University of Aberdeen

Microscopic plants, or phytoplankton, use the sun's energy to combine atmospheric carbon dioxide (CO2) and water to produce particulate organic matter (POM). A proportion of this sinks to the seabed, where bacteria and animals use it for energy production, maintenance of biomass and growth. These organisms maintain a strict balance of carbon (C), nitrogen (N) and polyunsaturated fatty acids (PUFAs) in their tissues. Their growth can thus be limited by any of these substrates, depending on the quantity and biochemical composition, or 'quality', of the POM available. Limitation by a single substrate necessitates that all others are in excess and must therefore be released to the environment. This can be achieved by adjusting the efficiencies with which C and N are assimilated or by liberating them as CO2 or dissolved inorganic N (DIN). It follows that the quantity and quality of POM arriving at the seabed, and the dietary demands of its inhabitants, influence the fate of C and N in marine sediments. Any POM that escapes ingestion, or that which is released as faecal pellets, may persist in the sediments for thousands of years. Geological storage of POM in marine sediments represents a means by which the ever-increasing atmospheric concentration of CO2 can be reduced. However, it also removes N from the biosphere. This reduces the potential for further phytoplankton growth, which is limited by the supply of DIN. If DIN was only supplied via the recycling of POM in marine sediments, it follows that a sustained net biological drawdown of atmospheric CO2 into the oceans can only occur when the C:N ratio of the POM arriving at the seabed is greater than the ratio of CO2:DIN released. Essentially nothing is known about how POM quantity and quality affect the offset between these ratios, or how it is liable to change in the future. Manmade nutrient enrichment and climate change are already changing the quantity and quality of POM arriving at the seabed, but we do not understand, or have the capacity to predict, how this influences the roles that marine sediments play in C storage and DIN release. In turn, this greatly restricts our ability to accurately represent the cycles of C and N in global models that are designed to make meaningful forecasts about future climate change. I will grow species of phytoplankton with different ratios of C, N and PUFAs, which therefore differ in terms of their quality. The C and N in the phytoplankton will be replaced with 13C and 15N, stable isotopes (SIs) of these elements that behave in an identical manner, but differ subtly in their mass. They are scarce and hence easy to follow in the natural environment. My research will, for the first time, introduce increasing quantities of the different, dual SI-labelled algae onto the seabed and follow the fate of 13C and 15N into 13CO2, DI15N, bacterial and animal biomass and the sediments, thereby providing a detailed insight into the ways in which the quantity and quality of POM influence the burial of C and the release of DIN. The experiments will be conducted in coastal and deep-sea (> 1 km deep) habitats as these are considered to be the most important areas for global seabed C turnover. My research will also provide information on the relative roles that bacteria and animals play in elemental cycling in shallow and deep-water habitats, a topic that currently remains hotly debated. The ultimate goal of this project is to generate a mechanistic understanding of the ways in which POM quantity and quality affect the fate of C and N in marine sediments. This will be used to produce a mathematical model that is capable of predicting the quantities of C stored, and DIN released by the seabed, given a known quantity and quality of POM. This will represent a significant step towards being able to accurately represent the role of marine sediments in global climate models.

Marine plants draw down CO2, and in a world of rising atmospheric CO2 levels carbon sinks in vegetated coastal ecosystems can sequester CO2 on geological time scales and are now referred to as 'Blue Carbon'. Marine macroalgae (MA) are highly productive macrophytes that currently cover approximately 3.5 million km2 of sublittoral seabed and provide 1521 Tg C yr-1 of net primary production globally. Nevertheless, MA have largely been excluded from estimates of blue carbon sequestration because they predominantly grow on hard substrates, which prevent the accumulation of detritus-rich sediments. But while MA carbon (CMA) cannot accumulate within the source ecosystem, it has been estimated that up to 82% of CMA is exported from the source ecosystem to seabed habitats at greater depths, providing a significant carbon subsidy to marine seabed ecosystems beyond the coastal zone and/or contributing to long-term carbon burial in the ocean seabed. Shelf seas are known for their significant stocks of carbon, and marine fjords have recently been proposed as major C sinks of global significance, despite their low area coverage accounting for 11 % of global annual carbon sequestration. Per unit area, fjord organic carbon burial rates are one hundred times as large as the global ocean average, and fjord sediments contain twice as much organic carbon as biogeneous sediments underlying the upwelling regions of the ocean. Studies in Arctic fjords suggest that CMA contributes up to 60% to C sequestration which would render macroalgae a major contributor to blue carbon sequestration, and global biogeochemical cycles in general. Both remineralisation and sequestration of CMA are poorly constrained, however, and the actual importance of MA detritus, and hence carbon derived from macroalgae, as a major ecosystem service to deep allochthonous (sink) benthic biota and sediments still needs to be addressed in a quantitative, geographically well-constrained investigation. This project will quantify the contribution of MA carbon to C sequestration and as food subsidy to benthic fauna in Scottish fjords through a combination of camera surveys, sediment coring and analysis, stable isotope analysis and isotope tracing experiments. Following the methodology of Smeaton et al. (2017), the project will deliver a predictive (first-order) assessment of the MA carbon subsidy into sea loch sediments. Climate change is a global issue, and improved understanding and management of Blue Carbon ecosystems is important for climate mitigation action, and directly relevant to UN Sustainable Development Goals 3, 13 and 14. Close collaboration with the Scottish Blue Carbon Forum will facilitate quick translation into national climate mitigation and adaptation policy. In addition to SUPER specific training events, the candidate will be trained in a wide range of field and laboratory techniques, including the design and conduction of novel isotope tracing experiments, food web modelling and GIS. It is envisaged that the student will also spend some time in the laboratory of our external collaborator Dr. Inka Bartsch at AWI Bremerhaven.

Background - In many areas of Sub-Saharan Africa, organic "wastes", including crop residues, food waste and excreta, are critical to food production, providing organic fertilisers for farmers who do not have the means to buy inorganic fertilisers, and reducing the need for irrigation by increasing the water holding capacity of the soil. However, these wastes also provide an important source of household energy. The way that organic wastes are used for energy directly affects both food and water security. Burning organic wastes removes carbon and nutrients, whereas residues from anaerobic digestion are nutrient-rich and can provide a valuable organic fertiliser as well as an energy source. Water quality can be improved by using organic wastes in energy production, so removing pathogens from the wider environment, but some energy technologies require extra water and so could impact the availability of water. Finding efficient ways to use organic wastes is crucial to achievement of the post-2015 Sustainable Development Goals to eradicate poverty, improve food security and nutrition, promote sustainable agriculture, ensure sustainable management of water, and ensure provision of affordable energy. However, optimum use of organic wastes is highly context specific, depending on complex interactions between what science makes possible and how decisions are made by individuals. Determination of realistic measures requires input from farmers, industry, civil society, and local and regional policy makers. Enhancement of the efficiency of energy transformations becomes vital in socio-economic environments where there is competition for water and organic feedstocks. Aim - to determine the how uses of organic wastes in rural areas of Southern Ethiopia impact food, energy and water security, and to design optimum solutions to improve access Objectives - 1. Find out how organic wastes are currently used in rural areas of Southern Ethiopia and determine what alternative uses would be culturally acceptable; 2. Complete a lab-based analysis of exergy (energy that can be used), carbon, nutrient, water and pathogen for the full life cycle of the different uses of organic wastes; 3. Make recommendations for better uses of organic wastes to minimise loss of resources. Methods A combination of focus group discussions and surveys will be used to better understand how farmers in Southern Ethiopia are currently using organic wastes and the acceptability of new options for improved use of such wastes. Options to be considered will include 1. Energy release technologies (unimproved cookstoves / 3-stone fire; self-made improved cookstoves; pre-manufactured improved cookstoves; biogas digesters and gas stoves); and 2. Methods for using organic wastes as fertilisers (fresh wastes, composts, bioslurry and biochar). Lab-based replicas of the different options for using organic wastes will be setup at UoA. The mass, energy and exergy balances will be studied, and carbon, nutrients, water and pathogens will be measured in samples of organic wastes, before and after treatment, losses being captured by analysis of any gases or liquids lost from the systems. Pathogens will be assessed by detection of faecal indicator organisms and clostridium. This analysis will follow the change in exergy, carbon, nutrients, water and pathogens from the initial organic wastes through the technology used to transform them, to the use of energy in cooking, and carbon and nutrients in organic fertilisers. The results from the lab will be brought together in a systems model, describing the flows of exergy, carbon, nutrients, water and pathogens through different systems. This will allow the use of organic wastes to be optimized for different outcomes, and recommendations to be made for better use of organic wastes.

Peatlands represent one of the largest stores of terrestrial carbon, accounting for ~21% of the global total soil carbon stock. Drained peatlands contribute to around 4% of UK's estimated total anthropogenic greenhouse gas emissions each year. Climate warming, increased drought occurrences and fires in these fragile ecosystems exacerbate uncertainty over the fate of peatland carbon. Increased effort is therefore required to develop sustainable management approaches for peatlands, which is expected to make an important contribution to climate change mitigation in Scotland. Drainage and climate stressors such as drought and warming impact the hydrology of wetlands such that the removal of water-logged anoxic conditions leads to increased decomposition of the otherwise preserved peat organic matter and release of CO2 back to the atmosphere (Kitson & Bell, 2020; Tiemeyer et al., 2016). Such conditions may reduce methane emissions but increased CO2 release outweighs the climate benefits of methane reduction in terms of long-term global warming potential (Huang et al., 2021). Fires, on the other hand, primarily affect belowground carbon cycling through change in aboveground organic matter and therefore decomposition rates and CO2 flux. Microbes (bacteria, archaea, viruses, fungi and other microeukaryotes) act as gatekeepers of soil-atmosphere carbon exchange because their growth, activity and interactions with the environment control the fate of carbon inputs (Malik et al., 2018). However, there is a lack of mechanistic understanding of the microbial physiological processes in peatlands that are responsible for carbon cycling, and their sensitivity to multiple climate stressors such as warming, drought and fire (Ritson et al., 2021). Therefore, there is an urgent need to understand the ecology and physiology of soil decomposer communities in response to changes in land use and climate together. The project aims to investigate microbial carbon cycling processes in intact and degraded systems that are under the influence of climate extremes, which are becoming increasingly frequent. There is a general consensus that degraded peatlands are less resilient to climate extremes such as severe droughts, heatwaves and fires in comparison to intact peatlands (Page & Baird, 2016). This PhD project will rigorously test the response of microbial functions and carbon sequestration rates to climate extremes. A combination of genotypic and phenotypic measurements will enable the project to link microbial traits to carbon sequestration rates under different treatment combinations. Taken together, this knowledge will provide the basis for better prediction and management of microbial processes in peatlands to enhance carbon storage under future climate.

There is a vast amount of research on how the visual system processes form and shape. However, much less is known about how object properties, such as surface texture, that indicate important material properties (eg its roughness or softness), are processed by the perceptual system. This is surprising because our perceptual systems recognise these properties with ease, but also surprising because many of our behavioural decisions, how we grasp or interact with an object, depend on the accurate perception of material properties. Human imaging studies have shown that specific areas within the ventral stream, that are distinct from those processing shape, form and colour, are responsible for texture discrimination (Cant, Arnott, & Goodale, 2009). Yet, it is unknown which visual information is used to estimate the roughness or softness of a material. Recently, Giesel and Zaidi (2013) suggested that the visual system estimates roughness in pictures using monocular/image based cues which are related to the 3D structure of the material. Taken together, this raises several interesting questions: 1) Are there any differences in how the visual system determines material properties for 2D and 3D stimuli; 2) How is the perceptual estimation of material properties used to inform the visuomotor system and vice versa? and 3) How does the visuomotor system use material properties to adapt its actions accordingly? To address these questions, we will use a range of techniques, including behavioural reaching and grasping studies, perceptual ratings, computational analysis of material properties and neuropsychology. In the perceptual experiments, parameters that are known to relate to the perceived roughness of a texture (ie height, density, sharpness and pointedness of the bumps) will be varied independently and systematically in both 2D and 3D stimuli and the effects on the perceptual system will be measured in psychophysical experiments. In the visuomotor experiments, the visual and haptic information about texture will be dissociated using a mirror setup such that participants grasp an object (placed behind a mirror) that feels different (eg rougher or smoother) than the object they see (placed in front of a mirror). Perceptual estimates of object properties will be assessed prior to and after object interactions. This will allow us to determine whether material perceptions change depending on the haptic experience. This is important as currently little is known about if and to what extent vision is affected by tactile experience or vice versa. In addition, a neuropsychological study is planned to investigate if a patient with ventral stream damage that is known to be unable to perceive shape and form but has no problems discriminating between different textures and colours (Cavina-Pratesi, Kentridge, Heywood, & Milner, 2009), is able to determine material properties such as rough and soft textures. If these estimations, as suggested, rely on inferring 3D structure (relating to form) from pictorial cues, the patient should be unable to identify specific material properties such as soft or rough for unfamiliar stimuli even when texture discrimination is still possible. Together these experiments will provide an increased understanding of which cues the perceptual and visuomotor system use to estimate material properties and how this information is processed in the human brain.