Abstract
Symbioses between ciliate hosts and prokaryote or unicellular eukaryote symbionts are widespread. Here, we report on a novel ciliate species within the genus Zoothamnium Bory de St. Vincent, 1824, isolated from shallow-water sunken wood in the North Adriatic Sea (Mediterranean Sea), proposed as Zoothamnium ignavum sp. nov. We found this ciliate species to be associated with a novel genus of bacteria, here proposed as “Candidatus Navis piranensis” gen. nov., sp. nov. The descriptions of host and symbiont species are based on morphological and ultrastructural studies, the SSU rRNA sequences, and in situ hybridization with symbiont-specific probes. The host is characterized by alternate microzooids on alternate branches arising from a long, common stalk with an adhesive disc. Three different types of zooids are present: microzooids with a bulgy oral side, roundish to ellipsoid macrozooids, and terminal zooids ellipsoid when dividing or bulgy when undividing. The oral ciliature of the microzooids runs 1¼ turns in a clockwise direction around the peristomial disc when viewed from inside the cell and runs into the infundibulum, where it makes another ¾ turn. The ciliature consists of a paroral membrane (haplokinety), three adoral membranelles (polykineties), and one stomatogenic kinety (germinal kinety). One circular row of barren kinetosomes is present aborally (trochal band). Phylogenetic analyses placed Z. ignavum sp. nov. within the clade II of the polyphyletic family Zoothamniidae (Oligohymenophorea). The ectosymbiont was found to occur in two different morphotypes, as rods with pointed ends and coccoid rods. It forms a monophyletic group with two uncultured Gammaproteobacteria within an unclassified group of Gammaproteobacteria, and is only distantly related to the ectosymbiont of the closely related peritrich Z. niveum (Hemprich and Ehrenberg, 1831) Ehrenberg, 1838.
Highlights
Ciliates exhibit a broad spectrum of symbiotic associations with unicellular eukaryotes and prokaryotes
Zoothamnium species with alternately branched stalk; zooids alternate on branches; three different types of zooids: microzooids (“trophic stage”), macrozooids (“telotroch stage”), terminal zooids; microzooids bulgy, inverted bell-shaped; macrozooids roundish to ellipsoid, located only on the most proximal part of the branches; top terminal zooid on the tip of the stalk, terminal zooids of the branches on the proximal end of each branch; undividing terminal zooids similar to microzooids in shape, dividing terminal zooids ellipsoid; macronucleus of microzooids S-shaped, showing irregular thickness; macronucleus of macrozooids extended through the whole cell, band-like with constant diameter; macronucleus of dividing terminal zooids S-shaped, regular thickness, filling almost the entire cell; in each zooid one orally located contractile vacuole present; a telotrochal band of one circular row of barren kinetosomes present aborally
The holotype and 12 paratypes of Zoothamnium ignavum sp. nov. were collected in July 2015 from sunken, degrading wood found in about 1 m depth in the canal of Sv
Summary
Ciliates exhibit a broad spectrum of symbiotic associations with unicellular eukaryotes and prokaryotes. Prokaryotic symbionts with phototrophic, chemoautotrophic and heterotrophic metabolism in ciliates The presence of an inverse gradient of reduced energy sources, such as sulfide and methane, in close proximity to oxidants, such as oxygen, nitrate, and sulfate, is a common denominator to all these environments [22]. It is at the oxicanoxic interface where chemosynthetic symbioses prosper [20,23,24]. A variety of animal hosts from several different phyla with chemosynthetic symbionts are known [22]. 1824 (Oligohymenophorea) [32– 37] were found being associated with chemosynthetic bacteria
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