Last week, I headed out to the field to help a labmate set up a new experiment. He’s researching the impact of sediment on estuary species, which requires simulating high levels of sedimentation in plots of seagrass. “Simulating sedimentation” is a lovely turn-of-phrase suitable for a scientific publication — but really, it just means hauling mud from one side of the estuary to dump it in plots on the other side.
After a couple of hours of mud-hauling, our work was done. I washed my hands in the tidal channel, but noticed something unusual. My ring, once silver, had tarnished to a dull bronze. I’d seen silver undergo this transformation, gradually, with old and disused pitchers in the cupboard, but never thought it could happen over the course of a few hours. What was going on?
Well, to answer that question, we can start by looking at the initial reactions people have to estuaries. Think about marshes– what do you associate with that landscape? Sweeping vistas, wind through the rushes, the cry of seagulls overhead– and, perhaps, a certain fragrance permeating the air. A stink, you could say. The smell of rotten eggs. As it turns out, that quintessential eau-de-estuary is closely linked to the mysterious transformation of my ring. (Side note: I sort of love this smell, actually. The power of positive associations?)
The estuary smell is caused, in large part, by the microbial community in the sediment. This mud is full of a wide array and a high number of microbes. Bacteria, protists, archae, cyanobacteria, fungi– all working away to decompose dead plant and animal matter, or photosynthesize, or cycle nitrogen and sulfur back up the food web. This microbial community is the reason estuaries are so highly productive. They’re great.
But some members of this choir of microbial production are also quite stinky. And to understand that, we’ll return for a moment to high school biology. Remember learning about respiration, the process by which organisms turn sugars into energy? Humans are aerobic respirators, meaning we require oxygen as a key chemical in our respiration process. That’s the case for most of the visible organisms we encounter in our daily lives. Mushrooms, fish, trees, birds, crabs, insects, algae…all of these organisms rely primarily on aerobic respiration to live. But that oxygen-dependency doesn’t really work in the mud of the estuary. As you move deeper into the sediment, oxygen levels drop dramatically. Just a few centimeters below the sediment surface, the possibility of thriving as an aerobic respirator is pretty much nil. But anaerobic respiration– which occurs in the absence of oxygen– provides an alternative that powers billions of anaerobic microbes in these anoxic sediments. While we breathe oxygen, these microbes “breathe” sulfates. And, through anaerobic respiration, they use those sulfates to create energy and grow and thrive.
What we care about, though, is the waste product of this anaerobic respiration. Microbes that “inhale” sulfates are “exhaling” H2S, or hydrogen sulfide. That’s the gas associated with the smell of rotting eggs, or industrial pollution. Basically, it’s stinky.
So, you’ve got your estuary mud: dark and deep and wet and full of fragrant sulfides. Enter the field ecologist mucking about, wearing a silver ring. When the silver ions encounter hydrogen sulfide, they scoop up the sulfur atoms and leave plain hydrogen behind. The result: silver becomes silver sulfide, or Ag2S. That’s the dark, solid material that tarnished my ring in the course of just a few hours.
But wait, you say, the silver pitcher in my basement tarnishes, too, but it’s not exposed to estuary mud! That’s absolutely true (well– depending on the location of your basement). However, sulfate-reducing microbes are found wherever decomposition occurs (which is to say: everywhere), and they release H2S into the air everywhere they’re found. What amazes me is just how many there are in the estuarine muck. My ring changed color in just a few hours– that mud is absolutely teeming with microbial life.
So it might just look like mud, and smell like rotten eggs, and have the unfortunate side-effect of discoloring your finest field-jewelry– but all of that sediment is a soupy, gloppy, gushy muck of life. And what’s more, it’s life that’s operating on a completely different framework than our oxygen-captivated selves. What a world!
Until next time,