The ocean holds fifty times as much carbon as is in the atmosphere. Biological processes contribute to storing carbon in the ocean on climate-relevant timescales (hundreds to thousands of years). Marine phytoplankton (drifting microscopic plants) use sunlight and carbon dioxide in the upper ocean to form their bodies which are rich in carbon. When phytoplankton die they might clump together and sink into the ocean interior, or they could be eaten by zooplankton (tiny animals) which produce fecal pellets that can sink rapidly. Once this organic carbon is deeper in the water, bacteria might colonise the particles and break them down, or they could be broken apart by zooplankton feeding on them. These processes all act to reduce the amount of organic carbon reaching the deep ocean, however the deeper it goes the longer it will remain out of contact with the atmosphere. This "biological carbon pump" helps to regulate our climate, and without biology in the ocean it has been shown that atmospheric carbon dioxide levels could be nearly double what they are today. Earth system models have differing, but all fairly simple, representations of the biological carbon pump due to a lack of understanding of how the processes contributing to particle formation and respiration function. The suite of models that contribute to the Intergovernmental Panel on Climate Change reports do not agree on the magnitude or direction of change for ocean carbon storage under future climate scenarios. This means we have low confidence for our future projections of ocean carbon storage, which is further impeded by a growing discrepancy between models and observations. In this project we will examine how much carbon has been respired during the transit from the upper ocean, and in what ways. We will measure the important processes of particle fragmentation and aggregation, microbial respiration, and zooplankton vertical migration and respiration. We will do this using a process cruise and autonomous underwater vehicles. We seek to answer the question: How is organic matter transformed and respired by biotic interactions in the mesopelagic, how does that vary with depth, location and season, and what are the consequences for ocean carbon storage? The ultimate goal is to generate new detailed understanding of important processes that influence the rate and depth of interior respiration which we will scale up to provide the globally-resolved information needed to develop the next generation of biogeochemical models.

The balance between the production of organic carbon during phytoplankton photosynthesis and its consumption by bacterial, zooplankton and phytoplankton respiration determines how much carbon can be stored in the ocean and how much remains in the atmosphere as carbon dioxide. The amount of organic carbon stored in the ocean is as large as the amount of carbon dioxide in the atmosphere, and so is a key component in two global carbon cycle calculations needed to avoid a global temperature rise of more than 1.5 degrees C: the calculation of the technological and societal efforts required to achieve net zero carbon emissions and the calculation of the efficiency of ocean-based engineering approaches to directly remove carbon dioxide from the atmosphere. Yet, despite its vital role, our ability to predict how ocean carbon storage will change in the future is severely limited by our lack of understanding of how plankton respiration varies in time and space, how it is apportioned between bacteria and zooplankton and how sensitive it is to climate change-induced shifts in environmental conditions such as increasing temperature and decreasing oxygen. This woeful situation is due to the significant challenge of measuring respiration in the deep-sea and the uncoordinated way in which these respiration data are archived. This project will directly address these two problems. We will take advantage of our leadership and participation in an international programme which deploys thousands of oceanic floats measuring temperature, oxygen and organic carbon in the global ocean, in an international team of experts focused on quantifying deep-sea microbial respiration, and our experience of collating international datasets, to produce an unprecedented dataset of bacterial and zooplankton respiration. We will derive estimates of respiration based on data from floats, so that together with estimates derived from recently developed methods including underwater gliders, the new database will include respiration measurements calculated over a range of time and space scales. Crucially, respiration rates will be coupled with concurrent environmental data such as temperature, oxygen and organic carbon. This dataset will enable us to quantify the seasonal and spatial variability of respiration and derive equations describing how respiration changes with the proportion of bacteria and zooplankton present and with the chemical and physical properties of the water. These equations can then be used in climate models to better predict how respiration and therefore ocean carbon storage will change in the future with climate-change induced shifts in temperature, oxygen, organic carbon and plankton community. We will take part in a hybrid hands-on and online international training course on observations and models of deep-water respiration targeted to early career researchers from developing and developed countries to showcase the useability of the respiration database and the global array of oceanic floats. We will also prepare Science Festival exhibits on observing life in the deep ocean for schoolchildren. The deliverables of the project - a unique global open-access database of respiration measurements, new equations describing the sensitivity of respiration to changing temperature and oxygen suitable for climate models and online training materials for early career researchers - are of benefit to scientists who aim to predict how a changing climate will affect the storage of carbon in the ocean, educators who train the next generation of ocean scientists and practitioners, policy makers who need to quantify nationally determined contributions to actions limiting global warming, and scientists, engineers, lawyers, governing bodies and commercial companies designing, evaluating and implementing ocean-based carbon dioxide removal technologies.