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More on How Microbes Make Earth Habitable

Tara Oceans.jpg

Last week we discussed how microbes maintain the viability of life on our planet ("How Microbes Make Earth Habitable"). In the same vein, more news just arrived about the carbon and nitrogen cycles.

Following up on MIT's findings about plankton taking carbon to the ocean bottom, Ohio State has just concluded from the three-year Tara Oceans oceanography expedition that plankton -- and some viruses -- are "key to carrying carbon" from the atmosphere down to the seafloor sediments. They took pictures of microbes at different depths to learn which ones are carrying carbon downward as they sink to the bottom. The research has helped scientists understand "the interplay of organisms in the ocean and of their role in the health of the planet."

As for nitrogen, Current Biology posted a Primer on "The Nitrogen Cycle," confirming that it "is entirely controlled by microbial activities." Before man started producing chemical fertilizers, "nearly all of the reactive nitrogen in the biosphere was generated and recycled by microorganisms."

Microbes Follow Icebergs

Another interesting interaction between geology and life was reported by the BBC News. Mark Kinver writes that "Giant icebergs play [a] 'major role' in [the] ocean carbon cycle." An aerial image shows a brilliant bloom of phytoplankton following one such giant iceberg in its wake. What's happening? The bloom is "triggered by the distribution of nutrients -- such as iron -- from an iceberg's meltwater." What this signifies is important:

"We estimate that giant icebergs account for between 10% and 20% of the actual vertical rate of carbon going from the surface to the deep (Southern) Ocean," he suggested....

Plankton scientist Dr Richard Kirby, who was not involved in this study, observed: "The phytoplankton at the sunlit surface of the sea has played a central role in the sequestration of carbon over millennia to affect the atmospheric concentration of this greenhouse gas, and so the Earth's climate.

"This interesting paper shows how much we still have to learn about these microscopic organisms, and how a changing climate may affect them, and also the food web they support." [Emphasis added.]

Bacteria Attack Lignin

Readers of Evolution News may remember Ann Gauger and Matt Leisola's design inference about lignin, the complex molecule that gives wood its strength. They argued that no organism has exploited this energy-rich substance -- which is good news for us, because "the indigestibility of lignin may be an essential requirement for the balance of life," Gauger wrote. "Lignin slows the degradation of wood, thus allowing the buildup of humus in the soil, which in turn permits plant growth and all resulting life that depends on plants."

Now, news from Rice University says that bacteria use a "tag team" approach to break apart this complex substance that locks up more than half a plant's sugar and holds a third of the carbon in biomass. This finding does not impact the conclusions of Gauger and Leisola, because the bacteria "chew through" the lignin to get to the cellulose they can digest. But "it's a very slow process, which is why it can take years for dead trees to decompose." The rate of decomposition of this energy-rich substance is what's interesting for considerations of planetary carbon cycling and habitability.

Bacteria Prepare the Land for Habitation

Another interesting interaction of microbes with geology happens on land. Biocrusts are accumulations of cryptic bacteria that inhabit the surfaces of arid soils. A paper in Nature Communications notes the ecological impact of these microbial habitats:

Much of the arid soil surfaces can be populated by cryptic photosynthetic assemblages known as biological soil crusts (or biocrusts), which are known to impart stability against erosion, to modify the hydrological properties of soils, and to contribute significantly to arid land fertility.

Researchers from Arizona and California found that the bacteria, mostly the nitrogen-fixing variety, produce a secondary metabolite called scytonemin that acts as a "sunscreen". It strongly absorbs solar radiation and dissipates it as heat, raising soil temperatures as much as 10°C, an effect that is "not without consequences" for soil microbial communities. While potentially making deserts more arid, this effect is good for temperate and arctic zones: "biocrusts in cold climates, that is, in polar settings or during winter months when activity is limited by temperature, the warming can be expected to be largely beneficial to biological entities."

About 41 percent of the earth's surface is arid, and this is where biocrusts figure prominently. By producing this sunscreen molecule, bacteria protect the soil from harmful radiation while simultaneously raising the temperature in cold arid lands to promote the growth of complex plants in ecological succession -- all this while stabilizing the soil, reducing erosion and fixing nitrogen that higher plants need. It's another case that has just come to light of global benefit from the world's smallest organisms.

For these reasons, microbial effects on land surface albedo may have global scale repercussions historically and in the present, and should be evaluated in models of planetary radiation budgets as a new twist of biosphere-climate feedback interactions. This may shed some light into the apparent inconsistency in temperature changes recorded in arid lands. Contrary to model predictions, temperature has been shown to increase when removing vegetation there, an apparent paradox that could potentially be explained by biocrust colonization. Based on estimates of the global biomass of cyanobacteria in soil biocrusts, one can easily calculate that there must currently exist about 15 million metric tons of scytonemin at work, warming soil surfaces worldwide.

Big Animals Help Earth's Habitability, Too

From orbit, a bear would appear as small as a microbe. Current Biology published another indication that all living things, even the big ones, have a stake in keeping the earth habitable. In "Megafauna move nutrients uphill," Michael Gross points to research about whales, salmon, birds and bears that reveal large animals' roles in moving phosphorus and nitrogenous compounds from the deep ocean to the mountains. These "ecosystem engineers" perform an "important ecosystem service" by distributing nutrients to higher elevations.

Given the laws of gravity and the hydrological cycle, there is a strong likelihood that nutrients available on land, even though they may go through many cycles around the food web, will eventually be washed out to the sea. In the oceans, there is the risk that they will drop out of the photic zone and reach the sea floor, where they will be buried in sediment that may only be returned to circulation on geological timescales, some tens or hundreds of millions of years later.

Animals can make important contributions to stem this flow, as was first reported for whales back in 2010 (Curr. Biol. (2010) 20, R541). Researchers studying the ecology of sperm whales found that they harvest nutrients such as iron from great depths (often more than 1,000 metres), where they hunt cephalopods, but release them when they defecate near the surface.

The nutrients are incorporated into microbes, plankton, and fish that become prey for birds and mammals. These larger animals, in turn, transport the nutrients up rivers and mountains. Although Michael Gross focuses on how humans are disrupting this natural system, one can only stand in awe of how the living web distributes the very atoms and molecules needed for complex life around the globe.

Tiny Creatures, Major Roles

It takes more than a rocky planet in a habitable zone to sustain life. Some twenty factors are listed in the film The Privileged Planet relating to geology, the atmosphere, the magnetic field, and other abiotic phenomena. We've seen that that life itself maintains the habitability of the earth in surprising ways. By means of coded instructions in their genomes, the tiniest of creatures play major roles in maintaining the essential cycles of the planet: the hydrologic cycle, nitrogen cycle, oxygen cycle, carbon cycle, and more.

Evolutionists may weave stories about how these remarkable interactions arose gradually as life emerged and progressed. The instances above, though, should call into question whether any life could have subsisted on a bare earth without at least some of these processes already in operation. When these observations are combined with the other evidences of fine-tuning in the earth, the laws of physics, and the universe, the inference to design seems irresistible.

Image credit: © S. Bollet, via Ohio State University.