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Abstract

Our main objectives are as follow: (1) Improve the fundamental understanding of water metabolism in ruminants by exploring hydrogen isotope relationships between tissues and compounds (e.g. fatty acids) within tissues and in excretory products (urine, faeces and breath (methane)). (2) Determine the proportion of hydrogen in ruminant fatty acids that originates from the drinking water and feed using compound-specific stable hydrogen isotope analysis to calibrate a novel palaeoclimate proxy. (3) Determine the routing of hydrogen from water and feed to enteric methane.

Description

Carbon metabolism is well studied in ruminants, but the routing of hydrogen from drinking water and feed to ruminant tissues and enteric methane remains largely unexplored. Hydrogen is involved in key metabolic processes of interest in agriculture and veterinary science, such as fermentation, biohydrogenation and methanogenesis in the rumen (Baldwin and Allison, 1983; Harfoot, 1981; McAllister et al., 1996) and fatty acid (FA) biosynthesis in adipose tissues (Engelking, 2015). A fundamental understanding of the routing of hydrogen in these pathways is therefore important in improving ruminant production, animal health and reducing environmental impact. This experiment will determine the routing of hydrogen from drinking water to ruminant FAs and enteric methane using a stable isotope approach. This approach also allows the relationship between the local climate (precipitation) and ruminant tissues to be explored. The hydrogen isotopic composition (recorded as δD or δ2H value) of a material is the relative abundance of the two stable isotopes of hydrogen within the material: protium (1affect the δD values of the water that animals consume. The δD values of ruminant tissues are determined by that of the drinking water (local precipitation) but also by that of pasture/fodder. Leaf water and leaf biomolecules have very different δD values from that of the local precipitation because of various physical and biochemical fractionation events that occur within the plant (Cormier et al., 2018). Therefore, the δD values of ruminant tissues are the result of the combination of the hydrogen sources and isotope fractionations. The link between bulk animal tissue and environmental hydrogen signals has been used in various ecological (Lott et al., 2003; Hobson et al., 2004; Meehan et al., 2001), food fraud (Heaton et al., 2008) and forensic (Podlesak et al., 2012; Ehleringer et al., 2008) studies. However, remarkably no studies have investigated the link between hydrogen signals in water, feed and mammal tissues at the biomolecular level. Similarly, no studies have investigated the routing of hydrogen from water and feed to enteric methane. Hydrogen isotopic analysis of ruminant FAs preserved in pottery at archaeological sites has revealed that they capture climate change events, such as the 8.2 ka global cooling event (Roffet-Salque et al., 2018). Therefore, there is potential to use archaeological ruminant FAs as a proxy for palaeoclimate reconstruction. Such proxy would complement other paleoclimate proxies such as ice cores by giving very localised climate information (Sachse et al., 2012). Furthermore, since the FAs are associated with prehistoric people, the proxy would give an insight into how prehistoric farmers responded to climate change events (Roffet-Salque et al., 2018). We are aiming to calculate δD values of past precipitation from archaeological animal fats in order to compare them to outputs of isotope-enabled climate models for palaeoclimate reconstruction. It is therefore essential to determine the proportion of hydrogen atoms in ruminant FA that are derived from the drinking water. Methane emission models incorporate data on the concentration and stable isotope (δD and δ13C) ratios of atmospheric methane and those of the various methane sources, and ruminant livestock production is the dominant source of anthropogenic methane, through enteric rumen fermentation, followed by rice paddies (Sherwood et al., 2017). The considerable climatic variation in precipitation δD values is an important consideration when determining the contribution of enteric methane to global emissions. A substantial amount of work has been carried out investigating the relationships between diet and methane production in cattle using δ13C values (Chang et al., 2019; Klevenhusen et al., 2010) and, to a lesser extent, δD values (Bilek et al., 2001). Additionally, there is evidence that rumen water contributes to methanogenesis (Bilek et al., 2001). However, no work has focussed on the relationship between drinking water and enteric methane δD values, so emission models are lacking this localised aspect. Furthermore, understanding the routing of hydrogen from feed/water to enteric methane is crucial to help mitigate the release of this greenhouse gas through dietary manipulation and supplements (Patra et al., 2017). The routing of hydrogen from drinking water to ruminant FA and enteric methane will be investigated by carrying out a feeding experiment in which groups of sheep are given isotopically distinct drinking waters and diets with identical molecular and isotopic compositions. Isotopic analysis of water, feed, sheep FAs and methane will enable the compound-specific quantification of drinking water-derived hydrogen in FAs and enteric methane. A similar feeding experiment was carried out in quail and it was determined that approximately 20 % of the hydrogen atoms in FA are water-derived (Hobson et al., 1999), but this will be the first study of its kind in mammals and particularly ruminants. Furthermore, compound-specific analysis will allow variations between different FAs to be identified. Ruminant lipids are subject to rumen microbial lipolysis and biohydrogenation and thus we expect that the proportion of water-derived hydrogen in ruminant FA will be higher than for birds. Overall, studying this modern system will enhance our understanding of water use in biochemistry and animal energetics, provide information about animal adaptation to warmer climates, improve climate models, and potentially lead to novel or improved methane mitigation strategies.

 

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