Methane as a novel energy subsidy in rivers: old or new carbon?

Lead Research Organisation: Queen Mary University of London
Department Name: Sch of Biological and Chemical Sciences


We are probably all familiar with the basic principle that life on earth is reliant on plant production driven by the sun's energy. 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, far below the influence of the sun's rays. These organisms were being fuelled, not by light, but by chemical energy from the core of the Earth itself. Such a mode of life would seem rather strange in the classic chalk rivers of southern England fished by J.R. Hartley! However, using a technique known as stable isotope analysis, we have demonstrated that this may actually be the case. We measured the stable carbon isotope values of common aquatic invertebrates (insects and crustaceans) and their food sources in the River Lambourn. Some invertebrates such as Gammarus shrimps and Simulium fly larvae reflected that of the dominant plant food sources in the river, whereas the cased larvae of the caddisflies Agapetus, Silo and Drusus indicated that their food source was likely to be derived partly from methane as a carbon source (up to 30%). We have identified such alternative 'fuelling' of food webs before in stagnant lakes but not in 'pristine' chalk rivers. 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. What could be even more exciting is the prospect that the original source of methane in groundwater fed rivers is both a subsidy to the system and ancient i.e. potentially formed thousands of years ago. Here we ask a simple question: Is there or is there not an ancient signal in the invertebrates from CH4? If we can show that a typically <1 year old invertebrate is made up in part by >100+ year old carbon, then this would change our perception of the ecology of such commonplace habitats. Hence, our proposal to combine the expertise from Queen Mary University of London in stable isotopes and biogeochemical cycling, with the expertise from Queen's University Belfast in radiocarbon dating to pursue such research.


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Description The concept of carbon fixation via chemosynthetic rather than photosynthetic pathways i.e. sunlight is nothing new (Sergei Winogradsky 1856 to 1953). Perhaps not surprisingly then, data (chemical signatures as _13C values) from deep- sea vents and methane seeps where methane is available at a 'point-source', indicate ~100% reliance upon chemosynthetic production for gastropod snails and sea stars - rather than that coupled to sunlight. Yet few would expect significant chemosynthetic energy in the iconic, clear water, lowland chalk rivers of Southern England. Curiously though, these iconic habitats are 'supersaturated with methane, the supersaturation, in part, coming from methane in the underlying groundwater. Previously we revealed that the larvae of two caddis flies (Trichoptera: Agapetus fuscipes and Silo nigricornis) had consistently depleted 'chemical signatures' (_13C) and that chemosynthesis coupled to methane oxidation provided the caddis larvae with up to 30% of their carbon or 'energy'. In this Small Grant the crux was: does the groundwater methane represent an 'old' or even 'ancient' carbon subsidy? And we asked a simple question: Is there or is there not an ancient signal in the invertebrates of pristine chalk streams from CH4?

The answer to our simple principle question is yes: the two caddis flies (Trichoptera) that we sampled (n = 24) in five chalk streams in both winter and summer are comprised of carbon which is at least 1800 years old! Further, this ancient carbon is not constrained to the key caddis fly species alone but it was also present throughout the main elements of the food-web, but to varying degrees. Just as we hypothesised, the greatest proportions of ancient carbon in any part of the food-web were correlated with a greater methane signature.
Exploitation Route Such pure fundamentle blue sky science is hard to exploit in any real tangible sense. That said, our work alters our perception of the goods and services provided by rivers and streams at a fundamentle level.
Sectors Environment