ALBERT

Description: The anthropogenic emissions of CO2 and other climate-active gases lead to a steep increase of global temperatures. Global climate change is particularly amplified in the Arctic (e.g., Serreze et al., 2009; Serreze and Barry, 2011). Increasing temperatures and the rapid sea ice decline have shown profound effects on life in the Arctic ecosystem (Wassmann et al., 2011). Climate model predictions suggest a seasonally sea ice-free Arctic well before the first half of this century (Overland and Wang, 2013; Docquier and Koenigk, 2021). The composition, structure and function of the Arctic microbiome will be altered with distinct effects on the marine system, on primary productivity, carbon fluxes and food web structures. Changes in the composition and structure of primary producers were already observed in Fram Strait (Nöthig et al., 2015), the boundary and highly dynamic zone between the Atlantic and the Arctic Ocean. These changes were reflected in the export flux of particulate organic matter (Lalande et al., 2013), also observable in the benthic communities (Jacob, 2014). Thus, understanding how the microbial communities changed over time under different environmental conditions is a scientific task needed to assess future changes in the Arctic ecosystem. This thesis aimed to understand the composition, distribution and function of bacteria, archaea and eukaryotic communities in Fram Strait across different spatial and temporal scales and their relationship with environmental variables. The overall objective was to identify signature groups and key factors of change, to provide a baseline to the effects of climate change and sea ice retreat. It provides a comprehensive overview of the Arctic microbiome by the incorporation of seawater, sinking particles and sea ice samples to identify key microbial indicators of change and environmental drivers in these communities. Samples were obtained in the frame work of the Long-Term Ecological Research (LTER) site HAUSGARTEN and the FRontiers in Marine Monitoring (FRAM) program. The results of Chapter I and Chapter II highlight the usage of methods free of compositional- bias and meta’omics approaches necessary to understand the role of microbial communities. The observations in Chapter I revealed that different water masses characterized by different physicochemical conditions harboured different active microbial communities. A late phytoplankton bloom dominated by diatoms in the surface waters of the eastern Fram Strait was identified, where members of the Bacteroidetes, Alteromonadales, Oceanospirillales and Rhodobacterales were significantly active. Abundant transcripts of transporters and fundamental cellular functions supported the degradation of organic matter. The deeper waters of Atlantic origin were marked by strong chemolithotrophic activities by members of Thaumarchaeota. In Chapter II I analysed bacterial and archaeal groups in deep-sea waters that benefitted from a phytoplankton bloom at the surface. Chapter III studied the development of microbial composition of sinking particles using a 12-year time-series study. The presence of sea ice and the passing warm anomaly were the drivers of change in these communities. In Chapter IV, microcosm experiments revealed bacterial taxa that responded to eukaryotes and substrates sourced from the sea ice during sea ice melt in seawater. Altogether, the results of this thesis provide baseline knowledge to better assess the effects of climate change on the Arctic microbiome and the consequences for ecosystem functioning and carbon cycling.

Description: The Mekong River drains a catchment of over 800,000 km2 and is the world's 12th longest river (4800 km), the 8th largest water discharge (470 × 106 m3/year), and the 10th largest sediment load (160 × 106 tons/year). The Mekong starts on the Qinghai–Tibet Plateau with a maximal elevation of 5220 m, flows through six countries (China with 16% of its basin, Myanmar with 5% of its basin, Laos with 35% of its basin, Thailand with 18% of its basin, Cambodia with 18% of its basin, and Vietnam with 11% its basin), and empties into the Vietnam East Sea (South China Sea). The Mekong River basin (MRB) has the world's most diverse river ecosystem. It is the world's largest inland fishery. Its biodiversity is fundamental to agricultural production and the food security of 90 million people in the Lower Mekong basin, including about 18 million people in the Vietnamese Mekong delta.

Description: Biodiversity generally increases productivity in ecosystems; however, this is mediated by the specific functional traits that come with biodiversity loss or gain and how these traits interact with environmental conditions. Most biodiversity studies evaluate the effects of species richness alone, despite our increasing understanding that intraspecific diversity can have equally strong impacts. Here, we manipulate both species richness and intraspecific richness (i.e., number of distinct strains) in marine diatom communities to explicitly test the relative importance of species and strain richness for biomass and trait diversity in six distinct temperature/nutrient environments. We show that species and strain richness both have significant effects on biomass and growth rates, but more importantly, they interact with each other, indicating that cross-species diversity effects depend on within-species diversity and vice versa. This intertwined relationship thus calls for more integrative approaches quantifying the relative importance of distinct biodiversity components and environmental context on ecosystem functioning.

Description: Solar-driven seawater electrolysis for hydrogen fuel production holds an outstanding potential towards the development of a carbon-neutral and sustainable energy infrastructure, but the development of green, efficient and stable photoelectrocatalysts selectively promoting oxygen evolution remains a formidable challenge. Motivated by this issue, in this work we propose a tailored combination of two economically viable materials, α-Fe2O3 and graphitic carbon nitride (gCN), to fabricate promising anodes – eventually decorated with cobalt phosphate (CoPi) particles – for alkaline seawater photosplitting. The target systems were fabricated via an original multi-step route, involving the plasma-enhanced chemical vapor deposition of iron(III) oxide on conducting glasses, the introduction of gCN in very small amounts by a rapid and facile electrophoretic process, and final annealing in air. A comprehensive characterization revealed the successful fabrication of composites featuring a tailored surface defectivity, a controlled nano-organization, and a close Fe2O3/gCN interfacial contact. After decoration with CoPi, the best performances corresponded to a Tafel slope of ≈100 mV dec−1 and overpotential values enabling us to rule out the competitive hypochlorite formation. In addition, photocurrent densities at 1.23 V vs. RHE showed a nearly 7-fold increase upon Fe2O3 functionalization with both gCN and CoPi. These amenable results, directly dependent on the electronic interplay at Fe2O3/gCN heterojunctions and on CoPi beneficial effects, are accompanied by a remarkable long-term stability, and may open up attractive avenues for clean energy production using natural resources.