Although covering only 0.2% of the ocean’s surface, coral reefs harbour 25% of ocean biodiversity and provide food to nearly a billion people. Ecological services from coral reefs are essential through fisheries, tourism, coastal protection and are estimated at about 30 billion USD per year. But corals are being stressed and recent estimates indicate that 20% of reefs have permanently disappeared and about 50% will be threatened by 2050. Coral reefs have often been at the forefront of research on climate change, due to bleaching, ocean acidification, and concerns about reef growth processes. This project entitled, “Genomic complexity of the coral holobiont across biodiversity gradients in the Pacific” is ambitious and seeks to investigate the complex diversity of the coral holobiont within the context of global change. Recent genomic developments have demonstrated the complexity of the coral genome that appears as complex as that for vertebrates. Reef coralsfurther add to this complexity through an obligatory photo-symbiosis developed with microalgae. The physiological consequences of the presence of these photosynthetic microalgae (called zooxanthellae), which further add to the complexity of the coral genome, and the deep physio-genomic impacts resulting from this symbiosis on both partners, have yet to be fully elucidated. Furthermore, corals are hosts of a still largely unknown world of associated bacteria, viruses and others protists, forming a complex symbiosis that biologists refer to as ‘holobiont’. This project will serve as the foundation for the new Tara Pacific expedition (2016-2018). It builds upon the experience of previous Tara-Oceans expeditions and will focus on coral reefs throughout the Pacific Ocean, drawing an east-west transect from Panama to Japan and a south-north transect from New Zealand to Japan, and will sample corals throughout roughly 40 island systems with local replicates. Tara-Pacific will develop and apply state-of-the-art technologies in very-high-throughput genetic sequencing to reveal the entire microbial diversity (i.e. full biological complexity) present within coral holobionts. The Tara-Pacific brings together a consortium of international experts in marine biology and ecology, cellular and molecular biology, genomics, and bioinformatics. Though this diverse team of global coral reef experts, we will have the expertise needed to build a comprehensive morpho-molecular inventory of the biodiversity of the coral holobiont, from viruses to prokaryotes, unicellular eukaryotes, and metazoans, which will include the biodiversity from both interstitial and surrounding sea waters. This very ambitious project will reveal a massive amount of cryptic and novel biodiversity, will shed light on the complex links between genomes, transcriptomes, metabolomes, organisms, and ecosystem functions in coral reefs, and will provide a reference of the biological state of modern coral reefs for the large research community dealing with coral adaptation to global and regional stressor.