Archaeon and archaeal virus diversity classification via sequence entropy and fractal dimension
2010
Archaea are important potential candidates in astrobiology as their metabolism includes solar, inorganic and
organic energy sources. Archaeal viruses would also be expected to be present in a sustainable archaeal
exobiological community. Genetic sequence Shannon entropy and fractal dimension can be used to establish a
two-dimensional measure for classification and phylogenetic study of these organisms. A sequence fractal
dimension can be calculated from a numerical series consisting of the atomic numbers of each nucleotide.
Archaeal 16S and 23S ribosomal RNA sequences were studied. Outliers in the 16S rRNA fractal dimension and
entropy plot were found to be halophilic archaea. Positive correlation (R-square ~ 0.75, N = 18) was observed
between fractal dimension and entropy across the studied species. The 16S ribosomal RNA sequence entropy
correlates with the 23S ribosomal RNA sequence entropy across species with R-square 0.93, N = 18. Entropy
values correspond positively with branch lengths of a published phylogeny. The studied archaeal virus
sequences have high fractal dimensions of 2.02 or more. A comparison of selected extremophile sequences with
archaeal sequences from the Humboldt Marine Ecosystem database (Wood-Hull Oceanography Institute, MIT)
suggests the presence of continuous sequence expression as inferred from distributions of entropy and fractal
dimension, consistent with the diversity expected in an exobiological archaeal community.
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