Abstract The culturability of a strictly anaerobic, extre- mely thermophilic archaeon, Thermococcus peptonophilus (optimal growth temperature: 85° C), was studied during survival stages at various temperatures (98, 85, 70, and 4°C). Total cell number (determined by DAPI staining), active cells (rhodamine-stained cells), and culturable cells (using most-probable-number) were counted over time. The number of culturable cells decreased under each con- dition tested. The total number of cells significantly de- creased only at temperatures close to the maximum for growth (98° C); at this temperature, the cells spontaneously lysed. Our results suggested that survival at 4° C in oxy- genated waters might be a mechanism for the dispersion of extreme thermophiles in the ocean. In addition, we proved the existence of T. peptonophilus cells in several physiological states: culturable cells, active non-cultur- able cells, inactive non-culturable cells, and dead cells. Cell death was caused by cellular lysis. Key words Thermococcus peptonophilus · Deep-sea · Hydrothermal vents · Survival · Extreme thermophiles · Culturability · Active cells · Mortality · Dead cells Introduction Survival strategies aimed to resist adverse environmental conditions have been found in every microorganism tested (Roszak and Colwell 1987a). Since microorganisms live in changing environments, they adapt in order to survive under adverse circumstances. Several strategies have been observed. Spore formation by microorganisms is one ex- ample of a clearly differentiated behavior for surviving extreme conditions. Non-spore-forming microorganisms are usually able to enter dormant (non-culturable) physio- logical states, allowing them to resist long terms under ad- verse conditions (Roszak and Colwell 1987a). In nature, the largest proportion of cells are in a dormant state (Roszak and Colwell 1987a) waiting for better conditions to come; thus, most of the cells present in natural envi- ronments cannot be cultured by common culturing tech- niques. In general, less than 0.1% of these microorgan- isms in natural samples can be cultured (Hobbie et al. 1977; Roszak and Colwell 1987a). It has not yet been reported whether extreme ther- mophilic archaea exist in different physiological states of survival, such as those found for bacteria (Roszak and Colwell 1987a and b). The objective of this study was to characterize the cells of an extremely thermophilic ar- chaeon, Thermococcus peptonophilus, in various physio- logical states of survival. Materials and methods Strain and growth conditions Thermococcus peptonophilus, strain OG1 (JCM 9653) (González et al. 1995), was used in this study and grown as described by González et al. (1995). Duplicate experiments were carried out at 98, 85, 70, and 4° C. Experiments were started at the beginning of the stationary phase of growth, once a plateau of cell numbers was observed in the growth curve. T. peptonophilus was grown at the selected temperature, except for the experiments performed at 4° C, where the cells were grown at 85° C. The effect of oxygen on cells incubated at 4° C was also tested; these cultures were grown at 85° C, cooled down on ice, and aerated for 10 min with air pumped through a 0.2-μm-pore-diameter filter. Cell counting and staining Samples for total counts were immediately fixed with formalin (1% final concentration) and kept on ice. Cells were stained with Juan M. González · Chiaki Kato · Koki Horikoshi Culturability and survival of an extreme thermophile isolated from deep-sea hydrothermal vents Arch Microbiol (1996) 166 : 64–67 © Springer-Verlag 1996 Received: 5 February 1996 / Accepted: 16 April 1996 SHORT COMMUNICATION J. M. González 1 · C. Kato () · K. Horikoshi The Deepstar Group, Japan Marine Science and Technology Center (JAMSTEC), 2–15 Natsushima-cho, Yokosuka 237, Japan Tel. +81-468-67-5555; Fax +81-468-66-6364 e-mail: katoc@jamstec.go.jp 1 Present address: COMB, Columbus Center, 701 E. Pratt St., Baltimore, MD 21202, USA