More than ten years ago, ER-ANT1 was shown to act as an ATP/ADP antiporter and to exist in the endoplasmic reticulum (ER) of higher plants. Because structurally different transporters generally mediate energy provision to the ER, the physiological function of ER-ANT1 was not directly evident. Interestingly, mutant plants lacking ER-ANT1 exhibit a photorespiratory phenotype. Although many research efforts were undertaken, the possible connection between the transporter and photorespiration also remained elusive. Here, a forward genetic approach was used to decipher the role of ER-ANT1 in the plant context and its association to photorespiration. This strategy identified that additional absence of a putative HAD-type phosphatase partially restored the photorespiratory phenotype. Localisation studies revealed that the corresponding protein is targeted to the chloroplast. Moreover, biochemical analyses demonstrate that the HAD-type phosphatase is specific for pyridoxal phosphate. These observations, together with transcriptional and metabolic data of corresponding single (ER-ANT1) and double (ER-ANT1, phosphatase) loss-of-function mutant plants revealed an unexpected connection of ER-ANT1 to vitamin B6 metabolism. Finally, a scenario is proposed, which explains how ER-ANT1 may influence B6 vitamer phosphorylation, by this affects photorespiration and causes several other physiological alterations observed in the corresponding loss-of-function mutant plants.

The transfer of substrates between to enzymes within a biosynthesis pathway is an effective way to synthesize the specific product and a good way to avoid metabolic interference. This process is called metabolic channeling and it describes the (in-)direct transfer of an intermediate molecule between the active sites of two enzymes. By forming multi-enzyme cascades the efficiency of product formation and the flux is elevated and intermediate products are transferred and converted in a correct manner by the enzymes. During tetrapyrrole biosynthesis several substrate transfer events occur and are prerequisite for an optimal pigment synthesis. In this project the metabolic channeling process during the pink pigment phycoerythrobilin (PEB) was investigated. The responsible ferredoxin-dependent bilin reductases (FDBR) for PEB formation are PebA and PebB. During the pigment synthesis the intermediate molecule 15,16-dihydrobiliverdin (DHBV) is formed and transferred from PebA to PebB. While in earlier studies a metabolic channeling of DHBV was postulated, this work revealed new insights into the requirements of this protein-protein interaction. It became clear, that the most important requirement for the PebA/PebB interaction is based on the affinity to their substrate/product DHBV. The already high affinity of both enzymes to each other is enhanced in the presence of DHBV in the binding pocket of PebA which leads to a rapid transfer to the subsequent enzyme PebB. DHBV is a labile molecule and needs to be rapidly channeled in order to get correctly further reduced to PEB. Fluorescence titration experiments and transfer assays confirmed the enhancement effect of DHBV for its own transfer. More insights became clear by creating an active fusion protein of PebA and PebB and comparing its reaction mechanism with standard FDBRs. This fusion protein was able to convert biliverdin IXα (BV IXα) to PEB similar to the PebS activity, which also can convert BV IXα via DHBV to PEB as a single enzyme. The product and intermediate of the reaction were identified via HPLC and UV-Vis spectroscopy. The results of this work revealed that PebA and PebB interact via a proximity channeling process where the intermediate DHBV plays an important role for the interaction. It also highlights the importance of substrate channeling in the synthesis of PEB to optimize the flux of intermediates through this metabolic pathway.

Herbivory is discussed as a key agent in maintaining dynamics and stability of tropical forested ecosystems. Accordingly increasing attention has been paid to the factors that structure tropical herbivore communities. The aim of this study was (1) to describe diversity, density, distribution and host range of the phasmid community (Phasmatodea) of a moist neotropical forest in Panamá, and (2) to experimentally assess bottom-up and top-down factors that may regulate populations of the phasmid Metriophasma diocles. The phasmid community of Barro Colorado Island was poor in species and low in density. Phasmids mainly occurred along forest edges and restricted host ranges of phasmid species reflected the successional status of their host plants. Only M. diocles that fed on early and late successional plants occurred regularly in the forest understory. A long generation time with a comparably low fecundity converted into a low biotic potential of M. diocles. However, modeled potential population density increased exponentially and exceeded the realized densities of this species already after one generation indicating that control factors continuously affect M. diocles natural populations. Egg hatching failure decreased potential population growth by 10 % but was of no marked effect at larger temporal scale. Interspecific differences in defensive physical and chemical leaf traits of M. diocles host plants, amongst them leaf toughness the supposedly most effective anti-herbivore defense, seemed not to affect adult female preference and nymph performance. Alternatively to these defenses, I suggest that the pattern of differential preference and performance may be based on interspecific differences in qualitative toxic compounds or in nutritive quality of leaves. The significant rejection of leaf tissue with a low artificial increase of natural phenol contents by nymphs indicated a qualitative defensive pathway in Piper evolution. In M. diocles, oviposition may not be linked to nymph performance, because the evolutionary prediction of a relation between female adult preference and nymph performance was missing. Consequently, the recruitment of nymphs into the reproductive adult phase may be crucially affected by differential performance of nymphs. Neonate M. diocles nymphs suffered strong predation pressure when exposed to natural levels of predation. Concluding from significantly increased predation-related mortality at night, I argue that arthropods may be the main predators of this nocturnal herbivore. Migratory behavior of nymphs seemed not to reflect predation avoidance. Instead, I provided first evidence that host plant quality may trigger off-plant migration. In conclusion, I suggest that predation pressure with its direct effects on nymph survival may be a stronger factor regulating M. diocles populations, compared to direct and indirect effects of host plant quality, particularly because slow growth and off-host migration both may feed back into an increase of predation related mortality.

Human forest modification is among the largest global drivers of terrestrial degradation of biodiversity, species interactions, and ecosystem functioning. One of the most pertinent components, forest fragmentation, has a long history in ecological research across the globe, particularly in lower latitudes. However, we still know little how fragmentation shapes temperate ecosystems, irrespective of the ancient status quo of European deforestation. Furthermore, its interaction with another pivotal component of European forests, silvicultural management, are practically unexplored. Hence, answering the question how anthropogenic modification of temperate forests affects fundamental components of forest ecosystems is essential basic research that has been neglected thus far. Most basal ecosystem elements are plants and their insect herbivores, as they form the energetic basis of the tropic pyramid. Furthermore, their respective biodiversity, functional traits, and the networks of interactions they establish are key for a multitude of ecosystem functions, not least ecosystem stability. Hence, the thesis at hand aimed to disentangle this complex system of interdependencies of human impacts, biodiversity, species traits and inter-species interactions. The first step lay in understanding how woody plant assemblages are shaped by human forest modification. For this purpose, field investigations in 57 plots in the hyperfragmented cultural landscape of the Northern Palatinate highlands (SW Germany) were conducted, censusing > 4,000 tree/shrub individuals from 34 species. Use of novel, integrative indices for different types of land-use allowed an accurate quantification of biotic responses. Intriguingly, woody tree/shrub communities reacted strikingly positive to forest fragmentation, with increases in alpha and beta diversity, as well as proliferation of heat/drought/light adapted pioneer species. Contrarily, managed interior forests were homogenized/constrained in biodiversity, with dominance of shade/cold adapted commercial tree species. Comparisons with recently unmanaged stands (> 40 a) revealed first indications for nascent conversion to oldgrowth conditions, with larger variability in light conditions and subsequent community composition. Reactions to microclimatic conditions, the relationship between associated species traits and the corresponding species pool, as well as facilitative/constraining effects by foresters were discussed as underlying mechanisms. Reactions of herbivore assemblages to forest fragmentation and the subsequent changes in host plant communities were assessed by comprehensive sampling of > 1,000 live herbivores from 134 species in the forest understory. Diversity was – similarly to plant communities - higher in fragmentation affected habitats, particularly in edges of continuous control forests. Furthermore, average trophic specialization showed an identical pattern. Mechanistically, benefits from microclimatic conditions, host availability, as well as pronounced niche differentiation are deemed responsible. While communities were heterogeneous, with no segregation across habitats, (smallforest fragments, edges, and interior of control forests), vegetation diversity, herbivore diversity, as well as trophic specialization were identified to shape community composition. This probably reflected a gradient from generalistic/species poor vs. specialist/species rich herbivore assemblages. Insect studies conducted in forest systems are doomed to incompleteness without considering ‘the last biological frontier’, the tree canopies. To access their biodiversity, relationship to edge effects, and their conservational value, the arboricolous arthropod fauna of 24 beech (Fagus sylvatica) canopies was sampled via insecticidal knockdown (‘fogging’). This resulted in an exhaustive collection of > 46,000 specimens from 24 major taxonomic/functional groups. Abundance distributions were markedly negative exponential, indicating high abundance variability in tree crowns. Individuals of six pertinent orders were identified to species level, returning > 3,100 individuals from 175 species and 52 families. This high diversity did marginally differ across habitats, with slightly higher species richness in edge canopies. However, communities in edge crowns were noticeably more heterogeneous than those in the forest interior, possibly due to higher variability in environmental edge conditions. In total, 49 species with protective value were identified, of which only one showed habitat preferences (for near-natural interior forests). Among them, six species (all beetles, Coleoptera) were classified as ‘priority species’ for conservation efforts. Hence, beech canopies of the Northern Palatinate highlands can be considered strongholds of insect biodiversity, incorporating many species of particular protective value. The intricacy of plant-herbivore interaction networks and their relationship to forest fragmentation is largely unexplored, particularly in Central Europe. Illumination of this matter is all the more important, as ecological networks are highly relevant for ecosystem stability, particularly in the face of additional anthropogenic disturbances, such as climate change. Hence, plant-herbivore interaction networks (PHNs) were constructed from woody plants and their associated herbivores, sampled alive in the understory. Herbivory verification was achieved using no-choice-feeding assays, as well as literature references. In total, networks across small forest fragments, edges, and the forest interior consisted of 696 interactions. Network complexity and trophic niche redundancy were compared across habitats using a rarefaction-like resampling procedure. PHNs in fragmentation affected forest habitats were significantly more complex, as well as more redundant in their realized niches, despite being composed of relatively more specialist species. Furthermore, network robustness to climate change was quantified utilizing four different scenarios for climate change susceptibility of involved plants. In this procedure, remaining herbivores in the network were measured upon successive loss of their host plant species. Consistently, PHNs in edges (and to a smaller degree in small fragments) withstood primary extinction of plant species longer, making them more robust. This was attributed to the high prevalence of heat/drought-adapted species, as well as to beneficial effects of network topography (complexity and redundancy). Consequently, strong correlative relationships were found between realized niche redundancy and climate change robustness of PHNs. This was both the first time that biologically realistic extinctions (instead of e.g.random extinctions) were used to measure network robustness, and that topographical network parameters were identified as potential indicators for network robustness against climate change. In synthesis, in the light of global biotic degradation due to human forest modification, the necessity to differentiate must be claimed. Ecosystems react differently to anthropogenic disturbances, and it seems the particular features present in Central European forests (ancient deforestation, extensive management, and, most importantly, high richness in open-forest plant species) cause partly opposed patterns to other biomes. Lenient microclimates and diverse plant communities facilitate equally diverse herbivore assemblages, and hence complex and robust networks, opposed to the forest interior. Therefore, in the reality of extensively used cultural landscapes, fragmentation affected forest ecosystems, particularly forest edges, can be perceived as reservoir for biodiversity, and ecosystem functionality. Nevertheless, as practically all forest habitats considered in this thesis are under human cultivation, recommendations for ecological enhancement of all forest habitats are discussed.

Regulation of metabolism is complex and involves enzymes and membrane transporters, which form networks to support energy dynamics. Lactate, as a metabolic intermediate from glucose or glycogen breakdown, appears to play a major role as additional energetic substrate, which is shuttled between glycolytic and oxidative cells, both under hypoxic and normoxic conditions. Transport of lactate across the cell membrane is mediated by monocarboxylate transporters (MCTs) in cotransport with H+, which is a substrate, a signal and a modulator of metabolic processes. MCTs form a “transport metabolon” with carbonic anhydrases (CAs), which not only provide a rapid equilibrium between CO2, HCO3– and H+, but, in addition, enhances lactate transport, as found in Xenopus oocytes, employed as heterologous expression system, as well as in astrocytes and cancer cells. Functional interactions between different CA isoforms and MCTs have been found to be isoform-specific, independent of the enzyme’s catalytic activity, and they require physical interaction between the proteins. CAs mediate between different states of metabolic acidosis, induced by glycolysis and oxidative phosphorylation, and play a relay function in coupling pH regulation and metabolism. In the brain, metabolic processes in astrocytes appear to be linked to bicarbonate transport and to neuronal activity. Here, we focus on physiological processes of energy dynamics in astrocytes as well as on the transfer of energetic substrates to neurons.