Distribution and Population of Free-Living Cells Related to Endosymbiont A Harbored in Oligobrachia mashikoi (a Siboglinid Polychaete) Inhabiting Tsukumo Bay

Abstract

Beard worms (Siboglinidae, Polychaeta), which lack a mouth and a digestive tract, harbor thioautotrophic or methanotrophic bacteria in special cells called bacteriocytes. These endosymbionts have been considered to be trapped at a specific larval stage from the environment. Although many species of beard worms have been discovered in various abyssal seas, Oligobrachia mashikoi inhabits Tsukumo Bay which is only 25 m deep. At least seven types of endosymbionts (endosymbiont A-G) have been distinguished in O. mashikoi. In this study, we investigated the distribution pattern of free-living cells related to the major endosymbiont (endosymbiont A) in Tsukumo Bay by quantitative PCR targeting the 16S rRNA gene. The endosymbiont A-related phylotype was detected in almost all sediment samples collected from 23 points in Tsukumo Bay, ranging in copy number of the 16S rRNA gene from 2.22×10 4 to 1.42×10 6 copies per gram of dry-sediment. Furthermore, the free-living cells made up less than 9% of the total eubacterial population, suggesting that the O. mashikoi larvae precisely select candidates for their endosymbiont from bacterial flora in the environment. This is the first report on the ecological characterization of a free-living bacterium related to the endosymbiont of the siboglinid polychaete, O. mashikoi.

Chemoautotrophic Carbon Fixation Rates and Active Bacterial Communities in Intertidal Marine Sediments

“…5). Although clade IA 1 sequences were most closely related to endosymbionts, they are also commonly found in related uncultured free-living bacteria [14], [72]. These two Rubisco clades are usually also detected in other environmental studies on marine sediments [32], [73].…”

New Perspectives on the Ecology and Evolution of Siboglinid Tubeworms

“…Available evidence supporting horizontal transmission as the primary mode for establishment of siboglinid symbioses includes: (1) a lack of symbionts in worms' gonadal tissues or larvae [13], [55], [122]–[124], (2) the presence of the motility-related flagellin gene in the vestimentiferan endosymbiont genome [117], [125], (3) the detection of highly similar bacterial phylotypes (based on 16S rRNA sequences analysis) in host and in the external environment [112], [126]–[129], (4) the presence of heterotrophic metabolic pathways in the vestimentiferan endosymbiont that are not expressed in hospite [117], (5) direct confirmation of horizontal transmission in Rifta pachyptila [26], and (6) the absence of reciprocal phylogenies (i.e., co-evolution) between host and symbiont [112], [130], [131]. Thus, following a non-symbiotic larval stage, siboglinids must establish a new symbiosis each generation in order to survive.…”

Direct Evidence That Extracellular Giant Hemoglobin is Produced in Chloragogen Tissues in a Beard Worm, Oligobrachia mashikoi (Frenulata, Siboglinidae, Annelida)

“…Furthermore, Numoto et al (2005 a, b) succeeded in analyzing the three-dimensional structure of this hemoglobin by X-ray crystallography, and this analysis suggested the positions of unpaired free cysteine residues to which the sulfur atom in hydrogen sulfide should bind. In addition, we clarified that in O. mashikoi, the bacteria oxidize sulfur (Aida et al, 2008). All these data imply that inside the host body, hydrogen sulfide is transported to the chemosynthetic symbiont as a non-toxic molecule bound to free cysteine residues in hemoglobin.…”