Washington DC -- A group of German, Spanish, and American researchers sampling sediment off the coast of Namibia have stumbled across the biggest bacteria ever known. The largest of these single-celled microbes is visible to the naked eye, about as big as the period at the end of this sentence and nearly 100 times larger than the previous bacterial record-holder. In addition to its giant size, the new microbe is an exotic organism that provides firmer evidence of coupling between two key environmental cycles thought until very recently to be mutually exclusive in the ocean: the sulfur and nitrogen cycles. The finding is described in the 16 April 1999 issue of Science.
"When I told them, my colleagues at first didn't believe me because the bacteria were so big," recalls Heide Schulz, the Ph.D. student at Max Planck Institute for Marine Microbiology who found the bacterial behemoths glistening in sediment they had pulled into the research boat. "But I've been working with exotic bacteria for a while now and I knew immediately that these were sulfur bacteria."
The researchers named the new bacteria Thiomargarita namibiensis, which means "Sulfur Pearl of Namibia." The microbes store elemental sulfur just under the cell wall as well as nitrate in a huge central sac, which shines with an opalesque, blue-green whiteness. They also grow loosely attached in strings, leading the researchers to compare them with strands of outlandish pearls. The largest cells are three-quarters of a millimeter in diameter. But to get a better idea of how big this is, it helps to make an analogy: If the largest Thiomargarita was a blue whale, then an ordinary bacterium would be a bit smaller than a new-born mouse. The largest previously known bacterium -- Epulopiscum fishelsoni, which lives in the guts of surgeonfish -- would be about as big as a lion.
Along with lead-author Schulz, the Science paper is co-authored by Timothy Ferdelman and Bo Barker Jorgensen, also of Max Planck Institute; Thorsten Brinkhoff of the University of Oldenburg in Germany; Mariona Hernández Mariné of the University of Barcelona in Spain; and Andreas Teske of the Woods Hole Oceanographic Institution in the United States. The scientists had originally gone to the Namibian coast looking for two other kinds of sulfur bacteria, Beggiatoa and Thioploca, which they had been studying off the Pacific coast of South America. The hydrography there and off the coast of Namibia is very much alike: both feature strong ocean currents running parallel to a north-south continental shelf. The eastward motion of the turning Earth pushes the currents to the west and causes an upwelling of deep ocean water that is unusually rich with the nutrients on which phytoplankton -- and thus other marine organisms -- thrive.
Once the scientists reached Namibia, however, they were surprised to find only scant levels of Beggiatoa and Thioploca. Instead, the sediment teemed with the previously unknown Thiomargarita.
Both Thioploca and Thiomargarita, which are close genetic cousins, face the same ecological challenge: how to oxidize ("eat") sulfide with the help of nitrate. Nitrate is present in sea water, but usually does not penetrate the oxygen-poor, sulfide-rich sediment where these bacteria are trying to eke out a living. What makes their survival possible is their unusual capacity to store both sulfur and nitrate: "This couples the sulfur and nitrogen cycles," says Schulz, "perhaps to a degree not previously given enough credence."
Such coupling is a matter of intrinsic interest to those wishing to understand the origin and nature of life. Earth's environment and the life it supports depend on the constant recycling of certain key elements, such as carbon, nitrogen, and sulfur. Microorganisms are major contributors to this recycling because they facilitate critical reactions known as reduction and oxidation. These reactions enable the easy transfer of elements to the oceans, sediments, and atmosphere, and eventually to other organisms.
The newly identified giant Thiomargarita bacteria join Thioploca in a small class of microbes now known to link the sulfur and nitrogen cycles. Given that sulfide is abundant in the plankton-rich upwelling regions where the microbes have been found, Schulz wonders whether "this way of oxidizing sulfide might be more important to the environment than we've thought."
Despite their commonalties, Thioploca and the giant Thiomargarita have developed strikingly different ways of scavenging. Thioploca cells form filaments that cling to each other and secrete an encompassing sheath of mucous film. The sheath serves as a kind of vertical tunnel through the sediment up to the overlying water, allowing the Thioploca filaments to glide up and down and thereby commute between their food source and the nitrate they need to metabolize it. Thiomargarita microbes, on the other hand, don't form filaments and are not mobile. They exist in strands of single, unattached cells evenly separated by a mucous sheath. Most of the strands observed by Schulz and her colleagues were linear and contained an average of 12 cells. The longest chains of 40 to 50 cells tended to break apart easily when manipulated; a few chains branched or coiled together in a ball.
Significantly, Thiomargarita appeared most abundantly (and Thioploca not at all) in an unusually loose, fluffy kind of plankton-rich sediment -- a fluid, green ooze easily suspended in shifting ocean currents. Schulz speculates that Thioploca's vertical sheaths aren't well-supported in such ooze, whereas the balloon-like Thiomargarita bacteria can float passively, storing sulfur until they happen to be brought into contact with the nitrate they need. Without the power to actively roam, however, Thiomargarita risk long stretches of nitrate starvation, something they appear to endure without much trouble. As Schulz says, "they can't move but, unlike Thioploca, they can hold their breath for months until they get what they need."
The giant microbes are also remarkable for their ability to withstand high levels of oxygen and sulfide that would prove fatal to their commuting cousins, the Thioploca. These and other odd characteristics, says Schulz, promise to make "the Sulfur Pearls of Namibia" a subject of much research in the years to come.
"Dense Populations of a Giant Sulfur Bacterium in Namibian Shelf Sediments" by H. N. Schulz, T. G. Ferdelman, B. B. Jorgensen at Max Planck Institute for Marine Microbiology in Bremen, Germany; T. Brinkhoff at University of Oldenburg in Oldenburg, Germany; M. Hernández Mariné at the University of Barcelona in Barcelona, Spain; A. Teske at Woods Hole Oceanographic Institution in Woods Hole, MA. Contact: Heide Schulz at 49-421-2028-646 (phone), 49-421-2028-690 (fax) or firstname.lastname@example.org
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