Elemental Universe

March 15, 2011

Areios is unique in this solar system in that it has liquid water on its surface, just like our Earth does. This liquid water helps to keep the climate moderate because of its high specific heat that allows water to absorb a tremendous amount of energy before it vaporizes. Water gets cycled throughout the environment through the perennial process of evaporation, condensation, and precipitation. Water also gets transformed chemically as it moves from the living to non-living environment. Water isn’t the only chemical to get recycled; nitrogen, phosphorus, sulfur, and carbon get converted into different forms through chemistry between the living and non-living parts of the environment. Areios undergoes more or less the same processes as the ones we find on Earth.

On Earth, the nitrogen cycle converts inert nitrogen gas into a form useable for life. Organisms called nitrogen fixers can break the triple bonds between the nitrogen atoms in N2 which few other organisms on Earth can do. These nitrifying bacteria appear mostly in the root of certain plants on earth and play a valuable role in supplying nitrogen to the environment, which is usually a limiting factor for plant growth. These bacteria can convert N2 to nitrate or nitrite, which plants can use. Other bacteria can convert nitrate or nitrite into ammonia. Before these organisms were around nitrate and nitrite were produced in lightning strikes, and production of usable nitrogen was limited. Still other bacteria called nitrifiers convert ammonia to nitrate while denitrifiers convert it back into nitrogen gas.

Phosphorus on Earth comes from rocks, which is released by weathering or through human intervention (think mining, extraction industries). When leached from rocks, phosphorus makes its way into the water and is quickly taken up by organisms because like nitrogen, phosphorus is a limiting factor for growth. When phosphorus gets into the water supply, it causes rapid algae growth to use up the available oxygen in the water, which causes the fish to suffocate in these dead zones at the mouths of major rivers. Right below phosphorus on the periodic table is arsenic; scientists recently discovered an organism that can substitute phosphorus in its DNA for arsenic. Because arsenic is so chemically similar to phosphorus, arsenic is deadly to humans because arsenic will replace any phosphorus in our bodies, but we can’t metabolize it. The arsenic cycle behaves analogously to the phosphorus cycle, which is what we would find on Areios instead of the phosphorus cycle. Instead of a life form that can tolerate higher levels of arsenic like GFAJ-1, life on Areios all but exclusively uses arsenic wherever phosphorus is used in biochemistry on Earth; in ATP, nucleic acids, and in the minerals that make up their bones and teeth. This alternative biochemistry makes the Areia fundamentally different from Terroa, or what I what classify as life from Earth, and this biochemical disparity effectively segregates these two trees of life because we Earthlings are toxic to the Areos as much as they are toxic to us. Contact with arsenic-life would be disastrous for both parties because of how poisonous we would be to one another. Perhaps even touching one another might be hazardous!

GFAJ-1 may be the first organism discovered that can replace the phosphorus in its DNA for a compound called arsenate.

The sulfur cycle is the most radically different from our world to Areios because sulfur plays a much bigger role in Areiosan metabolism. We’ll talk more about the role sulfur and sulfate-reducing bacteria in upcoming posts, but for now, understand that sulfur compounds are important in Areiosan ecosystems in a way that is significantly different from how it’s used on Earth. On Earth, sulfur is produced in hydrothermal vents in the form of hydrogen sulfide and in volcanoes as sulfur dioxide. Plants release minute traces of carbon vinyl sulfide in respiration and burning coal produces sulfur trioxide. These gases come in contact with water vapor and a series of reactions creates trace amounts of sulfuric acid droplets. This acid rain falls to the earth and reacts with the crust to form sulfide or sulfate minerals. These minerals eventually become part of the crust and get subducted into the mantle where sulfur gets spewed out by volcanoes and hydrothermal vents once more. Human presence alters this cycle through mining and burning coal, which has exacerbated acid rain damage, particularly in the Northeastern U.S. and Eastern Europe.

On Areios, the sulfur cycle is a bit more complex than on Earth. For one thing, some organisms build yellow mounds out of crystalline sulfur, like coral, or termite and ant hills on Earth. Some creatures incorporate a sulfur-eating bacterium on their skin that produces sulfuric acid to ward off predators. Areiosan cell metabolism relies more on sulfur compounds than our Terroan life. The earliest eukaryotes on Earth were thought to resemble an archean cell housing a rickettsia-esque bacteria (like the one that causes Rocky Mountain spotted fever). We’ll go more in depth on this endosymbiotic arrangement later, but it was thought that the rickettsia could detoxify peroxides in the cell, converting it to water while eliminating any oxygen that would be deadly to an anaerobe. On Areios, the first eukaryotic cells were meant to detoxify hydrogen sulfide and convert it to water and a solid sulfur precipitate.

The carbon cycle is the final biogeochemical cycle we’ll discuss and is probably the one most people are familiar with; with climate change such a hot issue right now for the political establishment, people have begun to pay more attention to the processes that release or sequester carbon in the environment. Carbon dioxide is a stable gas in the atmosphere that traps heat and gives both Earth and Areios a comfortable greenhouse effect that would otherwise freeze the planet. Our current climate predicament has less to do with the amount of carbon dioxide in the atmosphere (because carbon dioxide levels have fluctuated wildly over the last four billion years), but it’s dire because of the rapidity that the changes have been taking place and because human activities influence carbon dioxide levels to an unprecedented extent. The greenhouse gas methane is 40 times more powerful than CO2 and on Areios as well as Earth; this gas plays a much more powerful role in the climate. We’ll see why this is important in an upcoming post…

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