“CandidatusThiobios zoothamnicoli,” an Ectosymbiotic Bacterium Covering the Giant Marine CiliateZoothamnium niveum

Abstract

Zoothamnium niveum is a giant, colonial marine ciliate from sulfide-rich habitats obligatorily covered with chemoautotrophic, sulfide-oxidizing bacteria which appear as coccoid rods and rods with a series of intermediate shapes. Comparative 16S rRNA gene sequence analysis and fluorescence in situ hybridization showed that the ectosymbiont of Z. niveum belongs to only one pleomorphic phylotype. The Z. niveum ectosymbiont is only moderately related to previously identified groups of thiotrophic symbionts within the Gammaproteobacteria, and shows highest 16S rRNA sequence similarity with the free-living sulfur-oxidizing bacterial strain ODIII6 from shallowwater hydrothermal vents of the Mediterranean Sea (94.5%) and an endosymbiont from a deep-sea hydrothermal vent gastropod of the Indian Ocean Ridge (93.1%). A replacement of this specific ectosymbiont by a variety of other bacteria was observed only for senescent basal parts of the host colonies. The taxonomic status "Candidatus Thiobios zoothamnicoli" is proposed for the ectosymbiont of Z. niveum based on its ultrastructure, its 16S rRNA gene, the intergenic spacer region, and its partial 23S rRNA gene sequence.

Sulfide assimilation by ectosymbionts of the sessile ciliate, Zoothamnium niveum

“…This view of the Z. niveum niche on the peat banks of Belizean barrier islands resembles the conditions in locations on Corsica, France, where it recently has been found. Here it lives attached to rock surfaces in the vicinity of decaying sea grass (Rinke et al 2007). Micro-molar concentration of sulWde in norm-oxic seawater is also the medium in which Z. niveum has been successfully cultivated.…”

“…The sulWde concentration of 3 mol sulWde L ¡1 is where Rinke et al (2007) found optimal growth. This external concentration is well within the range measured between the colonies in situ by Vopel et al (2005).…”

“…Solute dynamics was calculated in the "Convection and DiVusion" application mode based on the results of the Navier-Stokes solution. Geometry of the zooids was based on Bauer-Nebelsick et al (1996a, b) and that of T. zoothamnicoli on Rinke et al (2007). The density of the calculation mesh was adjusted to give adequate calculation speed and tested not to inXuence the model results.…”

“…The morphology of the micro-zooid was taken from Bauer-Nebelsick et al (1996b) and the dimensions of the bacterial symbionts from Rinke et al 2007. Flow velocity through the ciliary apparatus (0.6 mm s…”

“…The side-branches of the "feather" carry several hundred microzooids (zooid = single ciliate) in a regular pattern (see BauerNebelsick et al 1996a, b for a detailed description). The entire surface of the zooids, stalk and branches are covered by a single layer of sulWde-oxidizing bacteria belonging to the Gamma-proteobacteria (Rinke et al 2007). The peritrich ciliates are Wlter-feeders, and the microzooids of Z. niveum possess a fully developed oral ciliature and cytopharynx, similar to Zoothamnium species that are not covered by sulWde oxidizing bacteria (Bauer-Nebelsick et al 1996b).…”