Symbiosis between bacteria and marine ciliates

The marine colonial ciliate Zoothamnium niveum (Hemprich & Ehrenberg, 1831) lives in symbiosis with sulfur- oxidizing chemolithoautotrophic bacteria which cover the entire surface of the colony. Since the redescription in 1996 from mangrove peat along a tidal channel in the Belize Barrier Reef system (Bauer-Nebelsick et al. 1996) populations of similar symbiotic colonies have been found on decaying plant material in the Mediterranean Sea. Z., niveum inhabits the sharpest oxygen/sulfide gradient of all known symbioses between sulfur bacteria and animals, including those described from deep sea hot vents. This gradient develops in the diffusive boundary layer on the surfaces where Z. niveum grows. Sulfide diffusing from sites of intensive sulfate reduction decreases over a few mm whereas oxygen supplied from the ambient free water increases. The contractile ciliate colony, which may extend 10-12 mm above the surface, is bathed in oxygen when expanded and is surrounded by sulfidic water, when contracted. It thus apparently supplies its symbionts alternatively with reduced sulfur compounds and oxygen as an electron acceptor. This project aims to map the microenvironment with high resolution around the colonies using microelectrodes for hydrogen sulfide and oxygen and follow the time course of the concentration of these dissolved gases during contraction and expansion of the colonies. Colonies appear to grow at discrete topographic features on the substrate surface, where sulfide flux is assumed to be high and will themselves manipulate the diffusive boundary layer due to their shape and behavior. According to morphological criteria Zoothamnium niveum appears to be closely related to the nonsymbiotic species Z. alternans, from which it differs (except for the possession of symbionts) mainly in size and growth speed. We have preliminary evidence, that aposymbiotic (symbiont-free) colonies of a Z. niveum-like species from the Mediterranean grows only to the size reported for Z. alternans. We therefore suggest, that the Z. niveum symbiosis is a very early stage in symbiosis evolution, where the host may be closely related and even identical with the non-symbiotic species. Differences between the descriptions of Z. alternans from localities on both sides of the Atlantic furthermore suggest, that there are several species involved, showing different degrees of dependence on associated bacteria. We will use molecular methods (RAPD, sequences of rRNA genes) to elucidate the systematic relationship of both ciliates and symbiotic bacteria from different localities, hoping to gain insight into early stages of symbiosis evolution.

Symbiosis is the coexistence of different species in which each plays an important role in the biology of the respective partner(s). In those cases in which the relationship is beneficial for all species involved this is called "mutualism". In this project the symbiosis of a ciliate which divides to form featherlike colonies consisting of up to 3000 individuals and sulfur bacteria has been studied. The bacteria which form a dense layer on the surface of the ciliates oxidize hydrogen sulfide (a highly poisonous product of organic decay) with oxygen and use the energy derived from this reaction to form organic matter from carbon dioxyde. Bacteria which detach from the surface are readily eaten by the ciliates. The ciliates settle and grow on places where there is a sharp boundary between oxygen and hydrogen sulfide in the water. In the Caribbean this is developed on the surface of mangrove peat, in the Mediterranean in the upper layer of accumulations of dead seagrass leaves on the sea bottom. Mirosensors were used to measure the concentration of oxygen and hydrogen sulfide in the vicinity of the colonies at high spatial and temporal resolution. It could be shown that the mixing of water containing the two dissolved gases by the ciliary beat of the animals efficiently supplies the bacteria with the necessary substances for their rapid growth and as a consequence also that of the ciliates. Molecular methods using similarities in certain genes to those of other species had to be used to characterize the bacteria which so far have defied cultivation. A similar approach was taken to study the relationship of those ciliates found in the Caribbean to those growing in the Mediterranean. It was shown that they belong to the same species, which probably has a cosmopolitan distribution in all warm seas.