I was digging through old files and found this biogeochemical cycle story by Michael G. Thought it covered things pretty well, and might be a good review:
A Sulfur Atom’s Tale
One could almost hear a sigh of boredom echoing through the subatomic landscape, the resigned wheeze of a sulfur atom on quarantine. When he first joined up with his four oxygen buddies, he thought being called “sulfate” would be a blast; however, after eons and eons of being shuffled around, pressed, pushed, prodded, heated, and pushed around some more, he had been pressed into the very middle of a patch of sedimentary rock. There was an expectation of numbness, an infinite determination not to resist; however, this sulfur atom just had to escape.
And then the rains came. The carbonic acid-laden water trickled down through the hairline fractures of the rock, quietly carving a path of runoff—of freedom—that the sulfur atom simply couldn’t pass up. Grabbing his four oxygens by the electrons, he dove in headfirst and rode the incalculably hectic waves of water molecules, grabbing on for dear life as he swirled with the whim of the trickling raindrop. A seemingly infinite number of water molecules soon surrounded the sulfate, curiously sniffing his negative charge and then backing away, a hectic herd that could not decide whether or not to stay next to the sulfate. Hmph! Who needs them; I’ll show you just what I can be, the sulfur atom—safely within his sulfate molecule—mused to himself, as his raindrop approached—
What was that? All those hairs? A living wedge in the rock? It couldn’t be, and yet it was. The grass root had snaked its way through the sedimentary rock, seeking out water and dissolved nutrients. The sulfur atom clenched its bond and held on, as the raindrop flew into the root as if entering an atomic Charybdis. Up the root; into the leaves; into the mouth of the rabbit; into the back left leg of the rabbit; into the fox’s mouth; into the fox’s paw: it flew by the sulfur atom as less than a blink. As soon as it had started, however, the sulfur atom could almost make out the looks of rock again: the fox had died, and decomposers and detritovores were returning the sulfur to the lithosphere. Oh, no; you can’t make me go back! he huffed and puffed, pulling his oxygen companions by his polar covalent bonds and plunging into another raindrop. This one had a chance to go somewhere, to be a part of something bigger than itself; indeed, the raindrop became a creek, and that creek became a river, eventually leading to something so large, so teeming with infinite infinities of water molecules, that the sulfur atom could not help but gasp.
The ocean: the sulfur atom’s new home. Still wearing his sulfate costume, he looked about amid the literal seas of water molecules, trying to find other sulfate molecules. He saw them, but he could do nothing but pity them: they were drifting down, down, down, down to the very bottom of the ocean floor to form sediment. He couldn’t help but chuckle to himself; their existences would be like his just was, an eternity of waiting, an instantaneous spurt of life in the marine food web, and then another eternity of waiting. As he drifted about, he soon found himself passing by some phospholipids. Hm; I haven’t seen those since—
Oh, no. I’m in a bacterium. And right he was; indeed, the bacterium saw right through the sulfur atom’s sulfate costume, heartlessly shearing off those four oxygen molecules with which the sulfur atom had spent billions of years. It was almost a sad moment for the sulfur atom, which had now to clumsily wave about its new appendages, two methyl groups. He felt like an Edward Scissorhands with clubs for arms—decidedly clumsy. And yet those clubs—those Mickey Mouse-like hands—were actually far from clumsy. He was rising! Up, up, and out of the water he rose, until he began to see some N2 and some O2. He realized that he must finally be in the air, and his new identity suddenly made sense: he was dimethyl sulfide, and he was king of the atmosphere—at least, he felt like it.
He did, at least, until he started to feel the worst sunburn he could’ve possibly felt, one that ripped his floating club-hands from his very covalent bonds. All of a sudden, he was flitting about, colliding with all sorts of other particles without a sense of being or sense of what in the world was happening. Then, as soon as it began, it ended, and the sulfur atom now found himself with four new oxygens. He was sulfate again. This time, however, he noticed some tagalongs, a ragtag duo of hydrogen atoms that pestered and followed him wherever he went. They were like the little brothers he never had—or wanted, for that matter—so the sulfur atom was quite confused about this addition. In his new form—H2SO4, or sulfuric acid—he suavely hitched a ride on a passing water molecule, gaining access to a raindrop on an express route downward. Gravity yanked him towards the earth’s surface with an exhilarating acceleration, though the moment was over before it could register on the sulfur atom’s sense of time.
As the little dots of green on the ground suddenly became increasingly clearer as he descended, he could just make out the very patch of grass he had once been a part of. The raindrop collided; his sulfuric acid burned the grass; and ricocheting off the now-injured leaf, the sulfur atom found himself in the same patch of soil he had been in a very short time ago. Water coursed by him on a determined course downward, and he followed, eventually entering the—
Wait a minute. I’ve been here before. And indeed he had: it was the very same sedimentary rock he had been a part of for as long as he could remember. His adventure a blip on the cosmic expanses of his time of existence, he pulled in his four oxygen atoms, wiggled himself into a space in the rock, and went to sleep.