Symbiont transmission in the hydrothermal vent tubeworms

Andrea Nussbaumer & Monika Bright

Department of Marine Biology, Institute of Ecology and Conservation Biology, University of Vienna, Austria

The uptake of symbionts in hydrothermal vent tubeworms (Vestimentifera, Siboglinidae) is facilitated by massive infection of the skin and simultaneous apoptosis. This mechanism, well known from pathogenic infection, leads here to the development of a new mesodermal organ, the trophosome, in the interior of the worm’s trunk and thus to the establishment of a nutritional association beneficial for both partners. We investigated settled tubeworm larvae and early juveniles from 200 to 500 µm, collected during three cruises to the East Pacific Rise, 9°50N, in 1998, 1999, 2001. Serial transmission electron microscopical sections showed infection of the epidermis, somatic muscles and mesenchym through symbiont like bacteria. Apoptosis was apparent in these infected as well as in adjacent cells by cell shrinkage, pycnotic nuclei with dense chromatin masses accumulating peripherally and dilated mitochondria. Fluorescence in situ Hybridization using a 16S rRNA symbiont-specific probe identified these bacteria as symbionts. Additional analyses with various group-specific eubacteria and archaea probes showed that neither the infected skin nor the developing trophosome harbored other than the symbiotic phylotype.

The current hypothesis proposes the establishment of the symbiosis being by uptake of symbionts through the digestive system of the early juvenile followed by proliferation of the endodermal midgut cells, transformation into bacteriocytes and development of the trophosome from endodermal tissue. Our findings point to a different pathway: bacteria infect the skin and trigger apoptosis, which probably facilitates migration into deeper layers until the visceral mesoderm is reached. The symbionts are then enclosed in vacuoles and the mesoderm proliferates to form the trophosome.

Infection and apoptosis facilitate transmission of symbionts and establishment of endosymbiosis in hydrothermal vent tubeworms

Bright M¹., Fisher C. R.², Nussbaumer A. ¹

¹Institute of Ecology and Conservation Biology, University of Vienna, Althanstr. 14, A-1090 Vienna, Austria

²The Pennysylvania State University, 208 Mueller Lab, University Park, PA 16802-5301, USA

The current hypothesis proposes that the transmission and establishment of the symbiosis between hydrothermal vent tubeworms (Vestimentifera, Siboglinidae) and thiotrophic endosymbionts is by uptake of symbionts through the digestive system in early juveniles into endodermal mitgut cells, followed by proliferations and transformation of midgut cells into bacteriocytes, and development of the trophosome. Our findings point to a different pathway: bacteria infect the skin and trigger apoptosis, which probably facilitates migration into deeper layers until the visceral mesoderm is reached. The symbionts are then enclosed in vacuoles and the mesoderm proliferates to form the trophosome. We investigated settled tubeworm larvae and early juveniles (200 - 500 µm in length), collected during three cruises to the East Pacific Rise, 9°50N, in 1998, 1999, and 2001. Serial transmission electron microscopical sections showed that the epidermis, somatic muscles, and mesenchym are infected with symbiont-like bacteria in early juveniles. Apoptosis was apparent in these tissues by cell shrinkage, pycnotic nuclei with dense chromatin masses accumulating peripherally and dilated mitochondria. Fluorescence in situ Hybridization was applied on a different set of animals using a 16S rRNA symbiont-specific probe. The bacteria in the skin were identified as symbionts. Additional analyses with various group-specific eubacteria and archaea probes showed that neither the infected skin nor the developing trophosome harbored other than the symbiotic phylotype.

Discovery of a new symbiosis from antarctic, shallow-water hydrothermal vents

Bright, M.¹, C. Arndt^2,3, H. Keckeis¹ and H. Felbeck³

¹Institute of Ecology and Conservation Biology, University of Vienna, Althanstr. 14, A-1090 Vienna, Austria

²Institute for Microbiology, Ernst-Moritz-Arndt University, Friedrich-Ludwig-Jahn Strasse 15, D-17489 Greifswald, Germany

³Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA 92093-0202, USA

A prominent not further identified species of Monocelidae (Plathelminthes, Proseriata) was identified in the vicinity of fumarole activity at Fumarole Bay (Deception Island, Antarctica). Apparently symbiotic, monotypic curved rods colonized the surface of the animals. Investigating the distribution of this animal and the metazoan meiobenthos in and in the vicinity of this area we infer that this species constitutes the most abundant species and the bulk of the biomass at these shallow water hydrothermal vent sediments. In contrast to the other metazoan meiofauna, the distribution of this species is positively correlated with the water temperature and gas emissions indicating a preference for the areas around fumaroles. The range of temperature tolerated by this symbiosis was determined in in vivo experiments to be at least 30°- 40°C.

The results of this study revealed a remarkable difference between shallow water and deep-sea hydrothermal vent meiobenthic communities. Generalists capable of tolerating extreme abiotic conditions appear to dominate shallow water vents, whereas endemism seems to be the rule in the deep-sea vents. A symbiotic life style apparently has evolved among meiofaunal representatives of shallow water vents, while it has not been described for any meiofaunal species from deep-sea vents.

The color of the trophosome: elemental sulfur distribution in the endosymbionts of Riftia pachyptila, Jones 1981 (Vestimentifera, Siboglinidae)

Pflugfelder B. ¹, Fisher C. R.², Bright M¹

¹Institute of Ecology and Conservation Biology, University of Vienna, Althanstr. 14, A-1090 Vienna, Austria

²The Pennysylvania State University, 208 Mueller Lab, University Park, PA 16802-5301, USA

The presence of elemental sulfur stored in vesicles of thiotrophic free-living or symbiotic bacteria is known to serve as energy source for sulfide oxidation. Such vesicles do occur in the chemoautotrophic sulfide-oxidizing symbionts of Riftia pachyptila (Vestimentifera, Siboglinidae), but neither the content of these vesicles was analyzed, nor their density in the different morphotypes in different regions of the trophosome from different animals. However, bulk chemical analyses showed that the specific color of the trophosome is related to the amount of elemental sulfur (Fisher xy?). The objectives of this study were to detect elemental sulfur in the vesicles of the symbionts by Electron Energy Loss Spectrography (EELS) and subsequently to investigate the density of sulfur vesicles (SD) by quantitative Transmission Electron Microscopy (TEM) and stereological techniques using SD values that express the percentage of the symbiotic cytoplasm area taken up by vesicles in ultrathin sections for comparisons: 1) three animals each with uniformly colored trophosomes; two animals with trophosome color changing from anterior to posterior; 2) three trophosomal regions (anterior, median, and porsterior), and 3) three trophosomal lobule zones (central rods, median small cocci, peripheral large cocci).

The SD highly correlated with the color of the trophosome. This was found in animals with uniformly colored trophosomes as well as trophosomes that exhibit a gradual color change from the anterior to the posterior. A relative SD decrease of 40% was found between animals with light and dark green trophosomes and another 40% decrease between dark green and black. All animals were collected from a single site extending a few m² but the amount of sulfur storage points to a highly variable supply of sulfide. In this respect, the color of the trophosome is a useful tool to estimate the chemical condition under which a specific tubeworm thrives.

In animals with light green trophosomes, we found a significant decrease in SD from the peripherally located cocci to the centrally located rods. This gradient, although it follows the direction of blood flow in the trophosome lobule and thus theoretically the supply of sulfide for sulfide oxidation and sulfur storage, cannot be explained as limitations of sulfide in rods. Support comes from animals with black and dark green trophosomes, in which no SD gradient in morphotypes was found. This points to a lower but similar supply of sulfide for all symbionts than in those animals with light green trophosome. Although the specific underlying mechanisms remain to be clarified, we hypothesize that the rods and cocci behave physiologically different when exposed to high sulfide concentrations but perform similar under lower sulfide concentrations.