marine microbial thiotrophic ectosymbioses - HYDRA-Institute

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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 <strong>ectosymbioses</strong> 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 <strong>marine</strong> 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.
2727_C04.fm Page 99 Wednesday, June 30, 2004 12:00 PM Marine Microbial Thiotrophic Ectosymbioses 99 Figure 1 Light microscopy (LM) micrograph of living specimen of Kentrophoros fistulosus; scale bar = 500 mm. Figure 2 Scanning electron microscopy (SEM) micrograph of midbody region with rods (r) on the dorsal side and cilia on the ventral side; scale bar = 20 mm. (Courtesy of W. Foissner.) Figure 3 LM micrograph of Zoothamnium niveum colony with stalk (s) and terminal zooid (t) on its tip and alternate branches with microzooids (mi) and macrozooids (ma); scale bar = 100 mm. Figure 4 SEM micrograph of contracted colony showing several microzooids (mi) and one macrozooid (ma) covered by symbionts; scale bar = 50 mm.
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