Pity the insect that tumbles into a pitcher plant’s trap. The slippery walls and waiting pool of water ensure it won’t clamber back out. There’s nothing left to do but wait to be digested.
The California pitcher plant (Darlingtonia californica) is also called the cobra lily for its curled-over shape that hides its exit from its victims. Unlike other pitcher plants, it doesn’t fill its trap from above with rainwater but from below, drawing water up with its roots. But like others, it seems to use bacteria living in that well to help digest its prey.
The bacteria perform another role too: making the liquid even harder for an insect to escape than ordinary water.
Nearly a century ago, scientists first noticed that the water in the traps of some pitcher plant species had unusually low surface tension. This means an insect that’s used to safely tiptoeing across puddles suddenly finds itself drowning inside a pitcher plant. But the reason for this extra-deadly water wasn’t clear. In 2007, Laurence Gaume and Yoel Forterre of France’s CNRS studied the liquid of the pitcher plant Nepenthes rafflesiana and found that it has so-called viscoelastic properties. This means “the power of gluing insects,” Gaume explains, and of forming watery filaments that cling to a struggling bug.
Inspired by that research, David Armitage, a graduate student at the University of California, Berkeley, wondered whether some of the unusual properties of a pitcher plant’s liquid might come from the bacteria residing there.
He gathered water from six D. californica wells and measured the liquid’s surface tension. It was significantly lower than the surface tension of plain water.
Then Armitage filtered the bacteria out of these fluids and used them to create artificial pitcher plant traps in the lab. He started with glass tubes and added sterile water and small amounts of pitcher bacteria, along with ground-up crickets to feed the bacteria. The resulting fluids had similar surface tension to the natural pitcher plant fluids. After leaving the tubes alone for a month, Armitage dropped ants into them.
In the video above, the tube all the way on the left contains plain water. The other tubes hold either natural or artificial pitcher plant fluid.
If you don’t have time to watch eight minutes of struggling arthropods set to Beethoven’s Moonlight Sonata, here’s a spoiler: none of them makes it out of the pitcher plant liquids. The ant in plain water stays at the top of the tube, while the rest sink. (Armitage says that despite appearances, all the ants in the video were totally fine about 20 minutes after he plucked them from their traps.)
No ants in plain water fell below the surface. Ants in pitcher fluids, or artificial pitcher fluids, mostly sank. But as Armitage made the artificial pitcher fluids using smaller and smaller concentrations of bacteria, the ants became more likely to escape.
Gaume, who didn’t work on Armitage’s study, says this convincingly shows that pitcher plant bacteria help to keep prey from escaping. She notes that the fluid of D. californica doesn’t have all the sticky properties of the N. rafflesiana fluid she studied, though; different pitcher plant species may use different sets of tricks to hold onto their victims. And, Gaume adds, it’s still possible that the plant itself makes a liquid with low surface tension.
There are about 200 to 500 species of bacteria present in pitcher plant fluid, says Armitage, who’s now at the University of Notre Dame. “A few common species seem to be members of groups known to produce compounds that affect the surface tension of their medium,” he says, but it will take more research to figure out exactly which bacteria make the water so dangerous.
“Alternatively,” Armitage adds, the low surface tension could be simply a side effect of the bacteria digesting bugs in the traps, “as fatty oils from the insect corpses are released into the water column.”
Either way, the partnership between pitcher plants and their resident bacteria seems to run deep. Carnivorous plants don’t necessarily work alone, and the cobra lily relies on microscopic partners for its deadly bite.
Image: by NoahElhardt (via Wikimedia Commons)
Armitage DW (2016). Bacteria facilitate prey retention by the pitcher plant Darlingtonia californica. Biology letters, 12 (11) PMID: 27881762