Discovery of a close relative of Thiomargarita nambiensis at Gulf of Mexico Cold Seeps

During a July 2002 research cruise to the Gulf of Mexico, we discoverd a giant vacuolate sulfur oxidizing bacteria that looked similar to Thiomargarita namibiensis. This giant bacteria is visible with the naked eye. The image below was taken using a digital camera focused through a geological magnifying glass. The tip diameter of the pH microelectrode is 500 microns and the opaque gray-white 'balls' are the new sulfur bacteria. Some cells exceed the tip diameter of the microelectrode.

These new microbes were first discovered in cores collected along the edge of the Brine Pool site (GC233). The gassy, sulfidic surficial sediments were coated by a microbial mat that looked like a crushed "oreo cookie" mat (description by Beth! Orcutt). The surface of the sediment was also home to a few lonely Thiothrix here and there, but mostly large spherical cells were present.

The size of cells varied between 180 and 375 µm in diameter and they were found at both brine-influenced and gas-hydrate influenced seeps in the Gulf of Mexico. These bacteria were characterized by an outer shell of sulfur-globule-filled cytoplasm that surrounded a central vacuole that accounted for about 80% of biovolume. The vacuole contained high concentrations of nitrate (~ 460 mM). About 800 base pairs of 16S rRNA gene sequence data, linked to this bacterium by fluorescent in situ hybridization, showed 99% identity with Thiomargarita namibiensis, previously described only from sediments collected off the coast of Namibia (Western Africa). While T. namibiensis cells form linear chains within a common sheath, the Gulf of Mexico strain occurred as single cells and clusters of 2, 4 and 8 cells, which were clearly the product of division in one to three planes (Kalenetra et al., manuscript submitted to Environmental Microbiology, September 2004)

Images of the novel Thiomargarita-like sulfur bacterium. Scale bar notes 100 µm unless otherwise noted. Top left image shows the 'fisheye' view of the large cells. Bottom left image is phase contrast light micrograph of an individual cell (courtesy of Karen Kalenetra). Top right image shows FITC stained cell illustrating the presence of large internal vacuoles (courtesy of Karen Kalenetra). Bottom right image shows large, spherical Thiomargarita-like bacterial cell adjacent to a Beggiatoa filament.

Images at the left and center show the elemental sulfur deposits along the periphery of the cells (images courtesy of K. Kalanetra). Middle images shows a quartet of cells (courtesy of K. Kalanetra). At the hydrate sites, these microbes were found living with both orange and white Beggiatoa, right image.

We thank the National Science Foundation's Life in Extreme Environment's program for supporting this work.

**Disclaimer** The content of this page is based in part on work supported by the National Science Foundation. Any opinions, findings, and conclusions or recommendations expressed here are those of the author (Mandy Joye) and do not necessarily reflect the views of the National Science Foundation.