|The ozone layer over Antarctica.
As you slather on the sunscreen before heading out for a day on the beach, are you repeating an action that first occurred over 2 billion years ago?
Some scientists believe that early life may have used sunscreen, although it was nothing like the white creams and lotions we use today. Instead, ancient sunscreens may have been mineral crusts made from iron, silica, and clay.
These heavy sunscreens would have been needed to block out the full spectrum of the sun’s ultraviolet (UV) rays. There are three kinds of UV rays: UV-A, UV-B, and UV-C. Modern sunscreens absorb UV-A and UV-B rays. UV-C rays are the most damaging, but they never reach the Earth because they are blocked by the ozone layer.
But over 2 billion years ago, the Earth had little oxygen and no ozone layer. UV-C rays blazed down on the Earth’s surface unimpeded, and would have fried to a crisp any life caught out sunbathing.
Before photosynthesis led to the rise of oxygen and the formation of the ozone layer, early life protected itself by living underwater. Water can block UV light while allowing enough visible light to shine through for photosynthesis to take place. Early photosynthetic life would have had to stay below a certain water depth to take advantage of visible light while avoiding UV.
But with a mineral-based sunscreen, early photosynthetic life may have been able to exist in much shallower waters, and perhaps even live out of the water entirely.
Kurt Konhauser of the University of Alberta studies life at mineral hot springs. Mounds of precipitated minerals called sinters form at these springs, but Konhauser has found that the sinters are actually composed of microbial communities as well as mineral deposits. As silica and iron passively precipitate onto microbial mats, the microbes migrate up, forming a new mat on top of the minerals. Over time the sinter layers accumulate high enough to lift life out of the water.
Credit: University of Alberta
In a study published in 2001 in the journal Geology, Konhauser and his colleagues described just how effective mineral-based sunscreens can be. The scientists grew cultures from cyanobacteria taken from hot springs in Iceland, and exposed them to silica-iron concentrations that mimicked hot spring effluent. They then exposed the mineral-coated bacteria to UV light comparable to the levels that would have occurred over 2 billion years ago.
They found that a control group of non-mineralized cyanobacteria experienced rapid damage to such UV exposure, with only 15 percent of the population remaining after 96 hours of radiation. The mineralized "sunscreened" bacteria, however, showed resistance to the UV, and after 384 hours of UV exposure, 90 percent of the population remained.
The mineralized cyanobacteria not only had higher rates of survival, but also higher rates of growth, photosynthesis, and oxygen production than the control group bacteria.
Curious to see just how heavy the sunscreen needed to be in order to be effective, Konhauser and his colleagues sliced a sinter into wafers of varying thickness.
"We wanted to test what thicknesses were necessary to shield UV, but let in certain wavelengths of visible light," says Konhauser. Just as early photosynthetic life in water had to live in a zone where UV light was blocked but visible light was accessible, early photosynthetic life would have needed a sunscreen layer that blocked UV yet allowed visible light to shine through.
They found this balancing act worked with even the thinnest wafer. Measuring a mere 0.15 millimeters, the wafer provided nearly 100 percent UV protection while still allowing visible light to penetrate.
|A layered stromatolite, produced by the activity of ancient cyanobacteria.
Credit: UC Berkeley
Konhauser and his colleagues have since tested hot springs in Kenya and New Zealand, and they have found that all types of microbes acquire a mineral coating.
"Microbes take advantage of the lack of predation and the high levels of nutrients at hot springs, but in doing so, cannot help but get covered in silica and iron precipitates," says Konhauser.
Such biomineralization seems to be an inevitable consequence of growing in geothermal solutions. Presumably, even the earliest photosynthetic life growing at hot springs would have been well coated with minerals.
"Given that shallow water stromatolites already existed 3.5 billion years ago, and they may have consisted of cyanobacteria, then the microbes likely were biomineralized at that early stage," he says.
Some have suggested that life could have originated at mineral hot springs. Konhauser says if life did start there, then biomineralization would have helped it survive.
"In fact, if life did indeed start at hot springs, one could argue that they never would have survived had it not been for a biomineral coating to protect from UV," he notes.
Over the past few years, astrobiologists have increasingly begun to see interconnections between early life and Earth’s geology. For instance, the evolution of photosynthetic organisms is recorded in the rock record – not from fossils, but from chemical indications in the rocks themselves. The oxygen produced by these organisms reacted with iron in the rocks, causing the iron to rust and producing red beds and banded iron formations (BIFs). BIFs are thought to be tangible proof of Earth’s transition from an anoxic to an oxygenated atmosphere, which took place between 2.5 and 2 billion years ago.
|Mars has huge quantities of iron, as evidenced by the rusty red color of its soil.
Janice Bishop of the SETI Institute has been studying this connection between iron, photosynthetic organisms, and atmospheric oxygen. She thinks iron oxide-bearing minerals like ferrihydrite and schwertmannite could have acted as a sunscreen for early photosynthetic life.
Both ferrihydrite and schwertmannite are "nanophase" minerals, which means the crystal size is only tens to hundreds of nanometers in size (a nanometer is one-millionth of a millimeter, or the length of ten hydrogen atoms laid side by side).
"The nanophase character of these minerals is partially responsible for the beneficial spectral properties," Bishop says.
In collaboration with Lynn Rothschild of NASA’s Ames Research Center, Bishop has found that iron oxides mixed with silica and clay provide the best sunscreen protection, perhaps because the silica and clays help disperse the iron oxide particles.
Bishop thinks small populations of early photosynthesizers lived in protected niches that contained nanophase iron oxide minerals. The iron oxide not only helped these early photosynthesizers survive, but may have actually promoted the evolution of such organisms.
"If this in fact was an important factor, than it may well be that without the iron oxide minerals, photosynthesis may not have become widespread until a later date," says Bishop.
The role iron may have played on early life naturally leads to questions about the potential for life on Mars. Mars has huge quantities of iron, as evidenced by the rusty red color of its soil. Mars also has silica, many volcanic features, large quantities of water ice at the poles, and perhaps even some water ice or liquid water underground in lower latitudes.
Most scientists think Mars is too cold and dry at present for life to exist, but the combination of water, heat, and sunscreen materials makes it tempting to think microbial life could have existed during a warmer, wetter period in the Red Planet’s history.
|The mineral hematite (shown above) is abundant on Mars.
Credit: Amethyst Galleries, Inc.
Mars has a thin atmosphere and almost no oxygen, so the surface is highly irradiated – just as the Earth’s surface was over 2 billion years ago. Konhauser says that studies of microbial sunscreen on Earth suggest that life could have once survived on the surface of Mars, as long as there had been some protection against direct irradiation.
"If life did exist on Mars when liquid water was available, then biomineralization would also have been likely," he says. "Along similar lines, those same biominerals may have led to their fossilization – and our current searches for life on Mars are looking for such fossils."
Bishop notes that a variety of forms of iron are present on Mars.
"In contrast to nanophase grains, the grain size of the gray hematite detected on Mars is probably tens of microns, and the grain size of the red hematite is probably a few microns," says Bishop. "We don’t know much yet about the specific iron oxide-bearing minerals present on Mars, although we suspect that there are a lot of nanophase iron oxides."
Bishop says that if life ever existed on Mars, it would not necessarily have needed sunscreen in order to survive. Life could have lived deep underground, protected from the irradiated surface, gaining energy from chemicals in the rocks. Like early life on Earth, the only organisms that would have needed to risk exposure to the sun were those that made their food from photosynthesis.
Bishop says she has a number of experiments planned to study the ability of iron oxide-bearing minerals to protect photosynthetic organisms from UV radiation. Thanks to funding through NASA’s Astrobiology Institute, she and Rothschild will take a closer look at the interactions between iron oxides and photosynthetic organisms at Yellowstone National Park and other field sites.
Konhauser is continuing to study hot spring sites in Kenya and New Zealand. He hopes to assess how microbes influence silica and calcium carbonate precipitation under different geothermal conditions.
His original experiments tested cyanobacteria, but there has been some doubt cast on whether cyanobacteria were the earliest photosynthesizing organisms. Konhauser and his colleagues are now conducting experiments on Aquificales, a deeply rooted bacterial lineage found in both terrestrial and deep-sea hydrothermal systems.