The response of the terrestrial biosphere to climate change is still largely unknown and represents a key uncertainty in climate change predictions. High latitude regions, including Arctic and boreal ecosystems, constitute a key component of the earth system due to significant soil carbon stocks. High latitude regions are net sources of greenhouse gases, such as methane (CH4) and nitrous oxide (N2O), but there is significant disagreement among flux estimates with further uncertainty due to a rapidly changing environment. Climate change effects are particularly strong during the non-growing season, altering the timing of spring snowmelt, fall freeze-up, and increasing winter temperatures. The changes have significant implications for biogeochemical cycles and ecosystem function across high latitude regions. Despite growing evidence of the importance of non-growing season greenhouse gas emissions, few measurements have been made in pristine Arctic and boreal ecosystems. Non-growing season CH4 emissions can account for 10-100% of annual CH4 flux, while next to nothing is known about emissions of N2O during this period. Process-based models miss non-growing season emissions of CH4, underestimating them by 67% and annual emissions by 25%. I will use complementary observations (WP1), modelling (WP2), and experiments (WP3) to quantify the annual magnitude of CH4 and N2O flux, identify controls on non-growing season flux, and assess why existing models of CH4 flux fail outside of the growing season. Are environmental conditions so different that existing model parameters fail, or is non-growing season biogeochemistry fundamentally different? The overall impact is to shift the paradigm from “nothing happens outside of the growing season” to “capturing non-growing season processes is key to understanding ecosystem dynamics.” Ultimately, results will provide novel insights into greenhouse gas budgets and transform our understanding of fundamental earth system dynamics.

The ocean is by far the largest reservoir for carbon dioxide (CO2) on Earth and represents a driving force for climate mitigation. Through photosynthesis, active marine microorganisms (e.g. phytoplankton) convert atmospheric CO2 into biomass, where the majority of it is cycled in the surface waters by diverse processes including bacterial respiration and hydrolysis. Some of this biomass is exported as particulate organic carbon (POC) into the deep ocean, where bacterial cells play a critical role in regulating the efficiency of carbon export because they colonize and enzymatically hydrolyze POC as it sinks . A recent study suggested that signaling mechanisms within particle-associated bacterial communities enhance the activity of hydrolytic proteins involved in POC degradation. This overlooked process, known as quorum sensing, might impact the amount of carbon sequestered in the marine environment and ultimately affect the rate that CO2 is removed from the atmosphere. Quorum sensing (QS) involves the excretion and reception of distinct signaling molecules, but the biogeochemical implications of these bacterial “conversations” are poorly understood. To date, only a few culture-independent studies on QS in the marine environment have been carried out. This project will elucidate the role of QS systems among marine bacteria in triggering the synthesis of specific infochemicals and hydrolytic proteins, as well as its impact on shaping particles and particle-associated bacterial communities. Proposed methods include mass spectrometry, proteomics, three-dimensional particle imaging, molecular assessment of bacterial assemblages, and in situ localization of bacteria on intact particles. The outcome of the project will provide critical information on the importance of QS in regulating the efficiency of POC degradation in the ocean, which is necessary to understand and predict future climate scenarios.

We expect that temperatures over the wintertime central Arctic will increase by 20 degrees - and precipitation will double - by the end of this century if greenhouse gas emissions continue to rise. Arctic sea-ice is projected to completely melt in summer within the next decades, and may cease to form in winter in the coming century. The traditional framework to understand this Arctic amplification of climate change focuses on the steady-state mean Arctic climate. However, the Arctic wintertime atmosphere has two preferred states that are largely controlled by initially warm and moist air masses that cool and dry after being advected from lower latitudes. We understand little about how these air masses cool and dry, and what controls the sudden transition from a cloudy state to a clear state along their trajectory. This lack of understanding is a major obstacle to scientific progress and improved climate models. To achieve groundbreaking progress, I will analyze the warm, moist poleward flows, cold, dry equatorward flows and the air-mass transformations that lead from one to the other. I will observe and model such air masses along their trajectories using recently developed air-mass following balloons and customized model setups. Cooling of air in the Arctic mirrors heating in the Tropics. Together, these drive the global atmospheric circulation, but the Arctic’s role in this picture has largely been overlooked. My team will investigate how the Arctic couples to the global climate system using a novel concept of averaging the atmospheric circulation. We will focus on how and why both the heat and moisture content and the amount of air transported into and out of the Arctic change in a warming world and contribute and respond to Arctic amplification. A3M-transform will deliver a step change in understanding the air-mass transformation processes that shape Arctic amplification and transform our view of how the Arctic couples to the global climate system.

Photosymbioses between heterotrophic hosts and phototrophic algal endosymbionts have repeatedly and independently evolved across the tree of life. The immense productivity of photosymbioses affects global nutrient cycles, yet global warming is disrupting these nutrient-exchange symbioses, destabilizing the ecosystems they support. Heat stress erodes the metabolic controls underlying the establishment and maintenance of mutualistic carbon recycling in these symbioses. A link between endosymbiont nutrient release and their intracellular maintenance could thus explain the evolutionary establishment as well as the ecological collapse of photosymbioses in the Anthropocene. Harnessing recent advances in cell isolation and culture techniques, PhagoPhoRe seeks to understand the processes underlying the establishment, maintenance, and breakdown of the cnidarian-algal symbiosis. Specifically, I will investigate the idea that the release of photosynthates enables algal endosymbionts to evade phagosomal digestion and autophagic immune responses of the host by mimicking ongoing digestion processes. In three complementary work packages that combine state-of-the-art single-cell transcriptomics, proteomics, and isotopic imaging techniques, I will: 1) Characterize the role of host phagosome maturation processes in the intracellular establishment and maintenance of algal endosymbionts. 2) Investigate the postulated link between algal nutrient release and host phagosome arrest and immune suppression. 3) Examine the drivers of co-evolution towards obligate metabolic interactions in photosymbioses. Deciphering the interplay between symbiotic nutrient cycling and host phagosome maturation will be key to understanding the establishment and subsequent breakdown of photosymbioses. These processes are unlikely restricted to photosymbioses and could reveal fundamental mechanisms enabling the establishment of endosymbioses and, as such, eukaryotic life itself.