C3 Seminar: David Bourne

Corals and the reef ecosystems that they support are in global decline in the face of ever-increasing anthropogenic pressures, including climate change. The design of effective conservation strategies is compromised by our ignorance of the detailed composition of the coral holobiont and how its components interact. Whilst the central role of the symbiotic dinoflagellate Symbiodinium in coral metabolism is widely recognised, recent work has also implicated a range of prokaryotes in coral health as well as disease. Transcriptomic evidence suggests that the symbiotic algae may provide not only fixed carbon, but also some amino acids, to their coral hosts. It has frequently been suggested that some of the bacteria closely associated with corals are able to provide essential nutrients, for example, by fixing nitrogen, implying that the microbiome might be critical to holobiont function.

However, the true extent of metabolic integration between the various eukaryotic and prokaryotic components of the coral holobiont is unknown. In this talk I will first provide an update of the first coral hologenome, comprising the genomes of the coral host Porites lutea, its endogenous Symbiodinium strain C15 and detailed insights into the associated bacterial and archaeal communities through reconstruction of 52 population genomes. Analyses of these data are consistent not only with recycling of N, P and S within the system, but also with more complex metabolic interactions between the partners, and suggest that the bacteria may play critical roles in buffering the association against environmental variation. Anthropogenic stresses such as increased sea surface temperatures, nutrient input and sedimentation shift coral-microbiota associations, thereby contributing to reduced coral reef health. Coral diseases are on the rise with disease outbreaks contributing to significant loss of both key reef organisms and coral cover. In this talk I will also explore the microbial dynamics underlying two of the most prevalent coral diseases White Syndrome (WS) and Black Band Disease (BBD). While the causative agent(s) underlying WSs remains elusive, the microbial and cellular processes are beginning to identify and differentiate visually similar but potentially distinct WS etiologies. Black band disease (BBD) is a virulent, globally-distributed infectious disease characterised by a darkly pigmented polymicrobial band at the interface between apparently normal coral tissue and freshly exposed skeleton. Microbial taxonomic profiles for BBD have been extensively characterised, but the underlying microbial metabolic functions that contribute to anoxic and sulphide-rich microenvironments within the lesion and govern its pathogenicity are poorly characterised. Through combined metagenomic and metatranscriptomic profiling of BBD lesions, the development of BBD pathogenicity was linked to relative increases in production of sulfide by sulfate-reducing bacteria, and to photosynthetic CO2-fixation by cyanobacteria. The latter play a pivotal role in pathogenesis by introducing organic carbon in addition to nutrients derived from dead coral tissue.

Furthermore, heterotrophic bacterial degradation of organic carbon produced by cyanobacteria represents a key mechanism for the depletion of dissolved oxygen within BBD lesions to levels harmful to underlying coral tissue, especially in the absence of light. The increased mechanistic understanding of BBD development highlights the complexity of microbial interactions within the polymicrobial communities involved in the pathogenesis of BBD.

About the speaker

Dr David Bourne – Australian Institute of Marine Science (AIMS)

Dr David Bourne is a Senior Lecturer at James Cook University and a Principal Research Scientists at the Australian Institute of Marine Science (AIMS). His training is in the area of molecular microbial ecology with his research focused on investigation of microbial diversity, structure and function in complex ecosystems. His research is divided essentially into two areas, the first investigating the normal microbial communities associated with corals and their functional roles in maintaining coral fitness. The second research focus is to elucidate pathogens and mechanism of disease onset in corals and the implications this has on a stressed reef ecosystem in light of climate change being a major driver of coral reef degradation. Current active projects combine use of amplicon sequencing, metagenomic sequencing and transcriptomic sequencing approaches with advance imaging and chemical analyses to address coral holobiont and coral disease questions. He is also actively engaged in citizen science programs, currently running an Earthwatch project assessing the recovery of reefs around Orpheus Island on the GBR.

Light refreshments provided after talk - All are welcome!