Metatranscriptomics sheds light on “who is doing what” in the coral Porites lutea

Global decline of coral reefs due to climate change calls for effective, nature-based strategies to protect these crucial ecosystems. Developing such strategies requires a thorough understanding of the complex roles and interactions of key inhabitants within coral holobionts. Using a metatranscriptomics approach, we investigated the transcriptionally active microbiome of the healthy stony coral Porites lutea , focusing on the coral tissue, the green endolithic algal layer ( Ostreobiu m layer), and the deeper coral skeleton. We show distinct, metabolically active communities within these compartments and highlight substantial metabolic redundancy across carbon, nitrogen, and sulphur pathways. Our study provides the first transcriptomic evidence of the ability of Ostreobium to transfer fixed carbon to other members of the holobiont and the coral host. Additionally, we identified critical roles of diverse coral holobiont members in nutrient cycling and maintaining homeostasis through scavenging of reactive oxygen and nitrogen species. This study provides novel molecular-level understanding of the functional roles played by diverse coral holobiont members in their respective compartments and underscores that corals harbour several distinct microbiomes with a wide array of functions.