The effects of sulphide on growth and behaviour of the thiotrophicZoothamnium niveumsymbiosis

Abstract

Zoothamnium niveum(Ciliophora, Oligohymenophora) is a giant, colonial marine ciliate from sulphide-rich, shallow-water habitats, obligatorily associated with the ectosymbiotic, chemoautotrophic, sulphide-oxidizing bacterium ‘CandidatusThiobios zoothamnicoli’. The aims of this study were to characterize the natural habitat and investigate growth, reproduction, survival and maintenance of the symbiosis fromCorsica, France (Mediterranean Sea) using a flow-through respirometer providing stable chemical conditions. We were able to successfully cultivate theZ. niveumsymbiosis during its entire lifespan and document reproduction, whereby the optimum conditions were found to range from 3 to 33 μmol l−1ΣH2S in normoxic seawater. Starting with an inoculum of 13 specimens, we found up to 173 new specimens that were asexually produced after only 11 days. Observed mean lifespan of theZ. niveumcolonies was approximately 11 days and mean colony size reached 51 branches, from which rapid host division rates of up to every 4.1 hours were calculated. Comparing the ectosymbiotic population fromZ. niveumcolonies collected from their natural habitat with those cultivated under optimal conditions, we found significant differences in the bacterial morphology and the frequency of dividing cells on distinct host parts, which is most likely caused by behaviour of the host ciliate. Applying different sulphide concentrations we revealed that the symbiosis was not able to survive without sulphide and was harmed by high sulphide conditions. To our knowledge, this study reports the first successful cultivation of a thiotrophic ectosymbiosis.