Bladderworts, carnivorous plants of the genus Utricularia, live in water or soggy soil. To snare their snacks, bladderworts set ingenious little traps, sometimes in the hundreds, among their waterborne leaves. The traps maintain an internal pressure lower than that outside; when passing prey triggers an exterior hair, a trapdoor snaps open, and inflowing water carries the prey inside to be digested.

Biologists have long noted algae among the insects, nematodes, and other minute animal prey in bladderwort traps. Are the algae symbionts? Are they swept in accidentally with animals?  Or could bladderworts actually eat algae?

To advance the debate, Marianne Peroutka of the University of Vienna and several colleagues analyzed 1,450 traps from four species of Utricularia. More than half the traps contained algae, often unaccompanied by animal prey. In fact, algae constituted as much as 80 percent of trap contents under certain conditions.

Intriguingly, the softer the water the plant inhabited, the more algae its bladders bore.

Soft water, low in minerals, supports less animal life than hard water does, and Peroutka thinks bladderworts may compensate for the lack of meat by eating more greens. Indeed, some of the entrapped algae appeared semi-digested, as others have noted. 

A few other carnivorous plants are known to eat plant matter, so perhaps we should start calling them omnivores.

The finding was detailed in the journal Plant Ecology.