On the footprints of an invasion: : the genetic paradox in the green algae Caulerpa taxifolia, a drifting hotspot of microbial diversity

The successful prevention and management of invasions require the identification of the very initial steps of dispersal and pathways of introduction, and the understanding of key elements in the metabolism of invasive species facilitating their sustainable settlement over a wide spectrum of environments. The green algae Caulerpa taxifolia native from Northern Australia has been introduced and characterized in a large range of geographic areas worldwide, where its ability to reproduce clonally and cope with environmental conditions deleterious to most other species may contribute to its success. The diversity of bacterial community associated to this algae and the putative metabolic function of some OTU might also have played a role. Here a strikingly congruent pattern of diversity of the algae genome and associated microbial communities emerged from their comparative characterization, revealing a tight symbiotic association of the eukaryotic host and part of its prokaryotic communities. Diversity patterns support a founding effect associated to an accidental introduction in the Mediterranean area while suggesting different origins and multiple events of dispersal in South Australia. These results might be related to the recent decline of the species in the Mediterranean, whereas its spread in Australia seems nowadays more acute. Besides the potential influence of eukaryotes adaptability, results reported here demonstrate that part of their bacterial communities can accompany their journey and facilitate their spread. The most representative OTU of the endophytic communities were assigned to nitrogen-fixing bacteria able to perform in anaerobic conditions, fitting the known sediment upheaval known to occur in locations invaded by Caulerpa sp. As a result, they can be effectively used as an element to trace introduction origin and reconstruct dispersal pathways, and seriously considered as a potential active player of the invasive trajectory of their meta-organism they are a component of. On the basis of those results we therefore advocate the study of the meta-organism including bacterial communities, rather than the eukaryotic species alone, as a key element for a comprehensive understanding and effective management of biological invasions.