Low-temperature adaptation of deep-sea bacteria isolated from the Japan Trench

Growth responses for bacteria culturable at 4 °C, isolated randomly from three different habitats comprising surface seawater, deep-sea water (5000 to 6000 m) and deep-sea sediments (ca. 2200 to 6000 m), were tested for growth at temperatures from 4 to 30 °C. All 67 isolates grew well at 4 and 20 °C, indicating that they were psychrotrophic. Compared with the surface seawater, psychrotrophic bacterial groups from the deep-sea were rich in bacteria with higher growth yield at 4 °C and lower doubling time at 30 °C. According to Ratkowsky's square root analysis, the deep-sea water group was characterized by lower predicted minimum growth temperature (mean Tmin = 266.2 K) than the surface-water group (269.9 K), while the lowest Tmin was measured in the deep-sea sediment group (263.9 K). Lower Tmin was also found in a deeper group among the sediment bacterial group. These data suggest that most of deep-sea bacterial isolates consist of members possessing higher adaptability to low-temperature environments. Psychrotrophs from deep-sea water also demonstrated an abundance of Gram-negative bacteria which were tentatively identified as belonging to the genus Moraxella, while psychrotrophs from surface water were more diverse. Bacillus spp. and Gram-positive coryneform bacteria dominated in deep-sea sediments, decreasing with increasing depth. Restriction fragment length polymorphism analysis of PCR-amplified 16S rDNA of Moraxella-like bacteria showed that deep-sea strains were different from surface strains and from the four known species tested belonging to the Moraxella–Psychrobacter group.