ALBERT

Description: The recognition that all macroorganisms live in symbiotic association with microbial communities has opened up a new field in biology. Animals, plants, and algae are now considered holobionts, complex ecosystems consisting of the host, the microbiota, and the interactions among them. Accordingly, ecological concepts can be applied to understand the host-derived and microbial processes that govern the dynamics of the interactive networks within the holobiont. In marine systems, holobionts are further integrated into larger and more complex communities and ecosystems, a concept referred to as “nested ecosystems.” In this review, we discuss the concept of holobionts as dynamic ecosystems that interact at multiple scales and respond to environmental change. We focus on the symbiosis of sponges with their microbial communities—a symbiosis that has resulted in one of the most diverse and complex holobionts in the marine environment. In recent years, the field of sponge microbiology has remarkably advanced in terms of curated databases, standardized protocols, and information on the functions of the microbiota. Like a Russian doll, these microbial processes are translated into sponge holobiont functions that impact the surrounding ecosystem. For example, the sponge-associated microbial metabolisms, fueled by the high filtering capacity of the sponge host, substantially affect the biogeochemical cycling of key nutrients like carbon, nitrogen, and phosphorous. Since sponge holobionts are increasingly threatened by anthropogenic stressors that jeopardize the stability of the holobiont ecosystem, we discuss the link between environmental perturbations, dysbiosis, and sponge diseases. Experimental studies suggest that the microbial community composition is tightly linked to holobiont health, but whether dysbiosis is a cause or a consequence of holobiont collapse remains unresolved. Moreover, the potential role of the microbiome in mediating the capacity for holobionts to acclimate and adapt to environmental change is unknown. Future studies should aim to identify the mechanisms underlying holobiont dynamics at multiple scales, from the microbiome to the ecosystem, and develop management strategies to preserve the key functions provided by the sponge holobiont in our present and future oceans.

Description: The reliable production of marine fish larvae is one of the major bottlenecks in aquaculture due to high mortalities mainly caused by infectious diseases. To evaluate if the compound poly-β-hydroxybutyrate (PHB) might be a suitable immunoprophylactic measure in fish larviculture, its capacity to improve immunity and performance in European sea bass (Dicentrarchus labrax) yolk-sac larvae was explored. PHB was applied from mouth opening onwards to stimulate the developing larval immune system at the earliest possible point in time. Larval survival, growth, microbiota composition, gene expression profiles and disease resistance were assessed. PHB administration improved larval survival and, furthermore, altered the larva-associated microbiota composition. The bacterial challenge test using pathogenic Vibrio anguillarum revealed that the larval disease resistance was not influenced by PHB. The expression profiles of 26 genes involved e.g. in the immune response showed that PHB affected the expression of the antimicrobial peptides ferritin (fer) and dicentracin (dic), however, the response to PHB was inconsistent and weaker than previously demonstrated for sea bass post-larvae. Hence, the present study highlights the need for more research focusing on the immunostimulation of different early developmental stages for gaining a more comprehensive picture and advancing a sustainable production of high quality fry.

Description: The stable production of high-quality fry in marine aquaculture is still hampered by unpredictable mortality caused by infectious diseases during larval rearing. Consequently, the development of new biocontrol agents is crucial for a viable aquaculture industry. The bacterial energy storage compound poly-β-hydroxybutyrate (PHB) has been shown to exhibit beneficial properties on aquatic organisms such as enhanced survival, growth, disease resistance and a controlling effect on the gastrointestinal microbiota. However, the effect of PHB on the developing immune system of fish larvae has so far not been investigated. In this study, the effect of feeding PHB-enriched Artemia nauplii on survival, growth and immune response in European sea bass (Dicentrarchus labrax) postlarvae was examined. Amorphous PHB was administered to 28-day-old sea bass postlarvae over a period of 10 days. The survival and growth performance were monitored, and the expression of 29 genes involved in immunity, growth, metabolism and stress-response was measured. While the expression of the insulin-like growth factor 1 (igf1), an indicator of relative growth, was upregulated in response to feeding PHB, the larval survival and growth performance remained unaffected. After 10 days of PHB treatment, the expression of the antimicrobial peptides dicentracin (dic) and hepcidin (hep) as well as mhc class IIa and mhc class IIb was elevated in the PHB fed postlarvae. This indicates that PHB is capable of stimulating the immune system of fish early life stages, which may be the cause of the increased resistance to diseases and robustness observed in previous studies.

Description: Introduction Various probiotics and immunostimulants have been shown to enhance the immune response and alter the disease resistance of aquaculture organisms. The bacterial energy storage compound poly-ß-hydroxybutyrate (PHB) for example improves resistance against pathogenic infections in shrimp (Laranja et al., 2014) and exhibits a controlling effect on the gut microbiota of juvenile sea bass which may result in the stimulation of immune functions (De Schryver et al., 2011). These properties might be of special importance for culturing early life stages since their immune system is not yet fully developed. Therefore, we assessed the potential immunostimulating effect of PHB in European sea bass (Dicentrarchus labrax) larvae in our study. Materials and methods We used rotifers as live carriers to feed PHB-accumulating bacteria (Alcaligenes eutrophus) to first-feeding larvae over a period of 14 days. Bacteria with a low (2.5%) and a high (75%) PHB content were used, respectively. Apart from the dose effect, we wanted to determine to which extend the point in time of the PHB administration matters. Therefore, PHB was added (in some experimental groups) directly to the water from the moment of mouth opening onwards. This led to the following experimental groups: 1) early and 2) later stimulation with a low PHB level, 3) early and 4) later stimulation with a high PHB level and 5) a control without any PHB. To estimate the immediate impact of PHB, larval mortality rates were monitored daily over the course of the experiment. Furthermore, larvae were sampled the first time after being fed for 3 days with PHB encapsulated in rotifers (and 5 days of PHB water treatment) and the second time after 14 days PHB via rotifers (and 16 days of PHB water treatment), respectively. Results and discussion Larval mortality rates were found to be the highest in the control group. During both sampling points larvae were slightly smaller and weighed less in the control group. The analysis of the gene expression profiles revealed that only certain immune genes such as cytokines (Interleukin-1ß, Interleukin 8 and TNFα) were affected by the PHB treatment. The data indicate that the application of PHB can provide a beneficial effect to sea bass larviculture in terms of higher survival rates. But further studies are required to verify the impact on the developing immune system of the larvae.

Description: Sponges—like all multicellular organisms—are holobionts, complex ecosystems comprising the host and its microbiota. The symbiosis of sponges with their microbial communities is a highly complex system, requiring interaction mechanisms and adaptation on both sides. The microbiome seems to rely on eukaryotic-like protein domains, such as ankyrins, modifications of the lipopolysaccharide structure, CRISPR-Cas, toxin-antitoxin, and restriction-modification systems, as well as secondary metabolism to communicate with the host and within the microbial community, evade phagocytosis, and defend itself against foreign DNA. Secondary metabolites produced by certain symbionts may even defend the entire holobiont against predators. On the other hand, the immune system of the sponge itself has evolved to discriminate not only between self and nonself but also between its associated microbiota and foreign microbes, such as food bacteria. Sponge holobionts are inextricably dependent on the surrounding environmental conditions due to their sessile nature. Thus, we discuss the link between environmental stress and sponge disease and dysbiosis, with a particular focus on the holobiont’s response to ongoing global change. While some species may be the “winners of climate change,” other species are adversely affected, e.g., by metabolic and immune suppression, as well as microbiome shifts resulting in loss of symbiotic functions. Hence, a much better understanding of sponge holobionts and the underlying molecular mechanisms of host-microbe interaction is required before the fate of sponge holobionts in a changing ocean can finally be validated.

Description: Global aquaculture production is growing rapidly, however, the industry is facing great challenges such as high mortality rates during early life stages. The lack of a fully mature immune system makes larvae highly sensitive to infectious diseases. One of the most common pathogenic bacteria in larviculture is Vibrio anguillarum causing worldwide severe economic losses. Due to the development of antibiotic resistances, the establishment of alternative methods to prevent and control diseases, ensure efficient growth and reach maximal survival rates is mandatory to optimize aquaculture productivity. A promising solution might be the early activation of the immature immune system of fish larvae by administration of immunostimulants as nutritional supplements. In our study we assessed the potential immunomodulatory effect of poly-ß-hydroxybutyrate (PHB) in European sea bass (Dicentrarchus labrax) larvae. PHB is a bacterial energy storage compound which may have a potential application as an immunostimulant in fish culture. In our experiment we used rotifers as live carriers to feed PHB-accumulating bacteria (Alcaligenes eutrophus) to first-feeding European sea bass larvae over a period of 14 days. To estimate the immediate impact of PHB, larval mortality rates were monitored daily during the course of the experiment. In order to assess the disease resistance of the larvae, survival rates after bath challenge with Vibrio anguillarum after 3 and 14 days of PHB treatment and 7 days post PHB treatment were monitored. Furthermore, we determined gene expression profiles for immune genes as well as metabolism- and stress-related genes. Results will be discussed in respect to the use of PHB in fish hatcheries and its effect on the immune system of first feeding sea bass.

Description: Background The interplay between hosts and their associated microbiome is now recognized as a fundamental basis of the ecology, evolution, and development of both players. These interdependencies inspired a new view of multicellular organisms as “metaorganisms.” The goal of the Collaborative Research Center “Origin and Function of Metaorganisms” is to understand why and how microbial communities form long-term associations with hosts from diverse taxonomic groups, ranging from sponges to humans in addition to plants. Methods In order to optimize the choice of analysis procedures, which may differ according to the host organism and question at hand, we systematically compared the two main technical approaches for profiling microbial communities, 16S rRNA gene amplicon and metagenomic shotgun sequencing across our panel of ten host taxa. This includes two commonly used 16S rRNA gene regions and two amplification procedures, thus totaling five different microbial profiles per host sample. Conclusion While 16S rRNA gene-based analyses are subject to much skepticism, we demonstrate that many aspects of bacterial community characterization are consistent across methods. The resulting insight facilitates the selection of appropriate methods across a wide range of host taxa. Overall, we recommend single- over multi-step amplification procedures, and although exceptions and trade-offs exist, the V3 V4 over the V1 V2 region of the 16S rRNA gene. Finally, by contrasting taxonomic and functional profiles and performing phylogenetic analysis, we provide important and novel insight into broad evolutionary patterns among metaorganisms, whereby the transition of animals from an aquatic to a terrestrial habitat marks a major event in the evolution of host-associated microbial composition.

Description: The reliable production of high quality juvenile fish is still hampered in marine aquaculture due to the enormous mortality in the larval stages, which is often caused by the outbreak of infectious diseases. Early life stages are extremely susceptible towards pathogens because they lack a mature immune system. Conventional disease prevention relies on the prophylactic use of antibiotics, selecting for antibiotic-resistant bacteria making treatments less and less efficient. Accordingly, the development of alternative health management strategies is one of the key factors for the progress towards a more sustainable aquaculture industry. A possibility to increase the disease resistance in early developmental stages is the stimulation of the immune system by steering the gastrointestinal microbiota. Even though it is recognized that a healthy gastrointestinal microbiota is a prerequisite for the host´s well-being, the mechanisms underlying the interdependency between gastrointestinal microbiota and immunity are not very well understood in fish. In my thesis, I explored different possibilities to stimulate the immune system in early life stages of European sea bass (Dicentrarchus labrax), a key species in European aquaculture. I investigated immunological processes on a cellular and molecular level as well as the larval microbiota, life history traits and disease resistance.