Wednesday May 3, 2023, 5 pm CEST
Jörg C. Frommle*
Evidence for an autoendolithic life history of free-living symbiodiniaceans
*Department of Biology and Center for Environmental and Marine Studies, University of Aveiro, 3810-193 Aveiro, Portugal
Dinoflagellates from the family Symbiodiniaceae engage in trophic endosymbioses with corals and various other marine hosts and play a pivotal role in sustaining the productivity and diversity of coral reef ecosystems. Yet, parallel to evolving symbiotic life histories, many symbiodiniaceans also preserved the ability to live as free-living cells outside of their hosts. The existence of these free-living populations and their importance as environmental pool from which hosts can acquire symbionts has been known for decades. Benthic habitats, especially reef sands, appear to be hotspots for free-living symbiodiniaceans, but their benthic life history has remained elusive.
A surprising new lead in the study of benthic symbiodiniaceans emerged when we discovered that in culture these dinoflagellates commonly form calcifying bacterial-algal biofilms that precipitate sand grain-like deposits and encase symbiodiniaceans as viable endolithic cells. The formation of these microbialites, we termed symbiolites, is induced by photosynthesis within the biofilm microenvironment, qualifying symbiodiniaceans as autoendoliths, i.e., organisms that actively construct their endolithic habitat through mineral precipitation. Endolithic symbiodiniaceans remain alive and photosynthetically active for weeks and upon a re-supply of nutrients they can partially dissolve the mineral and vacate the symbiolite. Because of this reversibility of the autoendolithic process, symbiolite formation does not appear to be a dead end for endolithic cells but instead points toward a transient autoendolithic phase in the life history of benthic symbiodiniaceans. Direct support for this hypothesis since emerged in form of diverse endolithic symbiodiniacean communities that we discovered across the globe.
In this seminar, I will give an overview of our work on this novel bacterial-algal calcification process, starting with its discovery in culture; over mechanistic and functional studies of the calcification process; to our latest field-based data. Further, I will discuss the potential function of creating an endolithic life stage; how endolithic populations could play an important role in the uptake of symbionts by corals; and what fate this life stage could have in the face of ocean acidification.