The Modern Concept of the Procaryote

In their seminal work, Stanier and van Niel (47) proposed that bacteria are distinguished from other forms of life, including viruses, protists, fungi, algae, plants, and animals, by their procaryotic cell structure. They defined the procaryotic cell by three major criteria: the absence of internal membranes that compartmentalize the nuclear material and the enzymatic machinery for respiration and photosynthesis, nuclear division which occurs by fission and not mitosis, and the presence of peptidoglycan in the cell wall. They also recognized that procaryotes possess enormous diversity, similar in scope to that of eucaryotic protists, and further proposed that the procaryotes represent a distinct mode of evolutionary diversification parallel to that found in the eucaryotes (47). Thus, the procaryotic-eucaryotic dichotomy was founded upon the recognition of two very different types of cellular organisms and not the phylogenetic relationships between them. Recently, five major criticisms of the concept of the procaryote have been proposed (36, 55): it fundamentally contradicts the three-domain model of life, the procaryotes are not monophyletic, the procaryotes are defined by negative characteristics, the term procaryote “sustain[s] the concept that procaryotes evolved into eucaryotes,” and the term is imprecise. As shown below, these criticisms are misstatements of the original proposal and modern descriptions of the concept or otherwise erroneous (47, 27). In fact, since its original proposal in 1962, the experimental evidence for the procaryote concept has been enormously enriched. Today, the concept of the procaryote includes a much greater understanding of the molecular basis for the differences between procaryotic and eucaryotic cells. In addition, it recognizes the antiquity, abundance, and diversity of procaryotes. Procaryotes likely dominated life on the early earth for over a billion years prior to the appearance of eucaryotes. Today, the biomass of the procaryotes is comparable to that of eucaryotes. The procaryotes are also extremely diverse, and representatives of two ancient domains, Bacteria and Archaea, are common today. Each domain includes organisms with many different metabolic and physiological capabilities, and the number of species is correspondingly so large that it has never been estimated accurately. The procaryotic cell. The cellular organization of procaryotes is of fundamental importance to their physiological and biochemical processes, and their differences from those of