Life Sciences Icon Life Sciences

With an Assist from Nature, DNA Travels the Globe

Evolution News

Like Johnny Appleseed, currents spread life far and wide. DNA doesn’t need wings to colonize the world. Earth supplies the transportation.

Desert Winds

Armando Azua-Bustos and his team weren’t expecting to discover a transportation system. The astrobiologist from the University of Madrid had laid out petri dishes in the Atacama Desert, one of the driest spots on earth. He knew that even in this inhospitable part of Chile, some microbes manage to survive. Curious how they get there, he laid out his dishes in long lines from the desert to the coast (see Astrobiology.com for maps). Michael Marshall reports on the discovery in New Scientist.

“I didn’t expect much,” says Azua-Bustos. But they found 28 species of microorganism growing on the plates, and extracted DNA from several more. 

The microbes came from near the coast. Azua-Bustos says such microbes may have been the first to colonise the Atacama. He highlights Oceanobacillus oncorhynchi, which lives in tidal pools. Because the pools dry out in the heat of the day, it can survive being dried out for hours — giving it a chance of surviving the Atacama. [Emphasis added.]

The findings show that “bacteria fly into the Atacama Desert every afternoon on the wind.” The microscopic passengers, loaded with DNA and molecular machines, rise 4,000 feet over mountains and valleys to their new homesteads 50 miles inland, carried in dirigibles made of fine dust, lofted by gusts of wind.

As an astrobiologist, Azua-Bustos wondered if life could colonize Mars in a similar way. Mars has wind, for sure, but no life has yet been found. The presence of perchlorates and salts everywhere makes it unlikely it Mars was ever inhabited. But he’s right about one thing: intelligent design could get it started.

If life can be transported around Mars, any contamination from our space probes could spread rapidly. “If we do carry contaminants to Mars, they could easily be dispersed by the Martian wind, as we have seen in the Atacama,” says Azua-Bustos.

 The team’s paper in Nature Scientific Reports adds:

Finally, our results are of critical application for planetary protection, as terrestrial microorganisms hitchhiking in rovers and landers (and their discarded landing material) may have been widely dispersed all over the Martian surface by planetary-level dust storms.

Vikings 1 and 2 were the first landers in July 1976. Before then, presumably Mars had a transportation system but no passengers.

Volcano Rafts

While sailing their catamaran from Australia to Fiji, Michael Hoult and Larissa Brill were in for a surprise. Instead of water, they found themselves surrounded by rocks. An underwater volcano had erupted vast quantities of pumice, the BBC News says, which floated to the surface and carpeted the sea in pebbles ranging in size from marbles to basketballs. The pumice raft, scientists estimated, was equal in area to 20,000 football fields. Two other sailors, Tom Whitehead and Shannon Lenz, said the rocky carpet was so thick, they couldn’t see the water. You can take a look at it in a video included in the article. 

Pumice rafts illustrate another geophysical, abiological transportation system for living creatures. Small marine organisms, including coral polyps, barnacles, and microbes, can attach to the floating rocks and drift far and wide. Live Science notes that the potential for this kind of transportation was only recently appreciated.

Pumice rafts are often teeming with sea life, such as new barnacles and corals, Scott Bryan, a professor of geology and geochemistry at Queensland University of Technology, told CNN. Bryan reported in a 2012 study that these rocky rafts can be a way to redistribute life across the ocean.

The pumice raft, with its passengers, is heading toward several islands in the South Pacific. It may reach the Great Barrier Reef, which has suffered a catastrophic “bleaching” over the last few years. Many feared the worst, “But scientists think that the structure has a chance of bouncing back, potentially with gifts brought in by its rocky visitor.” Coral polyps carried by the raft might ride on these natural delivery trucks. Bryan added, “This is a potential mechanism for restocking the Great Barrier Reef.”

Undersea volcanic eruptions may be more common than thought, because they are difficult to detect. One of the largest on record was described in Science Advances in January last year. Like the recent one, that eruption was first noticed in the form of a pumice raft covering 400 square kilometers. Undoubtedly its pebbles also ferried small passengers, with their DNA and molecular machines, to new destinations.

Underwater Networks

More cryptic than the cryptobiotic soils that transform deserts into gardens (see “Intelligent Design in the Dirt”) are cable bacteria living inside undersea sediments. These amazing bacteria, only recently described, have a global impact by regulating oxygen, carbon, and other chemicals required for life. Found in marine and freshwater sediments, they grow into filaments consisting of thousands of cells each. These filaments transport electrons like wire cables, forming a vast electrical grid under the sea. A new paper in PNAS tells about them:

Cable bacteria are globally occurring multicellular filamentous bacteria that are electrically conductive: they transfer electrons from sulfide oxidation at one end over centimeter distances to oxygen reduction at the other end. Unlike any other organism known, cable bacteria thus split their central energy-conserving redox reaction into 2 half-reactions that occur in different cells as far as several centimeters apart. Before this study, the molecular foundation, evolutionary origin, and genomic basis of this unique metabolism were unknown.

Good luck finding an “evolutionary origin” for cable bacteria. The authors say, “A consistent model for cable bacteria metabolism and intra- and intercellular electron transport is currently lacking, and the evolutionary origin of this unique lifestyle remains unclear.” It must have come about by some kind of “innovation,” they suggest. Paul Nelson will enjoy their quandary about the orphan genes discovered in these bacteria:

Approximately 20% of the cable bacteria-specific genes could not be assigned to an ancestral lineage, indicating an origin from a genomically undescribed donor lineage or that they arose from evolutionary innovation. The functional role of these cable bacteria-specific gene families could not be determined, as almost half of them encode hypothetical proteins and 71% could not be assigned a COG category… Remarkably, however, approximately one third of them has a predicted extracytoplasmic or membrane localization (SI Appendix, Fig. S1B), suggesting major innovations in the cell envelope, which is consistent with the conspicuous ultrastructure of the cable bacterium cell envelope.

The scientists eventually conclude that non-Darwinian processes led to these creatures. And if miracles of chance happened here, might they be happening elsewhere? The evidence “suggests… that

the divergence of the cable bacteria clade (Fig. 1B) and the evolution of the unique filamentous “electrogenic lifestyle” of its members were driven by neither massive gene loss nor major genome expansion. Rather, substantial horizontal gene transfer in combination with the moderate replacement and divergence of ancestral genes was key for the evolution of cable bacteria. This could imply that electric conduction along filaments may also have evolved in other phylogenetic lineages….

More interesting than evolutionary quandaries about cable bacteria is the significance of what they do for the planet. The authors begin by describing their key role in forming bedding surfaces with the right chemistry for other organisms, enhancing the habitability of the earth:

Cable bacteria can dominate benthic microbial communities, and the cable bacteria-associated electrogenic sulfur oxidation (e-SOx) can account for the larger part of a sediment’s oxygen consumption. The metabolic activity of cable bacteria has distinct effects on sediment biogeochemistry, especially on the cycling of sulfur, iron, phosphorus, and nitrogen, with implications for eutrophication and habitability of sediments. Notably, cable bacteria induce the formation of a 1- to 4-cm-deep suboxic zone devoid of O2 and H2S and a pronounced pH peak in the oxic zone, consistent with the consumption of protons during O2 reduction to water.

Articles in the references explain how these bacterial cables regulate oxygen and other minerals that sea life depends on. Another paper in Nature Communications explores “Subseafloor life and its biogeochemical impacts.” The open-access paper shows how life under the seafloor plays a significant role in carbon and oxygen cycles.

Oxidation-reduction (redox) reactions transfer electrons from one chemical to another. This transfer constitutes one of the most significant processes on Earth’s surface. The primary driver of redox activity on Earth’s surface is oxygenic photosynthesis, which converts water and carbon dioxide into oxidized and reduced compounds (O2 and organic matter, respectively). Organic-fueled respiration reverses this process by oxidizing the organic matter.

…. Consequences of this long-term oxidation include Earth’s astonishing diversity of oxygen-reliant organisms, metabolites, and oxidized minerals.

In short, “Because subseafloor communities reside at the interface between the biologically active surface world and the large geological reservoirs of biologically important chemicals, subseafloor microbial activities play a fundamental role in Earth’s biogeochemical cycles.” This role has not been appreciated before recent times, and much more needs to be learned about it. Would the earth even be habitable without this global electrical grid in the seafloor? That would make a good research project for design advocates.

Implications

Last month, New Scientist celebrated the 100th anniversary of James Lovelock’s “Gaia Hypothesis.” While frequently dismissed by scientists for its New Age flavor and Greek-goddess overtones, the Gaia hypothesis did have an influence on scientists:

Lovelock’s idea is that life on Earth acts to preserve its own existence, by stabilising conditions on the planet. Popularised in the 1970s, it inspired a generation of scientists who study Earth’s many systems, from the climate to forests, and how they interact as a whole.

One need not deviate into religious speculations, including pantheistic ideas like Gaia. The fact is, good observational science shows that the earth’s living and nonliving processes interact in remarkable ways to sustain life. Products of mindless evolution cannot “act to preserve their own existence.” Only a designing intelligence would have the Foresight to give a Privileged Planet the requirements to allow a Privileged Species, endowed with mind, to stand in awe at the power of intelligent design.

Photo: Atacama Desert, Chile, ESO/S. Lowery [CC BY 4.0], via Wikimedia Commons.