ABSTRACT
Various microorganisms, including prokaryotes and eukaryotes, contribute actively to certain geological processes, a fact that until not too long ago seems not always to have been suf ciently appreciated by some microbiologists and geologists. Geomicrobially active prokaryotes include members of the domain Bacteria (formerly designated Eubacteria) and the domain Archaea (formerly known as Archaebacteria or Archaeobacteria). Both Bacteria and Archaea are prokaryotes because they lack a true nucleus. Each has its genetic information encoded in a large circular polymeric molecule of deoxyribonucleic acid (DNA). This structure is often called the bacterial chromosome; but unlike the chromosomes of eukaryotic cells, it does not contain structural protein such as histone, nor is it surrounded by a nuclear membrane. The molecular size of a prokaryotic chromosome measures on the order of 109 Da. Some genetic information in prokaryotes may also be located in one or more extrachromosomal circularized DNA molecules, called plasmids. The exact molecular sizes of different plasmids vary, depending on the amount of genetic information they carry but generally range ∼107 Da.
