The plant that farms other plants for food

The genus Utricularia is a collection of over 200 species of carnivorous plant found around the world. Above the ground, they look pretty, but not particularly odd. However, below the leaf shoots come from a stolon, a runner, which holds something peculiar. The stolon holds lots of small ‘bladders’ which act as suction traps. When they’re triggered, a trap door opens and whatever was unlucky enough to be outside gets sucked in. Once inside the plant digests whatever came through the door before resetting the trap. It’s a staggeringly fast trap and excellent for catching prey. However, when you start looking closely at what the plant is catching, you see something peculiar. A lot of the time the bladderwort is catching algae.

About ten years ago Peroutka and colleagues recognised this was a problem. They calculated that up to 80% of possible prey in a bladderwort plant was algae. That’s a problem. If Utricularia is a carnivorous plant, then what’s it doing with all that extra matter? If 80% of what you put into your mouth weren’t food, then you’d be causing a lot of trouble for your digestive system, and the same would be true for a carnivorous plant. An alternative was that Utricularia was digesting the algae.

A few years ago Koller-Peroutka and colleagues were able to show that Utricularia benefitted from catching algae. Unlike animals, plants wouldn’t trigger the trap, but the bladders fired at random and opportunistically dragged in whatever was around, including algae – which explained how some traps had nothing but algae in them.

Ellwood and colleagues have recently tested this finding by Koller-Peroutka’s group. They examined the traps of a number of Utricularia plants living in Italy, to see what they were eating. They didn’t think the algae at their sites could trigger the traps. Nonetheless almost three-quarters of the traps had only phytoplankton in them – and no animals that triggered the trap. Looking at the algae in the traps the authors say: “the proportion of trapped algae either dead or decaying ranged from 5% to 65%.” They also found the variety of phytoplankton in the traps was pretty much like the phytoplankton you could find outside the traps. From this, they conclude that the traps are firing randomly to catch plant material and – because the cells are being broken down – that this is something that bladderwort is aiming to do. It will eat its greens as well as meat.

However, another recent paper shows that Utricularia isn’t purely a hunter. Sirová and colleagues have been looking at how Utricularia breaks down its food to digest it. They argue that Utricularia is also a microbial farmer.

The farming element is down to where the algae Utricularia eats comes from. Sirová and colleagues refer back to earlier work by Jennifer Richards. She notes, among other things: “Free‐floating plants generally have at least three whorls of mature leaves before they become completely covered with algae and appear to be senescent.” If Utricularia encourages the growth of algae outside the traps, then it has a supply of food to eat close to the traps when they fire.

Sirová’s team also shows that the traps host fungi, bacteria and protozoa. The combination of guests in the trap work on degrading the plant material and then each other into a form that Utricularia can digest. The authors compare the digestive teamwork to the kind of microbial community you could find in a ruminant. They go on to say that the traps with their microbial communities “…[R]epresent unique biodiversity and activity hotspots within the nutrient-poor, dystrophic environments, in which they grow…”

Linnaeus never accepted that plants could be carnivorous. Plants were prey for other organisms, not predators. This recent research shows that calling Utricularia ‘carnivorous’ is still applying a human idea to a plant. From Utricularia‘s point of view, there might not be too much difference between a microscopic animal and microscopic plant. Rather than thinking of it a carnivorous or herbivorous, Utricularia is simply opportunistic. The suggestion by Sirová and colleagues that it helps algae grow as a food source also indicates that it’s very good at making an opportunity for itself.