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于初级生产的基本原理，即由太阳能量驱动的光合植物。1977年，当奇怪的6英尺长的管虫和巨大的蛤蜊从太平洋深处浮出水面时，人们引起了极大的兴趣，因为它们揭示了大量的生产，实际上整个群落都依赖于化学能（化学合成）。很少有人（如果有的话）会怀疑这种化学合成生命在英格兰南部经典的白垩河中可能很重要。然而，一个偶然的发现，作为更广泛的NERC LOCAR项目的一部分，研究河水和地下水交换的生态意义，表明情况确实如此。我们在我们的一个重点模型系统（兰伯恩河）中测量了常见水生无脊椎动物（小甲壳类动物和昆虫）及其假定的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能量来源和食物网的通量。小虾和黑蝇幼虫的光合作用值反映了显性光合作用，而普通球蝇幼虫的光合作用值则有明显差异。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，地虱得到了20- 25%的化学合成碳“补贴”。淡水可能只占地球总水量的3%，而河流所占的比例则微乎其微，然而，我们认为我们最熟悉的就是这一小部分，我们的日常生活都依赖于这一小部分。我们早期的研究表明，我们对河流过程的了解并不像我们最初想象的那么多；实际上，这是一种全新的碳源，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，我们对它是如何产生和在环境中循环的了解越多越好。这些最初的发现促使我们在实验室中检查简单条件下化学合成与光合作用的相对比例，我们表明化学合成确实是一个重要的能源来源；大约6%，但在自然条件下可能会更高。我们现在需要做的是将这些实验室里的简单测量扩大到实际的实地试验中，这样我们就可以同时控制甲烷和阳光的数量。然后，我们可以将我们在昆虫中看到的稳定同位素“模式”直接映射到我们假设的驱动该模式的过程，并缩小这一知识差距。在淡水生物协会的河流实验室里，有许多河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对它们进行修改。河道里的水来自R河，我们之前发现那里夏季甲烷浓度最高，非常适合我们的实验。在一系列的实验中，我们将控制甲烷浓度、阳光和动物数量，同时测量同步的光合作用和化学合成生产。如果我们能证明包括植物在内的整个食物网最终都受到甲烷循环的影响，那么我们最初对甲烷补贴重要性的计算（20-25%）就被低估了，化学合成生产对这些河流中的生命甚至更重要。总之，我们将结合Hildrew和Woodward的传统河流生态专业知识，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控制的。

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.