Identifying a subsequent bleaching response in the Acropora genus through comparative transcriptomics

Abstract

Rising sea temperatures trigger coral bleaching, a process where corals expel vital symbiotic algae from their tissues, leading to reef degradation worldwide. Coral reefs, not only as a source of cultural, economic, and societal benefits, but also as a crucial ecosystem to fight climate change (due to a larger oxygen production in a fraction of space that a forest requires) constantly experience coral bleaching events worldwide with less time between such events, and more intensity in the response severity, as four mass bleaching events have occurred thus far, threatening to conduct many relevant coral species to mass extinction events, a particularly impactful truth for acroporid corals. This thesis addresses the critical need to classify and characterize coral bleaching process by depicting molecular signatures of this phenomenon in Acropora corals, a genus known as reef-builder that is crucial for healthy reef ecosystems due to structure formation and shelter provision in marine ecosystems. Although extensive research has explored the molecular mechanisms of coral bleaching through transcriptomic studies, characterization of the bleaching response remains a challenge in relation with variation in identified mechanisms and lack of integrative efforts between different studies, hindering the development of preventive solutions rather than curative ones. Firstly, we created a dataset with previously processed transcriptomic data from geographically diverse Acropora species using 40 samples from simulated heat stress experiments that corresponded to transcriptomic studies from adult coral colonies with reports of sampling time. Secondly, we analyzed chronological activation of transcriptome patterns through time series for control samples, thermal stress and bleaching in Acropora, aiming to identify conserved molecular signatures regardless of geographical variation. Results revealed that despite heterogeneity present in the dataset, subsequent gene expression responses were identified through functional analysis for both control and heatstressed scenarios with additional validation of time dependence through comparison with a bleaching group, nonetheless, species-specific expression was also identified with a relevant impact of the bacterial component of the coral holobiont. By classifying bleaching responses, we can pave the way for a more targeted intervention strategy to inhibit coral bleaching at a critical juncture defined by gene expression patterns, regardless of environmental variability. The present work could contribute to further management strategies for coral reefs in response to climate change with an informed perspective in molecular terms.