|MadSci Network: Microbiology|
Hi Celeste, and thanks very much for your question! You have specifically asked about one particular species of bacteria, but I'd like to start with a bit of background.
All types of bacterial cells die from one of two types of causes - those derived from problems either external or internal to the cell. The most typical external causes include 1) lack of nutrients (food), 2)not being able to create energy, 3) exposure to extreme conditions (such as temperature or pressure), 4) exposure to toxic materials or poisons, or 5) grazing, or being eaten by larger organisms. Reasons 1, 2, and 5 are by far the most common in the environment. Typical internal problems include 1) genetic mutation, or 2) cell "suicide" (apoptosis). It's also important to realize that bacteria don't die of old age like people do... they simply continue obtaining enough nutrients to grow and reproduce through cell division until one of the above problems occurs.
Now let's take a look at the species you were asking about... Thiomargarita namibiensis. This is the largest bacterium ever found, and has several strange characteristics, including the chemicals it uses to create energy, and its size. All bacteria need both an energy-rich chemical and an electron acceptor to create energy that they can use. The electron acceptor most commonly used by bacteria that live at Earth's surface is oxygen. However T. namibiensis lives in the anoxic zones of ocean sediments, which means no oxygen is available. So they use nitrate from the ocean water as an electron acceptor instead. The interesting thing is that since this bacterium lives in the sediment, they can't move around and easily access nitrate, which is most abundant in the water itself. Once they use up the nitrate that exists in the sediment pores in which they live they have to wait an occasional storm passes that stirs up the sea floor and mixes the sediment and water together. Then they can grab the new nitrate that has been mixed in. However, since this only happens occasionally, how do they survive those times when storms aren't occurring? It seems T. namibiensis cells evolved an unusually large hollow center in the middle of their cells. This space allows them to hoard a large amount of nitrate whenever a storm does pass. That means they can continue creating energy in between storms. This storage area is so large it makes their cell diameter up to 0.8mm!! This is visible by the naked eye, which is extremely unusual for a bacterium. More information is available here Thiomargarita namibiensis
So how do T. namibiensis cells die? Well let's consider the external reasons I listed earlier. 1) The ocean sediments in which T. namibiensis lives tend to have large amounts of nutrients available, so there is not a large risk of them running out. 2) Because of their unusual ability to hoard nitrate, T. namibiensis cells ensure stable conditions in which they can create energy. 3) The environmental conditions in sediments are generally quite stable, with the exception of mixing of water and sediments due to currents and storms. So extreme temperatures and pressures are unlikely. 4) Exposure to toxic chemicals is a possibility, but also rare in ocean sediments. 5) Death by grazing is also possible, but due to the very large size of T. namibiensis this may be difficult by all but the largest zooplankton. In addition since the cells live in the sediment they would be quite difficult to reach. What about the internal reasons? 1) Genetric mutations are random changes in the way an organisms' DNA is created and used. These can affect the ability of cell to perform any of the basic life function. Mutations occur frequently and are a common cause of cell death if a cell is well-suited to its local environmental conditions. 2) Cell "suicide", or apoptosis is described in detail on this page, although it may be a little advanced cell apoptosis.
So in summary, I think that T. namibiensis has adapted to its environment so well that the chances of cells dying due to environmental conditions are small. I think it is more likely that mutations or cell "suicide" are what kills them.
Try the links in the MadSci Library for more information on Microbiology.