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Background and Aim: Stomatal regulation allows plants to promptly respond to water stress. However, our understanding of the impact of above and belowground hydraulic traits on stomatal regulation remains incomplete. The objective of this study was to investigate how key plant hydraulic traits impact transpiration of maize during soil drying. We hypothesize that stomatal response to soil drying is related to a loss in soil hydraulic conductivity at the root-soil interface, which in turn depends on plant hydraulic traits. Methods: We investigate the response of 48 contrasting maize (Zea mays L.) genotypes to soil drying, utilizing a novel phenotyping facility. In this context, we measure the relationship between leaf water potential, soil water potential, soil water content and transpiration, as well as root, rhizosphere and aboveground plant traits. Key Results: Genotypes differed in their responsiveness to soil drying. The critical soil water potential at which plants started decreasing transpiration was related to a combination of above- and belowground traits: genotypes with a higher maximum transpiration and plant hydraulic conductance as well as a smaller root and rhizosphere system closed stomata at less negative soil water potentials. Conclusions: Our results demonstrate the importance of belowground hydraulics for stomatal regulation and hence drought responsiveness during soil drying. Furthermore, this finding supports the hypothesis that stomata start to close when soil hydraulic conductivity drops at the root-soil interface.

Background and Aims Stomatal regulation allows plants to promptly respond to water stress. However, our understanding of the impact of above and belowground hydraulic traits on stomatal regulation remains incomplete. The objective of this study was to investigate how key plant hydraulic traits impact transpiration of maize during soil drying. We hypothesize that the stomatal response to soil drying is related to a loss in soil hydraulic conductivity at the root-soil interface, which in turn depends on plant hydraulic traits. Methods We investigate the response of 48 contrasting maize (Zea mays) genotypes to soil drying, utilizing a novel phenotyping facility. In this context, we measure the relationship between leaf water potential, soil water potential, soil water content and transpiration, as well as root, rhizosphere and aboveground plant traits. Key Results Genotypes differed in their responsiveness to soil drying. The critical soil water potential at which plants started decreasing transpiration was related to a combination of above and belowground traits: genotypes with a higher maximum transpiration and plant hydraulic conductance as well as a smaller root and rhizosphere system closed stomata at less negative soil water potentials. Conclusions Our results demonstrate the importance of belowground hydraulics for stomatal regulation and hence drought responsiveness during soil drying. Furthermore, this finding supports the hypothesis that stomata start to close when soil hydraulic conductivity drops at the root-soil interface.

Impact statement: Methane oxidizing microbes play a key role in reducing the emission of this potent greenhouse gas to the atmosphere. The known versatility of the recently discovered anaerobic Methylomirabilota methanotrophs is limited. Here, we report a novel uncultured Methylomirabilis species, Candidatus Methylomirabilis iodofontis, with the genetic potential of iodate respiration from biofilm in iodine‐rich cavern spring water. Star‐like cells resembling Methylomirabilis oxyfera were directly observed from the biofilm and a high‐quality metagenome‐assembled genome (MAG) of Ca. M. iodofontis was assembled. In addition to oxygenic denitrification and aerobic methane oxidation pathways, the M. iodofontis MAG also indicated its iodatereducing potential, a capability that would enable the bacterium to use iodate other than nitrite as an electron acceptor, a hitherto unrecognized metabolic potential of Methylomirabilota methanotrophs. The results advance the current understanding of the ecophysiology of anaerobic Methylomirabilota methanotrophs and may suggest an additional methane sink, especially in iodate‐rich ecosystems.

The research of anaerobic degradation of non-substituted polycyclic aromatic hydrocarbons is still in its infancy and most processes therein only poorly understood. Due to the poor bacterial degradation capabilities of PAHs, only few cultures exist that can be used to explore the underlying mechanisms. Their growth times are considerably longer than those of compounds with a smaller molecular weight and the production of biomass is substantially lower as shown by strain-specific FISH analyses and flow cytometry. Elucidation of pathways started in enrichment cultures growing on naphthalene as sole carbon and electron source and is at a point where individual steps are fairly well characterized through years of research. The next logical step is to look at compounds with a higher molecular weight to find similarities or dissimilarities in the pathways. The compound of interest in this thesis is phenanthrene, a three-ringed PAH without known degradation steps apart from a carboxylation as initial reaction (Zhang and Young 1997; Davidova et al. 2007). Parallels to anaerobic naphthalene degradation are expected due to similarities in the aromatic ring structure. In this thesis I wanted to find insights into the anaerobic degradation of phenanthrene as the next sized PAH beyond the initial carboxylation reaction. Therefore, we isolated bacteria able to degrade hydrocarbons under anoxic conditions from a naturally contaminated ecosystem, the pitch lake in Trinidad, Trinidad & Tobago. An enrichment culture growing anaerobically under sulfate reducing conditions was set up from sediments from the pitch lake. This culture was able to degrade phenanthrene as sole carbon and energy source. It was used to elucidate the anaerobic phenanthrene degradation pathway. Beyond the carboxylation reaction in the C2 position, which could be shown indirectly within the framework of this thesis by metabolite analysis and biochemical enzyme assays, I was able to show the ligation reaction fro, Die anaeroben Abbauwege polyzyklischer aromatischer Kohlenwasserstoffe (PAKs) sind bisher weitestgehend unerforscht geblieben. Aufgrund des langsamen Wachstums und geringer Biomassebildung existieren nur wenige Kulturen, anhand derer die Abbauwege aufgeklärt werden können. Des Weiteren sind deutliche Unterschiede der am Abbau beteiligten bekannten Gene zwischen verschiedenen Spezies aufgedeckt worden, was überdies die Aufklärung durch Sequenzierungsdaten verhindert. Der einzig bislang untersuchte anaerobe Abbauweg eines PAKs ist der von Naphthalin, eines Aromaten mit zwei Ringen und einem Molekulargewicht von 128 g/mol. Für diesen Abbauweg wurden die initialisierende Carboxylierung, die Reduktion der aromatischen Strukturen sowie die letztendliche Ringöffnung bereits beschrieben. Der naheliegend folgende Schritt betrifft die Untersuchung der Abbauwege für Schadstoffe mit höherem Molekulargewicht auf Gemeinsamkeiten und Unterschiede. Das Augenmerk dieser Arbeit liegt hierbei auf dem Schadstoff Phenanthren, einem aus drei aromatischen Ringen aufgebauter Kohlenwasserstoff mit einem Molekulargewicht von 178,23 g/mol. Aufgrund der Ähnlichkeiten in der aromatischen Ringstruktur ist diesbezüglich mit Parallelen zum anaeroben Naphthalinabbau zu rechnen. Auf der Suche nach Spezialisten des Phenanthrenabbaus wurden Bakterien aus einem natürlichen Teersee in der Karibik isoliert und unter anoxischen Bedingungen angereichert. Diese Anreicherungskultur wächst auf Phenanthren als einziger Kohlenstoff- und Energiequelle und unter sulfatreduzierenden Bedingungen. Anhand dieser Kultur konnten weitere – über die initialisierende Carboxylierung hinausgehender Schritte – des anaerobe Kohlenwasserstoffabbaus für Phenanthren untersucht werden. Dabei wurde mittels einer Metabolitenanalyse die Carboxylierung in der C2-Position bestimmt. Die anschließende Ligase-Reaktion konnte in einem Enzym-Assay nachgewiesen werden, wobei nur 2-Phenanthroesäure zu 2-Phenanthroyl-CoA umgesetzt wurde, was

Complex microbial communities in environmental systems play a key role in the detoxification of chemical contaminants by transforming them into less active metabolites or by complete mineralization. Biotransformation, i.e., transformation by microbes, is well understood for a number of priority pollutants, but a similar level of understanding is lacking for many emerging contaminants encountered at low concentrations and in complex mixtures across natural and engineered systems. Any advanced approaches aiming to reduce environmental exposure to such contaminants (e.g., novel engineered biological water treatment systems, design of readily degradable chemicals, or improved regulatory assessment strategies to determine contaminant persistence a priori) will depend on understanding the causal links among contaminant removal, the key driving agents of biotransformation at low concentrations (i.e., relevant microbes and their metabolic activities), and how their presence and activity depend on environmental conditions. In this Perspective, we present the current understanding and recent methodological advances that can help to identify such links, even in complex environmental microbiomes and for contaminants present at low concentrations in complex chemical mixtures. We discuss the ensuing insights into contaminant biotransformation across varying environments and conditions and ask how much closer we have come to designing improved approaches to reducing environmental exposure to contaminants.

Complex microbial communities in environmental systems play a key role in the detoxification of chemical contaminants by transforming them into less active metabolites or by complete mineralization. Biotransformation, i.e., transformation by microbes, is well understood for a number of priority pollutants, but a similar level of understanding is lacking for many emerging contaminants encountered at low concentrations and in complex mixtures across natural and engineered systems. Any advanced approaches aiming to reduce environmental exposure to such contaminants (e.g., novel engineered biological water treatment systems, design of readily degradable chemicals, or improved regulatory assessment strategies to determine contaminant persistence a priori) will depend on understanding the causal links among contaminant removal, the key driving agents of biotransformation at low concentrations (i.e., relevant microbes and their metabolic activities), and how their presence and activity depend on environmental conditions. In this Perspective, we present the current understanding and recent methodological advances that can help to identify such links, even in complex environmental microbiomes and for contaminants present at low concentrations in complex chemical mixtures. We discuss the ensuing insights into contaminant biotransformation across varying environments and conditions and ask how much closer we have come to designing improved approaches to reducing environmental exposure to contaminants.

This thesis addresses fundamental questions towards the specific involvement of soil microbiota in plant-associated soil food webs. Bacterial micropredators and niche differentiation to protozoa were characterised in a maize soil. Specific root-associated bacterial populations were shown to be preferentially mobilised with seepage water after extreme precipitation events. These findings allow for an advanced understanding of links between carbon pools and microbial communities in different soil compartments., Diese Doktorarbeit beschäftigt sich mit grundlegenden Fragen zur Beteiligung von spezifischen Bodenmikrobiota in pflanzenassoziierten Nahrungsnetzen. Bakterielle Mikropräparatoren und die Nischendifferenzierung zu den Protozoen wurden für einen Maisfeldboden untersucht. Es wurde gezeigt, dass spezifische wurzelassoziierte Mikroben nach Starkregenereignissen selektiv mit dem Sickerwasser mobilisiert werden. Diese Ergebnisse erlauben ein verbessertes Verständnis der Verbindungen zwischen Kohlenstoffpools und mikrobiellen Gemeinschaften in Bodenkompartimenten.

Sulfate-reducing microorganisms (SRMs) often compete with methanogens for common substrates. Due to thermodynamic reasons, SRMs should outcompete methanogens in the presence of sulfate. However, many studies have documented coexistence of these microbial groups in natural environments, suggesting that thermodynamics alone cannot explain the interactions among them. In this study, we investigated how SRMs compete with the established methanogenic communities in sediment from a long-term, electron acceptor-depleted, asphalt-exposed ecosystem and how they affect the composition of the organic material. We hypothesized that, upon addition of sulfate, SRMs (i) outcompete the methanogenic communities and (ii) markedly contribute to transformations of the organic material. We sampled sediments from the test and proximate control sites under anoxic conditions and incubated them in seawater medium with or without sulfate. Abundance and activity pattern of SRMs and methanogens, as well as the total prokaryotic community, were followed for 6 weeks by using qPCR targeting selected marker genes. Some of these genes were also subjected to amplicon sequencing to assess potential shifts in diversity patterns. Alterations of the organic material in the microcosms were determined by mass spectrometry. Our results indicate that the competition of SRMs with methanogens upon sulfate addition strongly depends on the environment studied and the starting microbiome composition. In the asphalt-free sediments (control), the availability of easily degradable organic material (mainly plant-derived) allows SRMs to use a larger variety of substrates, reducing interspecies competition with methanogens. In contrast, the abundant presence of recalcitrant compounds in the asphalt-exposed sediment was associated with a strong competition between SRMs and methanogens, ultimately detrimental for the latter. Our data underpin the importance of the quality of bioavailable organic materials in anoxic e

The dissertation shows that Pelotomaculum candidate BPL is the first member of the genus that can perform full dissimilatory sulfate reduction, besides its potential to degrade benzene. Results from physiological, genomic, and proteomic analyses are presented indicating that environmental Spirochaetes recycle detrital biomass and convert it into end products or electrons that support growth of associated degraders. Lastly, a novel reverse stable isotope labelling analysis is developed as a sensitive means to monitor biodegradation., Die Dissertation zeigt, dass Pelotomaculum cand. BPL als erstes Mitglied dieser Gattung sowohl Benzol abbauen als auch Sulfat vollständig reduzieren kann. Ergebnisse von physiologischen, genomischen und proteomischen Analysen machen deutlich, dass Umwelt-Spirochäten tote Biomasse wiederverwerten. Dabei entstehen Endprodukte oder Elektronen, die das Wachstum von nahestehenden Bakterien unterstützen. Schließlich wird eine neue stabile Isotopen-Methode entwickelt, mit der man sehr niedrige Mineralisierungsraten von Mikroorganismen messen kann.