Marine-dwelling microbes and plants produce 8 billion tonnes of dimethylsulfoniopropionate (DMSP) per year in Earth's surface oceans alone, via enzymes we have identified. Organisms produce DMSP to protect against salinity, cold, turgor pressure, oxidative and drought stresses, and predation. DMSP released into the environment is also widely taken up by microbes for these anti-stress properties, and used as a key nutrient via distinct degradation pathways. DMSP has critically important roles in global sulfur and carbon cycling, signalling, and as a major source of climate-active gases (CAG) e.g. dimethylsulfide (DMS) and the foul-smelling gas methanethiol (MeSH). Each year millions of tonnes of DMS, the characteristic smell of the seaside and a potent foraging cue guiding diverse organisms (gulls, seals, zooplankton, etc) to food, is released from DMSP via microbial DMSP lyase enzymes that we also identified. Some DMS is released and oxidised to form aerosols and cloud condensation nuclei in the atmosphere, which reduce the global radiation budget and 'cool' local climate. Critically, these sulfate aerosols return to land in rain - the primary transfer of biogenic sulfur from the oceans to land. DMSP synthesis and degradation are thought to occur only in marine settings, so DMSP cycling in terrestrial environments has largely been unexplored. We challenged this dogma by revealing that DMSP synthesis is widespread in the plant Kingdom, ranging from common plants like grass, to agriculturally-important crops like maize, cabbage and sugarcane. Furthermore, our preliminary work shows that DMSP levels surpassing those in seawater exist in soils in which these key agricultural and bioenergy crops grow. Our work shows such soils liberate significant quantities of DMS and MeSH - processes ignored in climate models. We have also isolated novel bacteria and fungi from maize and sugarcane soils that utilise DMSP as a carbon source and show inducible DMSP-dependent DMS or MeSH production. Critically, these bacteria lack known DMSP degradation genes in their genomes, and thus likely possess novel DMSP catabolic enzymes and/or pathways. We have therefore uncovered a potentially large and virtually unexplored research area with profound implications for biogeochemical cycling. Our findings urgently require detailed study to establish the importance and influence of terrestrial DMSP cycling on the climate. We wish to answer the fundamentally important questions of how microbes associated to terrestrial plants degrade DMSP, and the ecological and global importance of the process, especially relating to CAG production. We will test the hypothesis that plant-made DMSP is a key nutrient for CAG-producing microbes. In an everyday context, are microbes degrading DMSP responsible for the rotten MeSH smell associated with cabbage fields, or the sweet DMS smell associated with sweetcorn? We will study microbial DMSP degradation and concomitant CAG production associated to plants known to produce low (maize) and high (sugarcane) levels of DMSP, which together cover >0.2 billion ha. Collaborations are in place to sample these plants, as are the model DMSP-producing bacteria we isolated to study microbial DMSP degradation mechanisms in terrestrial environments. Our major aims are to elucidate the enzymes, pathways, and mechanisms of DMSP degradation in terrestrial microbes and use this knowledge to define the magnitude of the process and factors regulating it. Furthermore, we will use cutting-edge microbial ecology, modelling and process work to answer fundamental ecological questions: what are the key microbes that degrade DMSP and emit CAG in terrestrial environments, and how do they influence the climate? We see our proposal as addressing a major new challenge that will reveal the importance of DMSP in terrestrial environments, uncovering new and unexpected research fields with far-reaching implications for current and future climate models.

The Southern Amazon faces the greatest climatic threat of all Amazon regions. This region is drier and warmer than 'core' areas of the Amazon and has been subject to the most pronounced drying and warming trends. It is also the region of the Amazon where increases in tree mortality have been most marked and where atmospheric measurements suggest forests are no longer acting as a carbon sink but as a net source of carbon to the atmosphere. Given that Southern Amazon is at the front line of the Amazon's battle against climate change, it is essential that we better understand how resistant its forest species are to climate stress. In Lethal Psi, we will construct a new 1-hectare drought experiment to better understand the physiological survival limits of southern Amazon trees. It has become increasingly clear that the process of hydraulic failure plays an important role in drought-induced tree mortality. Water is transported from the soils to the canopy under tension. As drought ensues and the soil dries, the tension in the xylem vessels that transport water intensifies and this can lead to the formation of air bubbles (embolism) in xylem vessels, disrupting water transport to the canopy and ultimately resulting in tree death. While this process is understood in general terms, one critical current knowledge gap is that we don't know the thresholds in embolism formation that result in the death of tropical trees. This lack of understanding of the physiological thresholds that result in death constitutes a key uncertainty for accurately modelling tree mortality under climate change. Determining the hydraulic thresholds of tree death is not an easy task and requires monitoring tree hydraulic status up to the point of death. In Lethal Psi, we track key indicators of hydraulic function (e.g. leaf water potentials and sap flux) from the beginning of our imposed drought all the way to the death of the tree to quantify how loss of xylem conductance translates into mortality risk. While other drought experiments have been set up in Amazonia, these did not monitor embolism status before and during the mortality process and were thus unable to provide insights into physiological thresholds of survival. Up to now, drought experiments have only been set up northeastern Amazonia, where annual rainfall is almost twice that of our study site and where changes in climate have been much less pronounced than in southern Amazonia. Given their ecotonal nature and the rapid climate change experienced in southern Amazonia, we expect that trees in this region are much closer to their climatic limits and will experience much more accentuated mortality under imposed drought than observed in northeastern experiments. Ultimately, we plan to use the newly acquired field data to develop improved mortality functions that we will apply more broadly across southern Amazonia to better predict drought mortality risk of this critically important region. This will be done by updating a unique trait-based model specifically developed to simulate Amazon forests and their responses to environmental change.

Tropical rainforests are one of the planets most important stores of carbon, as well as being essential to water cycling at large scales. Within tropical forests the largest trees, with diameters exceeding 70 cm, store between 25-45% of the carbon, yet represent <4% of the total number of trees. These large trees also transport disproportionately more water than smaller individuals do, making them a conservation priority for the future. Large tropical trees are likely to be very old, with many between 200-500 years and some estimated to be >1400 years old. Therefore, they have survived historical extreme climate events, including drought. Yet, recent evidence suggests water transport limitations are likely to make larger trees more vulnerable to the more extreme, more frequent drought events, which are predicted for the future. However, we still do not understand how large trees manage to overcome the huge resistances associated with transporting water such large vertical distances, against gravity, which substantially increase the hydraulic stress the tree experiences in a given climate. This information is essential to understanding how vulnerable these iconic tropical trees will be to the predicted future increases in drought frequency and intensity. Large trees can minimise the effects of increasing resistance to water transport with height through changing multiple leaf and stem hydraulic traits vertically through their stem and canopy. However, data on these vertical changes are rare and do not exist for tropical trees. Consequently, there is limited knowledge concerning whether trees can or cannot compensate for the negative effects being taller has on their water transport capacity and therefore their vulnerability to future drought events. In this project we will combine novel measurements of vertical changes in tree anatomical, structural and hydraulic properties on the world's tallest tropical trees, in two different tropical regions - Amazonia and Borneo - to achieve the following aims: Aim 1: Determine how vertical changes in tree hydraulic and anatomical traits regulate the capacity of tall trees to maintain water transport to their leaves under different environmental conditions. Aim 2: Determine if key structural and architectural properties of tropical trees control the vertical gradients of plant hydraulic and anatomical properties. Aim 3: Determine how accounting for vertical gradients in hydraulic properties in tall tropical trees alters predictions of tropical forest water and carbon cycling. To achieve these aims we will study the tallest tropical trees in the world. This will include trees in Amazonia discovered in 2019 that reach 88.5 m tall, ~30m taller than any other tree recorded in the neotropics. We will compare these to equivalent sized trees in Borneo from the dipterocarp family, the family containing the tallest angiosperm species in the world. On these trees we will measure vertical gradients in hydraulic and anatomical traits on 60 trees varying in height from 20-90 m. These trees will come from eight dominant species in Brazil and Borneo, allowing us to contrast the hydraulic adaptations of trees species from drier, more seasonal climates (Brazil), to those of species that have evolved in wetter, a-seasonal climates (Borneo). To realise the three aims above, our novel vertical hydraulic trait measurements will be combined with measures of whole-tree water transport and storage, tree architectural data derived from state-of-the-art ground-based laser scanning and vegetation models. Combining these techniques will allow us to make a step-change in our current understanding of the limits to water transport in the world's tallest tropical trees and the impact this may have on carbon and water cycling under future climate scenarios.