RT Journal Article SR Electronic T1 Coral symbiotic algae calcify ex hospite in partnership with bacteria JF Proceedings of the National Academy of Sciences JO Proc Natl Acad Sci USA FD National Academy of Sciences SP 6158 OP 6163 DO 10.1073/pnas.1420991112 VO 112 IS 19 A1 Frommlet, Jörg C. A1 Sousa, Maria L. A1 Alves, Artur A1 Vieira, Sandra I. A1 Suggett, David J. A1 Serôdio, João YR 2015 UL http://www.pnas.org/content/112/19/6158.abstract AB The dinoflagellate genus Symbiodinium is best known for harboring important endosymbiotic algae of marine invertebrates, notably reef-building corals. However, these dinoflagellates also live freely within coral reef waters and sediments and provide an important environmental pool for the colonization of new coral recruits. Although Symbiodinium facilitate coral calcification indirectly when in hospite, we show that they also can calcify in partnership with bacteria when free living. This discovery offers entirely new perspectives on fundamental questions regarding the life cycle and ecology of these dinoflagellates and could help explain how changes in ocean chemistry created a selective pressure that ultimately led Symbiodinium to establish an endosymbiotic life style. To our knowledge, our findings document the first identified dinoflagellate–bacterial calcifying community.Dinoflagellates of the genus Symbiodinium are commonly recognized as invertebrate endosymbionts that are of central importance for the functioning of coral reef ecosystems. However, the endosymbiotic phase within Symbiodinium life history is inherently tied to a more cryptic free-living (ex hospite) phase that remains largely unexplored. Here we show that free-living Symbiodinium spp. in culture commonly form calcifying bacterial–algal communities that produce aragonitic spherulites and encase the dinoflagellates as endolithic cells. This process is driven by Symbiodinium photosynthesis but occurs only in partnership with bacteria. Our findings not only place dinoflagellates on the map of microbial–algal organomineralization processes but also point toward an endolithic phase in the Symbiodinium life history, a phenomenon that may provide new perspectives on the biology and ecology of Symbiodinium spp. and the evolutionary history of the coral–dinoflagellate symbiosis.