ns 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.