Microbial degradation of isoprene in the terrestrial environment

澳大利亚的蓝山和弗吉尼亚州的蓝岭山脉之所以得名，是因为异戊二烯在大气中产生的蓝色雾霾，异戊二烯是一种由植物，特别是许多树种产生的大量气体。作为生物能源开始广泛种植的树木，即柳树和杨树，是异戊二烯排放量最高的树种之一。异戊二烯保护植物免受光和热的伤害，也可以作为信号分子。异戊二烯与低层大气中的其他化学物质反应非常活跃，限制了它们与甲烷反应的能力，还会产生臭氧。臭氧和甲烷都是强有力的温室气体，臭氧会损害植物的生长。从积极的一面来看，异戊二烯可以间接刺激云的形成，从而起到降温作用。数百项研究调查了异戊二烯的产生，主要是从树木中产生，并检查了环境变化对其从树木到大气的通量的影响。与之形成鲜明对比的是，只有少数几项研究表明，土壤中的微生物会消耗异戊二烯，其中一些微生物已经在实验室中培养出来了。微生物数量丰富(每茶匙土壤几十亿，每平方厘米叶子超过一百万)，是环境中化学物质循环的最重要催化剂。从对甲烷等其他气候重要气体的循环研究中，我们知道微生物的消耗是一个极其重要的过程，受到气候变化的很大影响。从培养的数百种消耗甲烷的细菌中，我们对它们的代谢途径有了广泛的了解，这使得研究工具的开发能够帮助为土地使用管理决策提供信息。对于与甲烷含量相似的异戊二烯，我们缺乏这方面的知识和工具。因此，除了我们已有的细菌外，我们建议培养异戊二烯降解菌，重点是土壤和树叶居民。利用强大的基因组技术，我们将确定与异戊二烯降解有关的基因的DNA序列。此外，我们将使用PI开发的工具来识别和调查那些不容易生长的异戊二烯降解物。大多数细菌看起来都很相似，所以我们经常使用DNA序列来研究它们在自然界中的作用。选定的独特DNA序列将用于识别、查看和计算自然样品中异戊二烯降解菌的关键物种。这将使我们能够准确地确定它们生活在哪里，例如，我们设想它们将在气孔(叶片中的气孔)周围特别丰富，大多数异戊二烯从气孔中逸出；使用最先进的成像技术(由我们的项目合作伙伴开发)将使我们能够识别哪些单个微生物正在积极降解土壤中或叶表面的异戊二烯。作为这项研究的补充，一名博士生将测量森林土壤中异戊二烯的消耗量，并首次在不同树种的树叶上测量异戊二烯的消耗量，将异戊二烯排放者与非排放者以及阳光和遮阳叶进行比较。我们将测试在叶表面添加异戊二烯降解微生物的排列是否会增加消耗，并通过测量微生物在叶片上生存或生长的能力，我们将获得关于这是否是减少异戊二烯通量的潜在战略的见解。该项目产生的所有数据将对与温室气体排放有关的天然林地和生物能源作物的管理有价值。

The Blue Mountains in Australia and Blue Ridge Mountains of Virginia, are so-named because of the blue haze that results from atmospheric reactions with isoprene, a gas produced in abundance by plants and especially many tree species. Trees that are starting to be grown widely as a source of bioenergy, namely willow and poplar, are among the highest isoprene emitters. Isoprene protects plants against heat and light-induced damage, and can also serve as a signaling molecule. Isoprene is so reactive with other chemicals in the lower atmosphere that it limits their capacity to react with methane and also generates ozone. Both ozone and methane are potent greenhouse gases, and ozone impairs plant growth. On the more positive side, isoprene can indirectly stimulate cloud formation which provides a cooling effect. Hundreds of studies have investigated isoprene production, primarily from trees, and examined the effect of a changing environment on its flux from tree to atmosphere. In stark contrast, only a handful of studies have shown that microbes in the soil consume isoprene, and a few of those microbes have been grown in the laboratory. Microbes are abundant (several billion per teaspoon of soil and more than a million per square cm of leaf) and the most important catalysts for cycling chemicals in the environment. We know from studying the cycles of other climatically important gases, like methane, that microbial consumption is an extremely important process that is greatly influenced by climate change. From hundreds of methane-consuming bacteria in culture, we have extensive knowledge of their metabolic pathways, which allows the development of investigative tools to help inform land-use management decisions. For isoprene, which is produced in similar abundance to methane, we lack this knowledge and tools.Therefore, in addition to those bacteria that we already have in culture, we propose to culture isoprene-degrading microbes, focusing on soil and leaf inhabitants. Using powerful genomic-based techniques, we will determine the DNA sequences of the genes involved in isoprene degradation. Additionally, we will use tools developed by the PI to identify and investigate those isoprene degraders that are not easy to grow. Most bacteria look alike and so we frequently use DNA sequences to study their roles in nature. Selected unique DNA sequences will be used to identify, view and count key species of isoprene-degrading bacteria in natural samples. This will enable us to determine precisely where they live, e.g. we envisage that they will be especially abundant around stomata (pores in the leaf) from where most isoprene escapes; and the use of state-of-the art imaging techniques (developed by our project partner) will allow us to identify which individual microbes are actively degrading isoprene in the soil or on the leaf surface.Complementing this study, a PhD student will measure isoprene consumption in forest soils, and for the first time, on leaves from various tree species, comparing isoprene emitters with non-emitters as well as sun and shade leaves. We will test whether adding permutations of isoprene-degrading microbes to leaf surfaces enhances consumption, and by measuring the microbes' ability to survive or grow on the leaves, we will obtain insights into whether this is a potential strategy for reducing isoprene flux. All of the data emanating from this project will be valuable for management of natural woodlands and bioenergy crops, in relation to greenhouse gas emissions.