问题有自己独特的看法；三是答题自己没有答好。上述问题的出现，都将对教与学产生一定的影响，有必要加以纠正。笔认为，答题时字尽管可以千差万别，但大的方面应该是一致的，也就是说，做问答题应该有一个基本的准则。     

谈地理个人数字化教学资料库的建立

一、个人数字化教学资料库的特点 在日常的教学工作中，地理教师要接触各种各样的资料，如图片、视频、试卷、教案、时事、心得等。假如在要用到时四处寻找，势必费时费力，效率很低。于是人们想到了一个办法，在平时搜集、制作一些与教学相关的资料，并把它们归类、集中在一起，就形成了所谓的个人教学资料库。这样一来，查找、使用教学资料就变得方便、快捷，效率大大提高。     

专题地质填图及有关问题讨论

面积性地质调查及填图是今后地质调查的主要工作,如何做好这个工作,并实现地质调查和科研一体化,特别是在已有填图基础上,如何部署和开展新的填图工作,是一个亟待回答的新问题。专题地质填图是针对存在的重要地质或资源、环境问题,或是针对特定目标地质体等,以解决特定问题或满足社会特定需求为主要目的、填图与研究融为一体的地质填图,填图的范围和比例尺依据解决问题和目标地质体而定。专题地质填图是以问题和需求为驱动,是对已有的综合地质填图的补充和提高,是高效实现地调科研一体化的一种方式。论述了专题填图的内涵、必要性,以及开展专题填图的方式和方法。