OK, so this is a little like the chicken and the egg, which comes first, a smaller genome, or a smaller cell. Is a cell small because it has a small genome, or is a genome small because the cell is small. My answer would favour the latter, though in terms of evolution, the processes will largely feedback and complement each other.

When you keep it simple, and a bacterial cell is (comparatively) simple, then you simply don't require the information required for more complex organisms. Though there are still many complex regulatory networks employed in bacteria that are still required, despite the small genome, for the appropriate and temporal expression of the encoded information.

It is not strictly true that introns do not exist in bacteria. A group of intron-like structures termed Group II introns have been identified in several bacterial genomes, though it is thought that they don't perform true intron functions, they are probably the ancestors of spliceosome introns, but the course of this evolution is not yet clear. The publications of a Canadian research group active in this area can be found here: [http://www.ucalgar y.ca/~zimmerly/Publication.htm]. Some more information on the nature and conservation of introns can also be found in an archived MadSci response [here ].

Now, back to small cells. The most appropriate response to why bacteria are small is this: because they are. This is not a dismissive answer. Simply put, natural selection is a voracious force on any organism, and with the mutation rate and generation time for bacteria, effects of envonment are rapidly accomodated, or the cells die out. If bacteria are still small after 3.x billion years, then that tells you that there is something eminently important about being small. In this case, it is due to the simple relationship between volume and surface area (we shall assume a sphere, though clearly not all bacteria are spherical).

Bacteria interact with their environments on a molecular level. Cells are constrained in their size because they have to exchange materials with the environment; materials such as water, gases, and nutrients. Cells pass these materials through their outer surface largely by diffusion as they lack many of the more complex membrane transport mechanisms found in the larger eukaryotic cells (which can be large for precisely this reason). The bigger a cell gets, the larger the volume in relation to the surface area - so at a certain point, the surface area is no longer sufficient to permit entry of nutrient required to maintain the greater volume - the same principle is also reponsible for the size limitation of insects and many other orders of invertebrates. Incidentally, the biggest bacterial cells are those of Thiomargarita namibiensis - this is 100x bigger than the previous record holder Epulopiscum fishelsoni~, which was itself 600x bigger than your average bacterial cell - basically, you can practically see it with the naked eye [See here for more info].

I hope this answer satisfies your interest in this area and as ever raises more questions than it has answered (that is of course the point of science, otherwise it'd be an awfully boring world).

Jim

MAD Scientist