marine microbial thiotrophic ectosymbioses - HYDRA-Institute

2727_C04.fm Page 98 Wednesday, June 30, 2004 12:00 PM
98 J. Ott, M. Bright & S. Bulgheresi
debris is much less stable and predictable than mangrove peat. It may, however, repeatedly collect
in defined places such as depressions in the vicinity of algae or sea grass stands or in crevices and
caves among rocks.
Thiotrophic ectosymbioses have been reported from mangrove peat and decomposing sea grass
and algae. In all cases the host is a sedentary peritrich ciliate belonging to the genus Zoothamnium.
Kentrophoros
Hosts
Ciliates
About 20 species of the ciliate genus Kentrophoros inhabit sheltered marine sands having an RPD
several centimetres beneath the surface (Fenchel & Finlay 1989). Soon after the description of the
first species (Sauerbrey 1928) it was recognised that the dorsal (left) surface of the cells is covered
by rod-shaped bacteria containing sulphur granules (Kahl 1935). Raikov (1971, 1974) suggested
a chemolithoautotrophic nature of the bacteria and showed that the bacteria are phagocytised by
the ciliate.
Specimens of Kentrophoros are ribbon shaped or tubularly involuted and, in the case of K.
fistulosus, can be up to 3 mm long, but are only 2–3 mm thick (Figure 1). The extremely flattened
shape is interpreted as an adaptation to provide ample space for the bacterial symbionts; it increases
the surface-to-volume ratio by a factor of 6–7 compared with other similarly sized ciliates. The
ventral (or right) side bears cilia arranged in longitudinal rows. The cells have only a vestigial
cytostome (Foissner 1995). Rod-shaped bacteria occupy the unciliated dorsal (or left) side of the
cell, which may be tubularly involuted in the central region (Figure 2).
The ciliates glide sluggishly between the sand grains. In the sediment they concentrate in the
oxic–anoxic chemocline. In an artificial oxygen gradient they aggregate around 5% saturation,
avoiding high oxygen tensions. The ciliates, however, do not react to sulphide but appear to be
randomly distributed in a sulphide gradient in the absence of oxygen (Fenchel & Finlay 1989).
Under experimental conditions and under the assumption that the ectosymbiotic bacteria constitute
its sole food, Kentrophoros was calculated to have a doubling time of 18 h at room
temperature, which is low compared with similarly sized ciliates. This difference was attributed to
suboptimal culture conditions (Fenchel & Finlay 1989).
Zoothamnium
The sedentary colonial peritrich ciliate Zoothamnium niveum (Hemprich & Ehrenberg 1831,
Ehrenberg 1838) was originally described from the Red Sea. Although the authors were struck
by its white appearance, they could not attribute it to the presence of bacteria. The ciliate has
been redescribed by Bauer-Nebelsick et al. (1996a) based on material collected from mangrove
peat in the Belize Barrier Reef system. There it grows on vertical to overhanging walls of tidal
channels, and lagoons cut into mangrove peat of backreef islands. Since then, Zoothamnium
niveum has been found in the western Mediterranean near Calvi, Corsica, where it grows on
decomposing subtidal accumulations of leaf debris and adjacent rocks near stands of the large
Mediterranean sea grass Posidonia oceanica.
Other reports of large, white Zoothamnium colonies
come from the Florida Keys (Bauer-Nebelsick et al. 1996a), Lanzarote (Canary Islands)
(Wirtz & Debelius 2003), Elba (own unpublished observations), Giglio (T. Pillen, personal
communication), and Greece (G. Scatolin, personal communication). The up to 15-mm-long
colonies are feather-shaped fans with alternating branches growing from a central stalk (Figure
3). Stalk and branches contain a contractile spasmoneme that allows the colony to contract
rapidly. Colony contraction occurs spontaneously or upon disturbance.