Macroalgae proliferations in tropical and subtropical coastal ecosystems represent a serious threat to the biologic diversity of these environments. In coral reefs, where the coral-algal competition can modify the ecosystem structure and functioning as well as the reef-scape, these interactions are particularly problematic. The macroalgae proliferation effects can be detrimental to the coral reef related ecosystem services and goods for several human populations depending on coastal ecosystems. The origin and mechanisms of these proliferations are poorly studied, and our knowledge of the number of the species involved is scanty. Hence, we proposed to address this subject through the case study of proliferations of the red algae (Rhodophyta) of the genus Asparagopsis, enlisted by IUCN among the introduced and invasive species in subtropical and temperate areas. Their status is still largely unknown (e.g., taxonomy, ecosystem effects, origin of the proliferations) and a global and multidisciplinary study is needed to understand and manage their proliferations.The ERA-NET NETBIOME programme provides the opportunity to submit an interdisciplinary research programme, involving French and Portuguese teams specialised in tropical and sub-tropical marine ecosystems. The eight partners, organised in a consortium, aim at establishing a study platform at the taxon distribution scale. A multi-scale approach will be put in place in two hemispheres and three oceans (the Portuguese islands of the Azores and of Madeira, La Réunion Island, New Caledonia, French Polynesia, Guadeloupe, and the Mediterranean). The first objective is to (i) assess the proliferation of algae populations in relation to the habitat and different environmental parameters, (ii) set up a reference list of the benthic organisms, cnidarians in particular, affected by the algal proliferations; (iii) monitor at certain study sites the spatio-temporal evolution of the algae populations. Results will be gathered in an online database which will allow the mapping of knowledge across three oceans.The second objective is to draw the profile of individuals from all the study areas using three complementary approaches: (i) a molecular taxonomy and phylogeography approach to identify the clades occurring in the study area (Asparagopsis sp. being a species complex) and determine the native vs introduced status of proliferating populations; (ii) a metabolomic approach to test the relevance of chemical signatures to differentiate taxa/clades (chemotaxonomic approach); (iii) an evaluation of the composition of microbial communities associated with algae using new-generation sequencing type 454 (e.g. community genomics). The combination of these different approaches is uncommon, particularly in marine systems. Such integrated approach is ambitious but feasible, and is one of the scientific challenges of the proposed study. For instance, it will be used to test the hypothesis that algal proliferation and their effects on cnidarians may be at least partially explained by the taxonomic/metabolomic/microbial identity. The third objective is to test experimentally, in situ and in the laboratory, the harmful nature of Asparagopsis and the ecological effects of its proliferation in cnidarians dominated benthic assemblages. We propose to examine the natural toxicity of algal species on coral health (holobiont) and its control by the action of herbivores. Asparagopsis bioactivity will be used as a proxy for the production of biologically active substances. Recorded bioactivities will be analyzed in light of different metabolic phenotypes defined elsewhere, and a potential link with the genetic profiles of algae and associated microbial communities will be sought. The expected results will help to fill the gaps on the status of the genus Asparagopsis at the biogeographic scale, to better understand the proliferation in different regions, to document interactions with cnidarians and assess the impacts in or