Ultrastructural changes in an obligately barophilic marine bacterium after decompression.

The bacterial isolate MT-41 from 10,476 m, nearly the greatest ocean depth, is obligately barophilic. The purpose of this study was to describe the morphological changes in MT-41 due to nearly isothermal decompression followed by incubation at atmospheric pressure. Two cultures were grown at 103.5 MPa and 2 degrees C and then decompressed to atmospheric pressure (0.101 MPa). One of the cultures was fixed just before decompression. The other culture, kept at 0 degrees C, was sampled immediately and four more times over 168 h. The number of CFU (assayed at 103.5 MPa and 2 degrees C) declined with incubation time at atmospheric pressure. Decompression itself did not lead to immediate morphological changes. The ultrastructure, however, was altered with increasing time at atmospheric pressure. The first aberrations were intracellular vesicles and membrane fragments in the medium. After these changes were plasmolysis, cell lysis, the formation of extracellular vesicles, and the formation of ghost cells. Intact cells in the longest incubation at atmospheric pressure had the normal cytoplasmic granularity suggestive of ribosomes but had few and poorly stained fibrils in the bacterial nucleoids. From the practical standpoint, samples of hadal deep-sea regions need to be fixed either in situ or shortly after arrival at the sea surface even when recovered in insulated sampling gear. This should prevent drastic structural degradation of sampled cells, thus allowing both accurate estimates of deep-sea benthic standing stock and realistic morphological descriptions.