Distribution and physiological characteristics of hyperthermophiles in theKubiki oil reservoir in Niigata, Japan

The distribution of culturable hyperthermophiles was studied in relation to environmental conditions in the Kubiki oil reservoir in Japan, where the t emperature was between 50 and 58 degrees C, Dominant hyperthermophilic cocc i and rods were isolated and shown to belong to the genera Thermococcus and Thermotoga, respectively, by 16S rDNA analyses. Using the most-probable-nu mber method, we found that hyperthermophilic cocci were widely distributed in several unconnected fault blocks in the Kubiki oil reservoir. In 1996 to 1997, their populations in the production waters from oil wells were 9.2 x 10(3) to 4.6 x 10(4) cells/ml. or 10 to 42% of total cocci, On the other h and, hyperthermophilic rods were found in only one fault block of the reser voir with populations less than 10 cells/ml. Dominant Thermococcus and Ther motoga spp, grew at reservoir temperatures and utilized amino acids and sug ars, respectively, as sole carbon sources. While organic carbon was plentif ul in the environment, these hyperthermophiles were unable to grow in the f ormation water due to lack of essential nutrients, Concentrations of some o rganic and inorganic substances differed among fault blocks, indicating tha t the movement of formation water between fault blocks was restricted. This finding suggests that the supply of nutrients via fluid current is limited in this subterranean environment and that the organisms are starved in the oil reservoir. Under starved conditions at 50 degrees C, culturable cells of Thermococcus sp, remained around the initial cell density for about 200 days, while those of Thermotoga sp, decreased exponentially to 0.01% of the initial cell density after incubation for the same period, The difference in survivability between these two hyperthermophiles seems to reflect their populations in the fault blocks, These results indicate that hyperthermoph ilic cocci and rods adapt to the subterranean environment of the Kubiki oil reservoir by developing an ability to survive under starved conditions.