Environmental impact on giant ciliate mutualism

Mutualism between microbial symbionts and eukaryote hosts is ubiquitous. One of the most exiting mutualisms is the association between a sulfur bacterium, called Candidatus Thiobios zoothamnicoli, and its host Zoothamnium niveum. These giant colonial ciliates live on submerged wood in marine shallow, tropical and temperate waters. As of yet, this is the only association involving a sulfur bacterium that can be cultivated. We could show that fitness of both partners is coupled and depends on the concentration of hydrogen sulfide. This chemical is leaking from wood and is, together with oxygen, and carbon dioxide used by the symbiont to produce organic carbon and to share the food with the host. However, sulfide emission from the wood is not stable and therefore the production of food for both partners is interrupted. This lack of sulfide can jeopardize the functioning of association and the survival of the microbe and the ciliate depending on the time frame. When sulfide lacks for about two days, the host sends off little swarmers to find a new habitat, but the old colony dies and the symbiont leaves the host. When sulfide lacks every winter in temperature waters, such as the Adriatic Sea, the symbiosis dissapears but whether the host and the symbiont survive separately and then re- join again in spring or the host encysts with or without the symbiont is not known. Further, little in known on the time frame of wood degradation in the sea, especially in shallow waters with wood used to build boats and piers in harbors. Therefore, it is unknown when sulfide is emitted and how long this stage lasts for the ciliate to colonize. We plan to study the mechanisms that have evolved in this mutualism to deal with the notoriously instable environment over short, annual, and long time scales. We plan to study 1) the gene expression of both partners in experiments in the lab under different environmental conditions to understand the molecular machinery goverining host-microbe functioning and interactions. 2) We will search in the environment for cysts, hosts without the symbiont, and free- living symbionts to elucidate overwinter survival stragegies 3) We will initiate a long-time experiment and plan to deploy fresh wood in the sea to follow the degradation of wood and the colonization of the giant ciliate and closely related species. These experiments will serve to decipher the mechanisms that have evolved in both partners to live together in a cooperation.

Most wood that ends in the ocean comes from land, either naturally by being transported through rivers or through human activities building wooden boats and harbor piers. Once in the ocean, usually it does not take too long to be degraded. We studied in this project the animals and microbes degrading and colonizing artificially deployed wood in the Northern Adriatic Sea over a period of three years. Following the colonization of wood and the succession of animal and microbial communities we found a high diversity of life for a brief period during wood degradation. Next to finding more than 200 taxa of animals and numerous microbes living on and in wood, only three shipworm, 2 gribble and one amphipod species were identified to directly feed on wood and therefore are, together with bacteria and fungi, the main wood degraders in the Northern Adriatic Sea. The shipworms do so with the help of symbiotic bacteria in their gills. In addition to this type of symbioses, we studied in detail another type of symbiosis, the sulfur-oxidizing chemoautotrophic association between a colonial ciliate Zoothamnium niveum and its symbiont Candidatus Thiobius zoothamnicola, that required the toxic chemical hydrogen sulfide to produce organic carbon and shares its food with the host. The genome of the symbiont was characterized during this project and revealed many pathways to utilize reduced sulfur species and store sulfur for the time when sulfide is not present. Therefore, we studied how this symbiosis reacts when sulfide emission is unstable over short and long-time scales. We characterized the chemical conditions using microsensor technology. We found that when sulfide lacks the association breaks down due to host death, microbial fouling and loss of symbionts. The offspring of the ciliates, the swarmers, however, can lose their symbionts but do not die. Instead, they can settle and grow into new colonies without their symbiont. When they do so they look very different in shape to the colonies associated with their symbionts. This phenomenon, called polyphenism, is well known from parasitic or predator-prey relationships but was unknown so far for beneficial symbioses. Currently progressing on the ciliate genome and gene expression pattern analyses of both partners and the ciliate without its symbiont this will further help to understand their ecology and evolution.