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.
Analog examples of what primeval oceans might have looked in the Precambrian are probably extant in various regions
and at various size scales in present day oceans albeit they have not been sufficiently recognized and/or studied. The
Eastern Boundary Current Ecosystems (EBCEs), with their characteristic high productivity-inducing coastal upwelling
events, their extensive and intensive anoxic/hypoxic water column and methane and sulfide-rich benthic environment,
appear to represent such analogs. Moreover, recent studies have shown that they possess diverse anaerobic prokaryotic
communities of mat-forming large multi-cellular filamentous bacteria similar to fossils found in Archean and Proterozoic
rocks. Observations in the Bay of Concepcion, central Chile (~36°S), inserted in the second most productive EBCE of
the world, suggests that given similar oceanographic dynamics, past oceans may have presented different predominant
colorations after the first probable "red" color of the reduced iron-rich Archean ocean and prior to the present day
"blue" color. In this coastal ecosystem a "black" coloration has been observed to form as the result of the floating to the
surface layer of sulfide-blackened benthic detritus together with chunks of microbial mats, and a "milky to turquoise"
coloration resulting from different concentrations of colloidal, nano-sized particles which may include elemental sulfur
and/or microorganisms. If the present is the key to the past we posit that "black" color oceans could have existed during
the Proterozoic "Canfield sulfidic ocean" followed by "milky to turquoise" colored oceans during later stages of the
Proterozoic. Meso-scale examples of "milky" and "turquoise" portions of oceans, caused by elemental sulfur from
oxidized hydrogen sulfide eruptions, have been described from off Namibia and there appear to also exist elsewhere.
Examples of "black" oceans have apparently not been reported but the name of the Black Sea, the largest permanent
anoxic basin on Earth, suggests that at some point in time it may have been black, at least locally and/or for short
periods, prompting the name. We conclude suggesting that analogous to the present "Blue Planet" denomination, in the
past our Earth could possibly have deserved the successive names of "Red", "Black" and "Milky-Turquoise" Planet.
In the soft reduced sediments of the continental shelf, below the oxygen minimum zone (OMZ) of the eastern
South Pacific (ESP), peculiar microbial communities have been disclosed which include a variety of large prokaryotes,
protists and small metazoans. Dominant among the prokaryotes are large multi-cellular filamentous bacteria which,
according to their size range, are roughly divided into megabacteria and macrobacteria. The former group is made up of
a few species of Gamma Proteobacteria of the genera Thioploca and Beggiatoa and the second group includes a diversity
of phenotypes. Protists include ciliates, flagellates, and foraminifers and the metazoans are mostly nematodes and small
polychaetes. A significant similarity has been found in the exploitation of the area/volume relationship among these large
bacteria and their fossil analog forms as described from pre-Cambrian rocks. For the same reason, the latter have mostly
been referred to as algae or cyanobacteria in the literature. The presence of these seemingly ancient bacteria in the
sediments of the oxygen minimum zones of the ESP, one of the most productive but also ecologically most inefficient
marine ecosystems of the world, suggests that such setting must have prevailed throughout the geological history of the
planet allowing for their survival and further that it might be considered an analog of Proterozoic ocean conditions. These
non-cyanobacterial communities offer an alternative hypothesis to students of the evolution of life on Earth and may be
of special interest to astrobiologists looking for life or traces of life in terrestrial or extraterrestrial environments since
these do not necessarily imply a photosynthesis-based metabolism.
Conference Committee Involvement (3)
Instruments, Methods, and Missions for Astrobiology XIII
3 August 2010 | San Diego, California, United States
Instruments, Methods, and Missions for Astrobiology XII
4 August 2009 | San Diego, California, United States
Instruments, Methods, and Missions for Astrobiology XI
12 August 2008 | San Diego, California, United States
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