In the Science journal article, GFAJ-1 is referred to as a strain of Halomonadaceae and not as a new species. The International Code of Nomenclature of Bacteria, the set of regulations which govern the taxonomy of bacteria, and certain articles in the International Journal of Systematic and Evolutionary Microbiology contain the guidelines and minimal standards to describe a new species, e.g. the minimal standards to describe a member of the Halomonadaceae. Organisms are described as new species if they meet certain physiological and genetic conditions, such as generally less than 97% 16S rRNA sequence identity to other known species and metabolic differences allowing them to be discerned apart. In addition to indicators to tell the novel species from other species, other analyses are required, such as fatty acid composition, respiratory quinone used and tolerance ranges and deposition of the strain in at least two microbiological repositories. New proposed names are given in italics followed by sp. nov. (and gen. nov. if it is a novel genus according to the descriptions of that clade).
In the instance of the GFAJ-1 strain these criteria are not met, and the strain is not claimed to be a new species. When a strain is not assigned to a species (e.g. due to insufficient data or choice) it is often labeled as the genus name followed by "sp." (i.e., undetermined species of that genus) and the strain name. In the case of GFAJ-1 the authors chose to refer to the strain by strain designation only. Strains closely related to GFAJ-1 include Halomonas sp. GTW and Halomonas sp. G27, neither of which were described as valid species. If the authors had formally assigned strain GFAJ-1 to the genus Halomonas, the name would be given as Halomonas sp. GFAJ-1.
The Genome Taxonomy Database assigns GFAJ-1 its own tentative species, Halomonas sp002966495. This means that the strain falls into Halomonas phylogenetically, and its whole-genome similarity compared to other defined species of the genus is low enough. Neither strain GTW nor strain G27 has a genome available for the database to run its classification.