Morphological, small subunit rRNA, and physiological characterization of Trimyema minutum (Kahl, 1931), an anaerobic ciliate from submarine hydrothermal vents growing from 28 degrees C to 52 degrees C.

A thermophilic strain of Trimyema minutum was isolated from the hydrothermally heated sea floor at Vulcano Island (Italy) and cultivated monoxenically on Marinobacter sp. and Methanococcus thermolithotrophicus. It can be propagated strictly anaerobically and is sensitive to oxygen: if exposed to air at 48 degrees C all cells die within 60 min. It grows from 0.45-7.2% (w/v) salt and at pH 6.0-8.0. The isolate is the most extreme thermophilic ciliate which ever has been cultivated, exhibiting an optimal growth temperature of 48 degrees C (doubling time 6 h). Growth occurs between 28 degrees C and 52 degrees C. Trimyema minutum is redescribed using live observation and silver impregnation. Its morphology and the small subunit ribosomal RNA sequence is distinctly different from that of T. compressum, but morphology is highly similar to that of T. shoalsia Nerad et al. 1995, which is thus probably a junior synonym of T. minutum. To stabilize the bewildering species taxonomy in Trimyema, we suggest to recognize our population as a neotype of T. minutum.