Manipulating the chemosynthetic and photosynthetic support of river food webs

我们可能都熟悉地球上的生命依赖初级生产的基本原理，即由来自太阳的能量驱动的光合作用植物。1977年，当太平洋深处出现奇怪的6英尺管虫和巨蚌的图像时，人们非常感兴趣，揭示了大量的产量，实际上是整个社区依赖化学能源(化学合成)。几乎没有人会怀疑，这种化学合成的生命可能在英格兰南部的经典白垩河中意义重大。然而，一个偶然的发现，作为一个更广泛的NERC Locar项目的一部分，研究河流水和地下水交换的生态意义，表明情况就是这样的。我们在我们的一个重点模式系统(兰伯恩河)中测量了常见水生无脊椎动物(小型甲壳类和昆虫)及其推定食物来源的稳定碳同位素值，因为我们可以使用稳定同位素来追踪通过食物网的能源和通量。而小对虾和黑蝇幼虫的值反映了优势光合产物的值，而普通瓢虫幼虫的值则明显不同。值得注意的是，这样的同位素值表征了甲烷衍生碳的输入，我们的计算表明，卡迪蝇得到了20%-25%的化学合成碳“补贴”。淡水可能只占地球总水量的3%，河流只占地球总水量的极小比例，但我们认为我们最熟悉的就是这一微小的比例，我们在日常生活中也依赖于这一比例。我们早期的研究表明，我们对河流中的过程并不像我们最初想象的那样了解得那么多；一种全新的碳来源，实际上是为河流中的生命提供燃料。当然，甲烷是一种强大的温室气体，我们对它在环境中是如何产生和循环的了解越多，就越好。这些最初的发现促使我们在实验室中检查了简单条件下化学合成与光合作用生产的相对比例，我们表明化学合成确实是一种重要的能量来源；大约6%，但在自然条件下有可能更高。我们现在需要做的是在实验室将这些简单的测量扩大到现实的野外试验中，在这种试验中，我们可以同时控制甲烷和阳光的量。然后，我们可以将我们在昆虫身上看到的稳定同位素“模式”直接映射到我们假设是该模式的驱动因素的过程中，并弥合这一知识差距。在淡水生物协会的河流实验室，有许多河流通道可以作为我们实验的基础，尽管我们需要根据我们的具体要求进行修改。这些通道用来自R Frome的水喂养，我们以前发现那里是夏季甲烷浓度最高的地方，这是我们实验的理想选择。在一系列实验中，我们将操纵甲烷浓度、日照和动物数量，同时测量同步的光合作用和化学合成产物。如果我们能够证明整个食物网，包括植物，最终都会受到甲烷循环的影响，那么我们最初对甲烷补贴重要性(20%-25%)的计算是低估了，化学合成生产对这些河流中的生命更加重要。总而言之，我们将结合希尔德鲁和伍德沃德的传统河流生态学专业知识、Grey的稳定同位素专业知识以及Trimmer的气体和营养循环专业知识，进行新的合作，重新评估我们河流的生产力是如何管理的。

We are probably all familiar with the basic principle that life on earth is reliant on primary production i.e. photosynthetic plants driven by energy from the sun. There was a great deal of interest in 1977 when images of bizarre 6ft tubeworms and giant clams came up from the depths of the Pacific to reveal significant production, indeed whole communities reliant upon chemical energy (chemosynthesis). Few, if any, would suspect that such chemosynthetic life may be significant in the classic chalk rivers of southern England. However, a fortuitous finding, as part of a wider NERC LOCAR project into the ecological significance of river water and groundwater exchange, suggests that this is the case. We measured the stable carbon isotope values of common aquatic invertebrates (small crustacea and insects) and their putative food sources in one of our focal model systems (the River Lambourn) because we can use stable isotopes to trace energy sources and fluxes through food webs. Whereas the values for small shrimps and blackfly larvae reflected that of the dominant photosynthetic production, the cased larvae of the common caddisflies were distinctly different. Remarkably, such isotope values characterise an input of methane-derived carbon and our calculations suggest that the caddisflies were receiving a 20-25 % chemosynthetic carbon 'subsidy'. Freshwater may comprise only 3% of the Earth's total water, and rivers a vanishingly small percentage of that, yet it is this tiny percentage with which we think we are most familiar, and upon which we rely in our everyday lives. Our earlier research suggests we do not know as much about the processes in rivers as we first thought; a completely novel source of carbon, in effect, fuelling life in the river. Of course, methane is a powerful greenhouse gas and the more we know about how it is produced and cycled in the environment, the better. These first findings prompted us to examine the relative proportion of chemosynthetic to photosynthetic production under simple conditions in the laboratory and we showed that chemosynthesis was indeed a significant source of energy; around 6% but with the potential to be higher under natural conditions. What we need to do now is to scale up these simple measurements in the laboratory to realistic field-trials in which we can manipulate both the amount of methane and sunlight. Then we can map the stable isotope 'patterns' we see in the insects directly onto the processes which we hypothesised were the drivers of that pattern and close this knowledge gap. At the River Laboratory of the Freshwater Biological Association, there are a number of stream channels which we can use as the basis for our experimentation, although we will need to modify those to our specific requirements. The channels are fed with water from the R Frome which we have previously found to have the highest summer concentration of methane, ideal for our experiments. In a series of experiments, we will manipulate methane concentration, sunlight and animal numbers, while measuring concurrent photosynthetic and chemosynthetic production. If we can demonstrate that the whole food web, including the plants, are ultimately affected by methane cycling, then our first calculations of the importance of methane subsidy (20-25%) are underestimates, and chemosynthetic production is even more important to the life in these rivers. In summary, we will combine the traditional river ecology expertise of Hildrew and Woodward, with the stable isotope expertise of Grey, and gas and nutrient cycling expertise of Trimmer in a new collaboration to re-appraise how productivity in our rivers is governed.