School of BioSciences - Theses
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ItemCoral-associated bacterial communities in early coral life stages: transmission mode and scope for manipulation( 2019)Global impacts of climate change and other anthropogenic disturbances are causing massive declines in coral reef ecosystems. As reef-forming scleractinian corals provide essential resources to a large part of the population, their degradation has severe ecological, social and economic consequences. Efforts are therefore urgently required to limit human impacts (e.g., by drastically reducing emissions of greenhouse gases), and also to assist coral adaptation to climate change. In this context, favourably adjusting coral-associated microbial communities could potentially benefit the host, as microbial symbionts are known to play critical roles in coral health. Probiotics have already proven effective in other organisms such as plants to increase crop yields, and humans to treat various bowel conditions. The successful application of probiotics in corals is contingent on the feasibility to manipulate the coral microbiome. Therefore, a central question of this thesis is whether the coral microbiome can be influenced by targeted bacterial inoculation in the laboratory. Initially, understanding how corals acquire and maintain their bacterial associates will assist in predicting whether probiotics could be taken up and retained across generations. The transmission mode of bacteria in corals with different reproductive strategies therefore constitutes an additional focus of this thesis. I start the thesis by providing information on the ecological importance of coral reefs, the threats that they are currently facing in the context of climate change and approaches that have been proposed to accelerate the adaptation of corals to environmental disturbances (Chapters 1 and 2). Within that scope, I focus on coral-associated microorganisms, highlight their roles for coral health and discuss the potential of microbial biotechnology to mitigate coral reef degradation. Chapter 2 includes experimental data providing proof-of-concept that the bacterial microbiome of juvenile corals can be influenced by exposing coral larvae to the mucus (which is known to contain a diverse microbiome) extracted from adult colonies of different coral species (Chapter 2). In Chapters 3 and 4, I investigate the transmission mode of coral-associated bacteria using 16S rRNA gene metabarcoding and fluorescence in situ hybridisation (FISH) microscopy. No evidence of direct vertical transmission of internalised bacteria was observed in the broadcast spawner Acropora tenuis (Chapter 3). However, metabarcoding shows that the gametes were already associated with diverse bacteria upon release and that early coral life stages successively associated with different bacterial communities, probably acquired from the environment. It is possible that the coral-associated mucus is removed during the FISH fixation procedure and therefore bacteria in the mucus would not be visualised by FISH microscopy. Parental colonies may thus drive the transfer of certain bacteria by releasing them to the water column upon spawning (horizontal transmission) and/or through the mucus layer coating the gametes while these are still inside the coral polyp (vertical transmission). Clear evidence for vertical transmission is present in the brooder, Pocillopora acuta, where newly released larvae contained internalised bacterial aggregates (Chapter 4). DNA metabarcoding provides evidence for vertical transmission as well as horizontal uptake of bacteria in this coral species. In Chapters 5 and 6, I explore the possibility of manipulating coral-associated bacterial communities by exposing coral recruits to fragments of adult corals (Chapter 5), as well as to a cocktail comprising a small number of pure bacterial cultures (Chapter 6). In the former approach, rearing P. acuta coral recruits in the vicinity of adult fragments of P. acuta and Platygyra daedalea under a flow-through system did not result in different bacterial associates developing in juveniles compared to control corals. The temporal changes of bacterial assemblages in early recruits suggest that their microbiome is dynamic, which may facilitate uptake of bacterial strains in an inoculum but also challenge their retention over time. In Chapter 6, I rear A. tenuis and P. daedalea recruits in the same aquaria and repeatedly inoculate them with a bacterial consortium generated in the laboratory. The seven bacterial strains present in the inoculum were enriched in inoculated recruits of the two coral species compared to the no-inoculum controls, and there is a significant effect of the inoculum on the coral-associated bacterial communities. Additionally, some of the structuring in the bacterial microbiomes is explained by host species, highlighting the role of host factors in shaping bacterial community composition. These results support proof-of- concept for the feasibility of coral microbiome manipulation as a first step towards developing probiotics aimed at enhancing coral climate resilience. In Chapter 7, the general discussion, characteristics of microbial inoculation strategies that could be implemented to corals are discussed, as well as their advantages and shortcomings. Emphasis is also drawn to current knowledge gaps and research priorities for the field of microbiome engineering in corals.