Opal is dominated by an ocean and atmosphere composed primarily of carbon dioxide. The total surface atmospheric pressure is approximately 73 bars, 70 of which are provided by carbon dioxide gas (the remainder being primarily oxygen and nitrogen). This thick blanket of greenhouse gasses is balanced, however, by Opal's distance from its star, which would leave an Earthlike world permanently frozen over. This thick atmosphere also efficiently distributes heat across the globe, resulting in balmy temperatures of 30 to 40 C worldwide and year-round.
That does not, however, mean that Opal is without weather! Slight variations in insolation over the year due to Opal's slightly eccentric orbit result in sea levels rising and falling as large portions of the atmosphere condense and then re-evaporate, with the annual fall storms providing the vast majority of liquid delivered to inland regions. Additionally, just as Earth's surface conditions happen to straddle the liquid-solid-gas triple point of water, Opal's surface conditions happen to straddle the liquid-gas-supercritical fluid boundary of CO2. Thus, the oceanic regions of Opal are frequently subject to waves of critical opalescence--which is what gives the planet its name!
Unsurprisingly, biochemistry on Opal is primarily based on the liquid CO2 solvent system, and opalescence storms thus pose a significant hazard to organisms living on or near the surface, as they can cause internal cavitation and radical short-term changes to the solvation properties of intracellular fluids. Like the biology of Rust, Opal's biochemical systems are in many ways similar to Earthling life, even using many identical macromolecules which are soluble in CO2; in fact, the relatively high compatibility of CO2 as a solvent with Earthling biomolecules has resulted in extensive industrial use of liquid and supercritical CO2 on Earth for organic synthesis and purification. There are, however, several key differences, mainly owing to the non-polar and aprotic nature of CO2.
The typical lipophilic tails seen in, for example, the phospholipids that form Earthling cell membranes have low affinity for CO2 despite being non-polar. Instead, Opal life makes use of a combination of oxygenated polymers with CO2-philic carbonyl groups and short polydimethylsiloxane neck sections with lipid tails to act as surfactants for interfacing lipophilic and CO2-philic phases, carboxylic acids heads joined to siloxane tails forming monolayers to mediate between CO2-phillic and hydrophillic (polar) phases, and normal fatty acid monolayers to interface between hydrophillic and lipophillic phases. The existence of a third separate CO2-phillic phase means that Opaline life's bilayer membranes cannot be easily identified with Earthling (or Rust, or Vitrium) style vesicles or the reverse-vesicles more typical of non-polar solvents. The equivalent of phospholipid membranes have CO2-phillic heads extended into the bulk medium, which may protecting either membrane-internal acid-based polar ends between the layers, or lipophilic alkane tails; these two membrane types are intermixed to achieve different structural properties, much as the fluorophillic phase is exploited in Vitrium biochemistry. Some simpler classes of organisms rely on the integration of salts to stabilize the interiors of their polar membranes, (a structure which is still seen in some organelle membranes in eukaryotic organisms, providing evidence for an endosymbiotic origin) but eukaryotic organisms on Opal have developed more complex hydrophillic units based on sugar esters which greatly reduces their reliance on salts which are insoluble in the CO2 medium. Additionally, rather than the highly charged and hydrophillic DNA used by Earthling life, Opaline life operates on Peptide Nucleic Acid, or PNA. The alternate membrane structures and replacement of the genetic molecule backbone mean that Opaline life has a vastly reduced reliance on phosphorus as a limiting nutrient compared to Earthing life. It is also worth noting that fluorocarbons, used extensively in Vitrium's biochemistry, also exhibit significant CO2-philic properties--but unlike Vitrium, Opal's environment does not make fluorine sufficiently bioavailable, so it remains exceptionally rarely used despite its functional advantages, parallel to Earthling biology.
Unlike on Blue Crystal and Vitrium, silicon is still decidedly a heteroatom in Opal's collection of common biopolymers, with carbon acting as the primary polymer backbone atom in Opaline biochemistry. Silicon is approximately as bioavailable on Opal, in the form of silicic acid weathered out of silicate rocks by carbonic acid, as it is on Earth; but, the solvation properties of siloxanes in Opal's environment provided the evolutionary pressure to develop efficient methods of breaking Si-O bonds and forming Si-C bonds, which have only been achieved by artificial directed evolution on Earth. Coincidentally, this ability is also used by many of Opal's creatures to construct opaline shells, just as on Vitrium, standing in for the calcium carbonate that is such a common structural material on Earth. This fact, however, was not discovered until well after the planet's initial discovery and naming.
The presence of carbonic acid of course implies the presence of water--and indeed, water is a common component of Opaline biochemistry. Photosynthesis operates almost exactly as on Earth, but with the roles of CO2 and water reversed--CO2 is the readily-available solvent while water, serving as the hydrogen source, is pulled from the atmosphere. And the aqueous vacuole--a reverse-micelle containing a supply of water in which aqueous chemical reactions can be performed--is a nearly-universal organelle in Opaline cells. Aside from the various aqueous reactions that it permits, the vacuole is central to most cells' energy metabolism, as it is both the source and sink of protons which allow Opaline life to make use of proton-pump mechanisms just like Earthling and Vitrium life--a class of molecular machinery that is unavailable to most biochemical systems operating in aprotic solvents (such as those of Cannonball or Blue Crystal). The astute student may well then ask: why did Opaline life not evolve to use water as its primary medium, rather than CO2?
The answer to this is in fact incredibly simple: density. Liquid CO2 is approximately 10% heavier than liquid water in similar conditions. Thus, excess water that cannot be solvated in the CO2 seas floats, keeping it out of contact with the solid substrates from which essential nutrients and catalytic substrates are acquired. This is the source of another of Opal's unique weather phenomena: rainpuddles. When the ocean or atmosphere become super-saturated with water, it does not condense into small drops with can fall out of suspension; rather, drops accumulate into large "puddles", which can "rain" upwards and drift around near the ocean surface for quite some time before being broken up and re-dissolved. The inverse phenomenon can occasionally be observed on Earth at sufficient depths, with excess CO2 that cannot be solvated in water forming puddles on the ocean floor. The near-critical nature of the overlying atmosphere, with density quite similar to that of the liquid oceans, also means that these rainpuddles can sometimes be found drifting through the air, presenting a navigation hazard to low-flying creatures. This should also highlight the ease of flight on Opal; it is hardly more difficult than swimming. The colonization of the sky was thus a more direct step in Opal's evolutionary history than the colonization of the land! And, since water, oxygen (produced by photosynthesis and filtered by lungs) and nitrogen (also easily filtered out of the ocean or air) are all lifting agents in Opal's thick CO2 atmosphere, aerostatic and hovering flight are far more common, and employed by creatures with a far wider range of morphologies, than are seen on Earth.
Despite their high-pressure environment, Opaline organisms are actually relatively easy to transport for study off-world. While most cannot remain active significantly below 30C and 70 atmospheres, due to extremely precise tuning of proteins to operate in that extremely stable temperature range, their fluids do not actually freeze above -50C, at which point pressures can be brought down to a mere 5.1 atmospheres with no structural damage to the organism, significantly reducing the structural and mass requirements for transport vessels. The vast majority of Opaline organisms enter a state of extreme torpor or suspended animation under these conditions, exhibiting no significant metabolic activity or need for life support functions, and will return to full function with few or no long-term ill effects upon being re-pressurized and brought back up to normal temperatures. Direct contact with Opaline organisms is generally safe, requiring only a drysuit and oxygen mask to enter their native environment for brief periods. Long term interaction, however, requires active cooling garments to prevent heatstroke in human researchers, and standard decompression protocols when exiting the Opaline environment.
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