Eavesdropping Universe

June 30, 2011

The Search for Extraterrestrial Intelligence (SETI) is on the brink of discovering habitable worlds in our own galaxy. With probes like the Kepler telescope already searching the stars in our galaxy and the Terrestrial Planet Finder probe soon to be launched, we have begun the search for worlds that resemble for own. While the thought of ever visiting these alien planets is still a fantasy, science fiction writers are already speculating on what we might find on these planets. Check out these links to discover more about the search for habitable worlds, the science beyond science fiction, and one curious collaboration between science and music that is truly out of this world.

 

Efficacious Universe

June 26, 2011

The earliest form of photosynthesis on Earth was thought to be the result of a mutated gene that coded for a sunscreen pigment. When the earliest life on Earth spread out from the hydrothermal vents, these organisms eventually spread across the entire ocean and found their way closer to the surface in search of food. However, the surface of the oceans were all but sterilized because the radiation being belched from our rambunctious Sun, which was more dangerous than today’s more placid Sun, would have killed anything that got too much exposure. Some cells adapted to this environment by capitalizing on a novel pigment that could protect the cell’s machinery by absorbing any UV radiation that might strike the cell’s surface. This pigment could transmit visible light away from anything inside the cell that could be damaged and over generations a species found a way to utilize that light to transport electrons through its membrane.

The electron transport chain is a metabolic pathway that creates ATP, the energy source for our cells. Animals use oxidative respiration to replenish a chemical called NADH that fuels the electron transport chain. Other organisms like bacteria use anaerobic respiration to generate NADH, but this isn’t as efficient and it doesn’t produce as much ATP from this older pathway. In either case, the NADH produced from these pathways donates its proton and carries electrons that can be harvested for use in the electron transport chain. These electrons are picked up by an electron carrier molecule that shuttles the electrons across a section of the cell membrane. The price to ferry these electrons through the cell membrane is to push a proton from outside the cytoplasm to inside of the cell membrane. This process of paying protons to move electrons culminates when those electrons get passed across the entire electron transport chain, then those electrons get attached up by an atom like an oxygen or sulfur along with two hydrogen atoms that got pumped inside the cell.

In photosynthesis, a photon that strikes a structure within a cell called a thylakoid gets passed through a pigment like chlorophyll until it reaches the end of the line and dumps that energy onto an electron; like a game of hot-potato that photon gets passed through proteins called antenna systems until it reaches that electron and excites it. Plants and other organisms that undergo photosynthesis have two separate and consecutive photosystems that work in series. Electrons are normally in a so-called ground state, but when they suck up the energy of a photon, it causes them to jump into a higher energy state. That electron flies off of the chlorophyll molecule and gets funneled into the electron transport chain. Photosystem II funnels photons through the thylakoid membrane of a chloroplast to split water apart; this replenishes the electrons lost when that photon carries an excited electron from the pigment molecule chlorophyll through the electron transport chain. This leaves the chlorophyll molecule with a positive charge and in order to reset that molecule back to a more stable form, water molecule gets ripped apart; one of its electrons gets incorporated into the chlorophyll. Photosystem I funnels another photon in to replenish NADH, an electron donor that powers the Calvin cycle that creates sugar for plants. The hydrogen atoms from water float around inside the cell until they get used to pay the ferry that moves electrons across the thylakoid membrane while the oxygen atom gets released into the atmosphere. This is how oxygen got built up in our atmosphere on Earth; the combined photosynthesis of early bacteria and plants belched out so much oxygen into the atmosphere that it poisoned most of the anaerobic life on Earth at the time. More on that later.

Photosynthesis went on before the advent of oxygen in our atmosphere; that and a process called chemosynthesis used a different molecule to accept electrons at the end of the electron transport chain. In these processes, sulfur and hydrogen sulfide are used instead of oxygen and water as the final recipient of electrons channeled through the electron transport chain. A form of anaerobic photosynthesis used by purple and green sulfur bacteria, for instance utilizes the same mechanisms of photosynthesis to create sugars from visible light, but this pathway only requires photosystem I and not II.

This diagram outlines the path an electron takes across the thylakoid membrane to complete the electron transport chain and power ATP synthesis in a cell.

 

Esoteric Universe

June 22, 2011

What did the first life on Earth look like? Research into the oldest lineages of single-celled life suggest that the first life form on Earth would likely have been an extremophile that survived in the hydrothermal vents (233) at the bottom of the ocean floor; this organisms would have lived off of the gases spewing from the vents and could have survived in near-boiling water. This cell would have been an archean life form; its cell wall would have been built out of peptidoglycan or a similar chemical meant to survive such daunting pressures and the acidity of the dissolved hydrothermal vent gases. Acidophilic cell membranes like this would be designed to pump hydrogen ions out of the cell and maintain a more neutral pH than the environment of the vents. Only an extremophile could survive the early environments on Earth and Areios.

The metabolisms of all animals are very similar and involve the same metabolic pathway; oxidative respiration. But archean life has such an eclectic set of anaerobic metabolic pathways; some organisms breathe hydrogen sulfide gas that bubbles out of the hydrothermal vents at the bottom of the ocean. These organisms are cut off from the Sun and form the base of an ecosystem that is wholly independent of photosynthesis. This world relies on chemosynthesis; instead of a utilizing a photon to start the electron transport chain, these creatures harvest electrons from hydrogen sulfide to kick start the process. Organisms need not use hydrogen sulfide, though. Some creatures have been known to use molten iron, arsenate, methane, or hydrogen gas.

Areiosan life too relies on sulfur to power the electron transport chain. One flaw of this system is that it doesn’t release as much energy as oxidative respiration. Animals simply can’t function off of anaerobic respiration; only in the rarest cases can anything bigger than the simplest multicellular organisms. Yet, there are complex organisms on Earth that have found a way survive off of anerobic respiration. Tubeworms living near hydrothermal vents have chemosynthetic organisms lining their gut to thank for their providing food. These bacteria use the hydrogen sulfide from the vents to produce ATP that they feed to the tubeworms. This symbiotic relationship is exceptional on Earth, but on Areios, it all but proves to be the rule for any macroscopic life.

Giant tubeworms can thrive in these environments, stretching up to 3 meters long. These organisms are extreme even in their ability to develop and grow so rapidly. In two years, some specimens can grow almost 10 feet under ideal conditions. Few creatures on Areios are ever as big as that because rather than devoting all of that energy into growth, Areiosan life forms devote the energy from their internal bacteria into mobility and procreation, so they are on the whole much smaller than life on Earth. Not only is the climate on Areios across-the-board colder than on Earth, owing to the higher levels of sulfur dioxide, but gravity on Areios is more intense and this keeps any organisms from growing too tall because the pull of gravity would limit anything from growing too tall. Anything but the sturdiest creatures would buckle under the weight of the atmosphere. For now, life on Areios is nothing but pond scum and singular bacteria, but with the advent of oxygen, organisms can evolve into the complex forms we recognize as animals like the ones found on Earth.

Yet, the environment on Areios keeps the organisms we could identify as animals from growing to the size of some organisms we find on Earth. There are no blue whales or sequoias on Areios; the largest animal to trammel its surface is perhaps the size of a horse. Low oxygen and high sulfur levels limit the size an organisms can reach and because oxygen is so scarce, endothermic organisms like mammals and birds with such a high metabolism are unlikely to be able to survive on Areios because of how much energy they would require to maintain their high-maintenance, warm-blooded metabolisms. Yet the largest organisms on Areios are more akin to fungi or coral reefs, many-headed colonial organisms that spread out over huge geographic expanses In fact, because of this harsh environment animals don’t arrive on the scene until much later in the evolutionary history of Areios. It takes almost 10 billion years for anything complex more complex than our primordial extremophile to arise on Areios, and by then, these novel animals are only ephemeral; as soon as they arrive, they are wiped out 4 billion years later.

 

Deep under the ocean in hydrothermal vents, there is an entire ecosystem built off of chemosynthetic bacteria that feed on sulfides. This could indicate life need not rely on the light from a star, but could thrive in other extraterrestrial environments.

 

Extant Universe

June 18, 2011

Paul Gilster of Centauri Dreams reports on the 100 Year Starship Study, an initiaitve launched by The Defense Advanced Research Projects Agency (DARPA) and NASA Ames Research Center (serving as execution agent), to discuss “the practical and fantastic issues man needs to address to achieve interstellar flight one hundred years from now“. The coordinating agencies will assemble a panel to discuss the implications of this endeavor at a symposium which will lay the groundwork for an organization that will one day create a  “self-sustaining organization that will tackle all the issues and challenges inherent in long duration interstellar space flight.”

Edge of the Universe

June 11, 2011

Check out this latest press release from the  NASA Science News webpage; the Voyager probes are beaming back some very interesting information on what its like at the far reaches of our own solar system.

 

Enduring Universe

June 6, 2011

We encountered this concept of a shadow biosphere that suggests some organisms on Earth may be relics of a previous genesis event. Right now, this is just speculation on the part of some astrobiologists who haven’t been able to find prove of this idea yet. And if life truly did arise on Earth more than once, where is it now? What does it look like and how does it compare to life as we know it now? How would we look for evidence that this actually happened? The most straightforward way to find answers would be to test the genome of bacteria from as many different locations as possible, and the more exotic, the better. (After all, it’s unlikely that something as big as a giraffe for example could have spawned from a different origin event, but something as small and elusive as a bacteria could go unnoticed unless we found it in the backdrop and sequenced its genome.) We would want to look for places that would be cut off from the rest of the biosphere; like miles underground. Scientists found bacteria colonies that consumed hydrogen bubbling from the crust miles below the surface in an abandoned South African mine. DNA analysis showed that this germ was related to all other life on Earth, but what does that mean? How would we know if life were alien to the planet?

If life as we know it didn’t use DNA or RNA, but relied on a different molecule for storing genetic information, then we might see something wildly different from life as we know it. Peptide nucleic acid (PNA) is composed of repeating N-(2-aminoethyl)-glycine units linked by peptide bonds; the structure is more stable under high temperatures and low pH so it’s possible that it could have been incorporated into early cells and that DNA only came later because DNA is more efficient at replication. Another analogue, threose nucleic acid (TNA) is a polymer used by biochemists in studies on DNA; this analogue can function like DNA or RNA, but TNA is simpler chemically, so it could have been a precursor to RNA.

While the evidence to support a TNA or PNA-dominated biosphere isn’t readily accepted by most molecular biologists, the RNA world hypothesis has enjoyed some support by researchers over the last 30 years. Because RNA can act as an enzyme protein and an information-storing nucleic acid, this versatile chemical is thought to have preceded the advent of DNA, leading some to suspect that early life ran off of RNA and not DNA. It wasn’t until later when a genetic accident could have spawned DNA, which would have dominated the life at the time because it could more efficiently copy itself.

Areiosan life runs off of PNA, but it has a triple helix structure. This idea isn’t new to biology; Linus Pauling suggested DNA might have a triple helix structure back in the 1950’s, but the discovery of DNA’s double helix structure by Watson and Crick completely shattered that idea. Still, when molecular biologists use PNA, it latches itself onto the double helix structure and forms a totally new triple helix that stabilizes DNA under higher temperatures and lower pH. The latter is imperative for Areiosan life because of the prevalence of sulfur in the environment. Sulfur dioxide and trioxide readily form sulfuric and sulfurous acid in contact with water, so increased acid rain has lowered the pH of Areiosan bodies of water. So while the acidified oceans would warp and denature Terroan DNA, Areiosan PNA doesn’t get bent out of shape by higher acidity.

DNA undergoes mutations regularly because the process of transcribing new DNA during cell replication is imperfect. DNA can be miscopied during transcription, but DNA polymerases that add base pairs to DNA during replication can correct some of these errors during a stage called elongation in eukaryotic cells. Because this PNA’s base pairs are harder to pry apart under moderate pH, the proofreading process in Areiosan cells is shorter than in Terroan cells. Because of this, a certain type of error called a mismatch error abounds in Areiosan cells because once replication is complete; there is no proofreading mechanism to follow like with our biology. Despite radically different mechanisms to mitigate mutations to the nucleic acid, both Earth life and Areiosan life experience identical rates of mutation. Mechanisms for repairing DNA are not perfect and these imperfections that allow mutations to occur spur the creation of novel genetic material. If there were no mutations, there would be no evolution and each individual would be a carbon copy of the parent; this would be disastrous because if every individual were exactly vulnerable to the same stresses, then a single event would wipe out an entire population. Mutation causes a varying ability for survival in individuals of a population, and this diversity can provide resilience to a population. Some of those mutations will prove to be deleterious to the individual, but the heritable mutations that don’t manage to wipe the individual tend to persist or even propagate within the population. These errors in DNA replication are the basis for new genetic traits, but if this process is too imprecise, it would kill off too many individuals and prevent that material from ever being propagated. The rate of mutation is therefore fine-tuned to allow “a balance between the evolution of species and the survival and reproductive success of individual organisms.”

This diagram shows the configuration a stable triple helix used on Earth in drugs for cancer treatment.

Evaporating Universe

June 2, 2011

A new study reveals bacterial genes that influence sulfur gas flux from seawater; the research could have implications for understanding the role of ocean bacteria in cloud formation. Check out this press release posted on Astrobiology Magazine.

 

A simplified graphic shows the process by which bacterioplankton send sulfur found in decaying algae into the food web or into the atmosphere, where it leads to water droplet formation—the basis of clouds that cool the Earth. Credit: Chris Reisch, University of Georgia