Question

Does the astrobiology community still hold with the 5 kingdom classification system, or have other kingdoms been introduced and discussed?

This is an excellent question which focuses on one of the most active areas of modern day biological and biochemical research. The short answer is that the old five Kingdom classification scheme has been superceded by a three "Domain" classification: Eubacteria, Eukaryota, and Archaea, with several so called 'kingdoms" within each domain. The Eubacteria (true bacteria) are prokaryotic - single celled organisms with no discreet nucleus. The Archea are also prokaryotic, but differ sharply from bacteria in their metabolic processes. The Eukaryota can be single- or multicellular, but are marked by the presence of a discreet, membrane-bound nucleus. This domain includes things from slime molds to diatoms to the familiar fungi, plants, and animals. The new classification was begun in the mid-1970's, about the time organisms termed "extremophiles" were first being identified from mid-ocean vents and other places where extreme environmental conditions (heat, pressure, salinity, pH) would suggest that no life forms could exist there. When the DNA sequences of some of these extremophiles was analyzed and compared with sequences from the other kingdoms, it was found that there were commonalities with bacteria and some of the higher plants and animals (Eukaryota), but that they had some cellular and molecular characteristics for which they are entirely unique in all the living world. These became the Archea.

Astrobiologists are interested in the Archaea (especially) because they grow and thrive in environments on earth that are analogous to environments that currently exist or previously existed on other planets or moons in our solar system.

For further reading, NAI suggests: Woese, C.R. Microbiology Review. 51: 221 (1987) for an early development and defense of the three Domain classification, and Doolittle, W.F. Phylogenetic Classification and the Universal Tree. Science. 284: 2124-2128 (25 June, 1999) for a modern summary of "molecular phylogenetics" and the tree of life.
October 30, 2001