We tend to stereotype extraterrestrial life as little green men, but there are much more instructive examples of alien weirdness right under our oceans. In the following article, Dr. Craig McClain of Deep Sea News and the National Evolutionary Synthesis Center discusses what the deep seas can teach us about potential astrobiological organisms.
We as humans have three fundamental questions. Where do we come from? Where are we going? Are we alone in the universe? The answers to these thrust at the core of our humanity and uniqueness. Through science we seek out replies to these inquiries.
In 1960 the National Academy of Sciences asked Frank Drake to gather a group of scientists to discuss the search for extraterrestrial intelligence, the program we now call SETI. "As I planned the meeting, I realized a few day[s] ahead of time we needed an agenda," recalled Drake at a NASA Forum held in 2003. He continued:
And so I wrote down all the things you needed to know to predict how hard it's going to be to detect extraterrestrial life. And looking at them it became pretty evident that if you multiplied all these together, you got a number, N, which is the number of detectable civilizations in our galaxy. This, of course, was aimed at the radio search, and not to search for primordial or primitive life forms.
What emerged was the now-famous Drake Equation:
N = R* • fp • ne • fℓ • fi • fc • L
Where N = the number of civilizations in our galaxy with which communication might be possible;
R* = the average rate of star formation per year in our galaxy
fp= the fraction of those stars that have planets
ne = the average number of planets that can potentially support life per star that has planets
fℓ = the fraction of the above that actually go on to develop life at some point
fi = the fraction of the above that actually go on to develop intelligent life
fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
L = the length of time for which such civilizations release detectable signals into space
It is ne (the average number of planets that can support life per star) and fℓ (the fraction planets with favorable conditions to develop life) that I have been the most intrigued by. ne is related to the habitable zone, the region around a star where a planet with sufficient atmospheric pressure can maintain liquid water on its surface. More specific criteria can also be used which can also alter the percentage from 0.5-20%. Some estimate fℓ at 100%; where life can evolve it will. In our sample size of one, Earth, life arose. [This image compares the habitable zone of the Kepler 22 system to our own solar system. Click here to learn more about the Kepler 22 system.]
Conversely, some argue that the value of fℓ approaches 0%. Life as we know arose only once on Earth; all life has a single origin. This points to a set of specific and very rare conditions unseen at other points in space and time in Earth's history. The chances of this set of conditions occurring on other planets are equally rare. This concept is formalized as the Rare Earth hypothesis; the emergence of complex multicellular life on Earth required a combination of events and circumstances so rare as to only occur once.
On the brink of Cameron being only the third person to ever visit the Challenger Deep in the Marianas Trench, I'm reminded how much exploration and scientific inquiry of the deep sea continues to challenge our perceptions of life and how life works. Insight gained form observing and investigating deep-sea life forced us to redefine and reexamine our theories of life, and potentially increase the values of fℓ and ne. Life — and the conditions necessary for life to emerge — may not be as rare as we think.
Life does not Require the Sun
Geologists exploring the seafloor near the Galapagos Rift in 1977 discovered a biological system wholly different than anything encountered on Earth before. Later that year, Peter Lonsdale published the first paper on the unique life at hydrothermal vents. He writes:
A community of abundant suspension-feeding organisms was photographed around an active hydrothermal vent at the Galapagos Rift. A site on the crest of the East Pacific Rise where hydrothermal discharge is suspected also has a dense colony of sessile organisms. The high standing crop of macrobenthos in these patches probably results from local increases of deep-sea food supply near hydrothermal plumes in the bottom water.
The venting hot water is rich in a variety of minerals, heavy metals, and toxic substances including hydrogen sulfide. Some bacteria have the ability to use the oxidation of hydrogen sulfide to provide energy through a mechanism other than photosynthesis. These bacteria, whether free-living or in symbiosis with such organisms as clams and tubeworms, form the base of food chains.
Life can survive on minimal food
The lack of light in the deep ocean prohibits photosynthesis. Animals that do not live near vents must therefore rely upon the minor amount of food that may sink from the productive ocean surface.
Each year approximately 16 gigatons of carbon fixed by phytoplankton sink to the ocean interior. This amount is a mere 3 percent of the total produced at the ocean surface. Consider that 3 percent of a five-pound bag of sugar is less that five-and-a-half tablespoons. This small amount of fluxed carbon, carried largely by "marine snow" [pictured here], dusts the seafloor and represents the only food source for the majority of organisms in the deep.
And yet, life not only thrives in the deep sea, it actually abounds with profound biodiversity. Adaptations flourish to allow animals to find rare mates, utilize novel food sources, reduce energy spent looking for food, and reducing overall energetic costs.
Life Can Survive at Extreme Temperatures, Pressures, and Toxicities
The fact that organisms have been discovered that can thrive in extreme temperatures, withstand up to 1100 atmospheres of pressure, and survive in the presence of hydrogen sulfide and heavy metal concentrations that would kill most animals is more than sufficient data to suggest that life can survive in even the most hostile of conditions. But how do these organisms do it?
Deep-sea organisms demonstrate quite well that extremes of temperature, pressure, and toxicity can be adapted to. Pressure increases 1atm for every 10m, so deep-sea organism can experience a range of pressures from 20atm at the shallowest point to 1100atm in the deepest. This results in a tighter packing of the phospholipids that make up cell membranes. Tighter packing, in turn, lowers the permeability of the membrane. Moreover, temperatures in the deep are typically near 4 degrees Celsius, and near the poles water can become supercooled to -1 degree Celsius. This also decreases the permeability of cell membranes. Deep-sea organisms deal with this by increasing their percentage of unsaturated fatty acids. Unsaturated fatty acids have kinks in their structure that prevent them from packing together tightly in the membrane.
We also see that temperatures and pressures select for proteins with different temperature sensitivities and pressure resistances. Changes in protein structure can influence their cellular function. Thus, selection in deep-sea animals has been for rigidity to counteract pressure. Proteins contain hydrogen and disulfide bonds between different subunits and parts of the amino acid chain that both dictate structure. A selection for proteins with increased bonding would minimize changes in shape due to pressure.
Metazoans Are Possible Without Oxygen
It was long believed that organisms living in regions of the deep sea where there is zero or little oxygen require, at a minimum, alterations to proteins that increase the efficiency at which oxygen is bound and transported throughout the body. But does complex multicellularity require oxygen? Research that I was involved in suggested that increases in oxygen were prerequisite for larger sized multicellular organisms.
Yet, in research published last year, three species of Loriciferans (multicellular organisms with heads, mouths, digestive systems, complex life cycles, and separate sexes) were found in completely oxygenless areas of the Mediterranean Sea [one of said species is featured here]. These species lack oxygen-requiring mitochondria and instead have organelles called hydrogenosomes, similar those of anaerobic bacteria. As the authors note:
This is the first evidence of a metazoan life cycle that is spent entirely in permanently anoxic sediments. Our findings allow us also to conclude that these metazoans live under anoxic conditions through an obligate anaerobic metabolism that is similar to that demonstrated so far only for unicellular eukaryotes. The discovery of these life forms opens new perspectives for the study of metazoan life in habitats lacking molecular oxygen.
Photosynthesis Does Not Require Light From the Sun
We all learned that photosynthesis requires sunlight back in grade school. It stands to reason, therefore, that life would be confined to habitats where solar light is available. But in 2005, researchers discovered a previously undescribed species of green sulfur bacteria (an organism that requires light for growth) living on a deep sea hydrothermal vent. The green sulfur bacteria was clearly running on photosynthesis — but at such depths, where was it getting the necessary light to do it? According to the researchers, "geothermal radiation that includes wavelengths absorbed by photosynthetic pigments of [green sulfur bacteria]."
Deep-Sea Origins of Life
Proposed environments for where life on Earth may have originated include primordial beaches, tide pools, hot springs, frozen oceans, the atmosphere, and others. There are, of course, competing models. Stanley Miller created an early Earth analog in the laboratory that produced organic molecules from water, methane, ammonia, hydrogen, and a shot of electricity. Miller and Urey's experiments in 1952 [depicted in the figure shown here], although quite distant from demonstrating how life evolved, pointed to the possibility that the conditions on the young Earth's surface could have produced the basic building blocks of life. (Though recent evidence suggests that Earth's early atmospheric conditions may not have favored the specific reactions considered by Miller and Urey.)
Fast forward to the 1970′s and the discovery of hydrothermal vents. The interface of cold and hot waters allow for unique reactions to occur. Moreover, the extreme pressure, protection from UV radiation, abundant geothermal energy, and the presence of both methane and sulfide provide the necessary conditions to serve as a cradle of life in the deep depths of the ocean. Attacks on this theory have taken the stance that life could not have begun at ocean vents because high temperatures would have destroyed amino acids; but in her book on vents, ecologist Cindy Lee Van Dover points to both empirical and laboratory evidence indicating this is not the case. Van Dover also presents an excellent figure of phylogenrtic tree of Bacteria, Eucarya, and Archaea, that point to the highly temperature-resistant nature of the basal taxa.
Add to this a a study from the journal Geology that shows certain types of clay mineral can convert simple carbon molecules to complex ones in conditions similar to the hot and wet environment of hydrothermal vents. The group simulated a vent in the laboratory by immersing various types of clay in pressurized water at 300 °C for several weeks and looking at the fate of methanol, a compound formed readily formed at vents. Having helped such delicate molecules to form, the clays can also protect them from getting broken down in the piping hot water issuing from the vents.
Evolution is Clever
One has to be in awe of what life through evolution has accomplished on our planet. The examples above are a mere fraction of the ways deep-sea life has adapted to extremes. In the 1800's, the belief was that the deep was inhospitable to life. In the early 1900's, the belief was that deep-sea life was present, but not diverse or abundant. Fast forward to today, and we have a much different view of the deep oceans — a view that expands our thinking on what life is capable of, and suggests that the planetary conditions needed to support life and the potential for life to develop are much greater than we have thought previously.
This post by Craig McClain originally appeared on Deep Sea News — an awesome blog dedicated to demystifying and humanizing science in an open conversation that instills passion, awe, and responsibility for the oceans.