This is the fourth in a series of posts by Associate Curator of Invertebrate Zoology Janet R. Voight as she heads out on an expedition to Norway. There, she and colleagues will look for a wood-boring clam, Xylophaga dorsalis, to study its, well, poo. Read the first post and stay tuned for upcoming posts to find out what they discover.
Hypothesis #1, as described in our list of hypotheses, says the clam feces in the borehole sustain microbes. The microbes (bacteria) may flourish as they help convert sulfide to sulfate, which benefits the clam!
My research at deep-sea hydrothermal vents taught me that bacteria and clams often work together: bacteria fix energy from chemical reactions and clams give the bacteria a place to live, so both benefit. This is called chemosynthesis, and it’s how these specialized, tiny, usually invisible to the naked eye, microbes or bacteria get energy. It contrasts with photosynthesis, which plants do to harness energy from sunlight.
When life at hydrothermal vents was first discovered at 2200 meters (1.3 miles) deep, far below where sunlight reaches, it was a mystery what animals ate at such depths. And these are big, densely packed animals, like giant tube worms and giant clams. Having a lot of big animals in one place in the deep sea is rare because food is hard to come by. Mostly, food sinks from the sunlit shallow water where plants use photosynthesis to convert energy from the sun to make sugars. A lot of that energy gets used as it sinks, so usually the deep sea just can’t support many animals at one spot. No one could explain the massive numbers of animals at seafloor vents and what they ate (in fact, things got weirder when they discovered that giant tube worms don’t even have mouths!). In time, biologists found that giant tube worms and giant clams host bacteria inside their bodies, which made sugars as they helped chemical reactions go forward. The bacteria share these sugars with their host animals and both flourish. When vents were discovered, no one thought bacteria could be so important—some even laughed at the idea—but test after test supported the hypothesis that bacteria fixed the energy for the ecosystem. In time, the hypothesis was accepted.
Since then, we’ve tested other places where it might apply. Repeatedly, microbial chemosynthesis has been shown to play a big role in energy fixation, especially where areas with and without oxygen meet. If you’ve ever been out in a creek and turned over a log sitting in mud, you might have smelled the nasty stink of rotten eggs—that is hydrogen sulfide, a potent toxin that our noses detect at very low levels. However nasty it seems to us, many types of microbes turn stinky sulfide into neutral sulfate and gain energy (sugars) from doing so. Our #1 hypothesis is that the clam feces in the borehole sustain microbes that do the same thing. I talked to someone who studies microbes, and her response was, “oh yeah, for sure! That sort of thing is really common.” So maybe it will be easy…but then again, who knows? No one has ever tried this before. Our hopes are high, but the odds may be long. Next time, I will tell you about how we are going to test that hypothesis and see if we can find something that shows it just can’t be that way.
Read on in Part 5: The Chemosynthesis Hypothesis: Do Clams and Bacteria Work Together?
Funding for this project was provided by the Robert A. Pritzker Center for Meteoritics and Polar Studies established by a grant from the Tawani Foundation.