It is necessary to realize that the term "photosynthetic prokaryotes"
encom- passes the widest and most diverse grouping of bacteria, but in
itself has no taxonomic or phylogenetic significance. It means those
organisms, other than eukaryotes, which require, obligately or
facultatively, light for growth. The re cent application 16S rRNA
sequencing to microbial phylogeny, asso- ciated mainly with the work of
Woese, emphasizes the evolutionarily dis- persed nature of purpIe and
green photosynthetic bacteria as weIl as the rather coherent
phylogenetic connections of the cyanobacteria. It is not surprising,
therefore, that a volume such as this which seeks to give an
introduction to this collection of organisms must be highly selective;
ac- cordingly, several important features are discussed only
superficiaIly, e. g., differentiation, life cycles, and biochemical
aspects of nutrition. Rather, we have attempted to provide adescription
of the essential features which are common and those which are
characteristic, e. g., the physiology of photo- synthesis and ecological
distribution. Bearing in mind the aim of the series, we have asked our
authors to emphasize aspects of the importance of these organisms in
nature and their industrial applications. As will be seen from the text,
these organisms have an ancient history in "biotechnology," having been
used as foodstuffs by several cultures, but their exploitation has been
limited to their natural patterns and products of growth.