Microbial hitchhikers ride thiobiotic nematodes

Jörg Ott

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

Marine nematodes are a clean lot: as a rule their cuticle is free of microbial fouling. An exception are members of the order Desmodoroidea where a fortuitous microbial epigrowth is common. In the mesopsammic Stilbonematinae, however, this epigrowth consists of sulphur oxidising, chemoautotrophic bacteria which form a monospecific, highly ordered coat which is species specific in all nematodes so far investigated. The motile worms transport the bacteria between the sulfidic deeper sediment layers and the oxidised surface thus supplying them alternately with sulphide and oxygen for chemolithotrophic CO₂ fixation. In turn the bacteria are fed upon by the worms and most probably constitute their sole food.

The worms comprise 8 genera with about 25 valid species. They are characterised by unique glandular sense organs in the epidermis and a peculiar pharynx - both characters probably related to the symbiotic monophagic life style.

The bacteria belong to the g-proteobacteria and form a clade with the symbionts of marine gutless oligochaetes. They are attached to the cuticle of the worms in a highly ordered fashion forming striking patterns. Some divide longitudinally, others form up to 100 µm long non-septate filaments. They usually contain numerous highly refractive inclusions consisting of elemental sulphur which makes them appear pure white under incident light.

There is no evidence of vertical transmission from parents to offspring in the worms, which furthermore moult 4 times loosing all symbionts in this process. The molecular recognition mechanism which allows the worms to pick up the correct bacteria from the free living microbial community in the sand is currently under investigation in the Vienna laboratory.

Zoothamnium niveum, a symbiotic ciliate with a wide geographical and ecological distribution: 18S ribosomal RNA gene phylogeny

Thomas G. Buchholz¹, Monika Bright¹, Silvia Bulgheresi¹, Johannes Fried², Wolfgang Ludwig², Joerg A. Ott¹

¹Institute of Ecology and Conservation Biology, University of Vienna, Althanstraße 14, A-1090 Vienna, Austria; thomas.buchholz@gmx.net

²Lehrstuhl für Mikrobiologie, Technische Universität München, Am Hochanger 4, D-85350 Freising, Germany; johannes.fried@uibk.ac.at

The sessile, colonial, marine ciliate Zoothamnium niveum (Oligohymenophorea, Peritrichia) lives in symbiosis with sulfur-oxidizing, chemolithoautotrophic bacteria covering its external surface. The symbiosis exploits the sulfide/oxygen chemocline developed on decaying macrophyte remains. While the Caribbean Z. niveum inhabits the surface of mangrove peat which provides a sufficient permanent sulfide supply year-round, Mediterranean Z. niveum-like symbiotic ciliates so far have been reported in the warm late summer months exclusively, when high amounts of sulfide are released from accumulated Posidonia debris.

Despite large geographical distances and remarkable differences in habitat and climate, the symbiotic Zoothamnium populations are morphologically indistinguishable. To find out, wether the Caribbean Z. niveum and the Mediterranean symbiotic Zoothamnium are different species, with the similarities being a convergence due to the symbiotic lifestyle, or the same species with a wide geographic and ecological distribution, the small subunit ribosomal RNA gene of two Caribbean populations and one Mediterranean population was sequenced, phylogenetically analyzed and compared with sequences of the limnic Z. arbuscula. Our results suggest, that all symbiotic populations form a closely related monophyletic taxon, Zoothamnium niveum. This symbiotic species differs distinctly from Z. arbuscula. Furthermore, the genus Zoothamnium appears to be paraphyletic within the Peritrichia.