Swimming in poison

I’ve had one career: scientist.

My dad, on the other hand, has been a musician, mobile home salesman, jeweler, car salesman, Chicken Delight manager, and a few other things besides. But one of his last jobs before retiring was to be a safety officer at the Shell Waterton natural gas plant in the foothills of southern Alberta.

A little while ago, I asked him about one of the biggest safety challenges. I remembered him talking about when he would come home from work, which they usually referred to by the chemical formula: H₂S.

Without a doubt H₂S is at the very top of the list for the deadliest safety hazard in a gas plant. The H₂S is contained in the gas (coming into the plant) before it is stripped out. Should a leak occur, the gas is colourless, odorless and heavier than air, so settles in low spots. The colourless and odorless properties make it extremely hard to detect without proper equipment. Exposure can cause death within three minutes.

Safety people wore H₂S detectors and monitors and breathing apparatus and air was available in all buildings. There was an alarm system for the entire plant.

This chemical, hydrogen sulfide, is so nasty because it messes with the energy production in your cells. Your mitochondria get messed up and can’t generate energy. It’s like the power switches in every cell of your body get flipped off, one at a time. Imagine turning off the lights in a big warehouse: one section goes dark. Then another. Then another...

Because all multicellular organisms have mitochondria, hydrogen sulfide in high enough concentrations should be a poison to almost any living creature.

But this unassuming little fish is able to survive levels that would kill almost anything else.

This fish is Poecilia mexicana, a relative of aquarium (and evolutionary biology) favourites like guppies and mollies. As you might guess from the second half of its name, it lives in Mexico – southern Mexico, to be exact, where there are several small rivers with very high levels of hydrogen sulfide. The high levels of hydrogen sulfide are due to the geology of the area: the hydrogen sulfide is in the ground springwater that feeds the rivers.

It’s not only that the hydrogen sulfide levels are high that makes these rivers difficult to live in. Because sulfur reacts with oxygen, the oxygen levels in these rivers are low.

And it’s not the only species; P. sulphuraria also lives in such rivers. In fact, between the two species, it looks like these fishes have invaded these dangerous rivers three separate times. The close relative of these fish do not withstand hydrogen sulfide, so the ancestors of all the locals didn’t have some built-in resistance to the poison.

Given than hydrogen sulfide’s main effect is at the subcellular level, it’s surprising that all these lineages of fish show substantial changes in their body shape: fishes in the high hydrogen sulfide streams have larger heads than those that do not. The selective pressure is probably not the hydrogen sulfide, but the low oxygen in the streams. Larger heads mean more space for bigger gills, which intuitively means more oxygen uptake. This hasn’t been tested physiollogicaly yet, though.

There are also some changes in the fin position in P. sulphuraria compared to their relatives in less toxic streams. This does not seem to have any adaptive significance. They do seem to indicate that this population is off on its own independent evolutionary pathway, however.

Curiously, small fish tolerated the hydrogen sulfide much better than large ones. The reason for this is not discussed in the discussion, and I can’t figure out a reasonable hypothesis for why.

How do these become superfish, impervious to this poison? All animals make some hydrogen sulfide naturally, and so naturally have chemical pathways to get rid of tiny amounts of hydrogen sulfide. Presumably, these pathways have been ramped up to the max in these fish. This remains to be tested physiologically.

Ramping up those defenses against the hydrogen sulfide may not come cheap, though. These are not “superfish,” whose genes are spreading through nearby streams because of their ability to tolerate this poison. The mechanism and the cost of tolerating the hydrogen sulfide remains to be seen. There’s still much to do to understand how these fish were able to make it in these hostile environments.

Reference

Tobler M, Palacios M, Chapman L, Mitrofanov I, Bierbach D, Plath M, Arias-Rodriguez L, García de León F, & Mateos M (2011). Evolution in extreme environments: replicated phenotypic differentiation in livebearing fish inhabiting sulfidic springs Evolution DOI: 10.1111/j.1558-5646.2011.01298.x

Photo from Michael Tobler’s own website.

This post is part of the National Evolutionary Synthesis Center (NESCent) competition for the Science Online 2012 conference.