  Host-microbe interactions play crucial roles in marine ecosystems, but we
still have very little understanding of the mechanisms that govern these
relationships, the evolutionary processes that shape them, and their ecological
consequences. The holobiont concept is a renewed paradigm in biology that can
help describe and understand these complex systems. It posits that a host and
its associated microbiota, living together in a long-lasting relationship, form
the holobiont, and have to be studied together, as a coherent biological and
functional unit, in order to understand the biology, ecology and evolution of
the organisms. Here we discuss critical concepts and opportunities in marine
holobiont research and identify key challenges in the field. We highlight the
potential economic, sociological, and environmental impacts of the holobiont
concept in marine biological, evolutionary, and environmental sciences with
comparisons to terrestrial science whenever appropriate. A deeper understanding
of such complex systems, however, will require further technological and
conceptual advances. The most significant challenge will be to bridge
functional research on simple and tractable model systems and global
approaches. This will require scientists to work together as an (inter)active
community in order to address, for instance, ecological and evolutionary
questions and the roles of holobionts in biogeochemical cycles.
