The Carnivorous Waterwheel

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Bladderworts (Utricularia spp.) aren't the only carnivorous plants stalking prey below the water surface. Meet the waterwheel (Aldrovanda vesiculosa). At first glance it looks rather unassuming but closer inspection will reveal that this carnivore is well equipped for capturing unsuspecting prey. 

The waterwheel never bothers with roots. Instead, it lives out its life as a free floating sprig, its stem is covered in whorls of filamentous leaves, each tipped with a tiny trap. The trapping mechanism is a bit different from its bladderwort neighbors. Instead of bladders, the waterwheel produces snap traps that closely resemble those of the Venus fly trap (Dionaea muscipula). These traps function in a similar way. When zooplankton or even a small fish trigger the bristles along the rim, the trap snaps shut and begins the digestion process.

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This similarity to the Venus fly trap is more than superficial. DNA analysis reveals that they are in fact close cousins. Together with the sundews, these plants make up the family Droseraceae. The evolutionary history of this clade is a bit confusing thanks to a limited fossil record. Today, the waterwheel is the only extant member of the genus Aldrovanda but fossilized seeds and pollen reveal that this group was once a bit more diverse during the Eocene. Whenever these genera diverged, it happened a long time ago and little evidence of it remains.

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At one point in time, the waterwheel could be found growing in wetland habitats throughout Africa, Europe, Asia, and even Australia. Today it is considered at risk of extinction. Its numbers have been severely reduced thanks to wetland degradation and destruction. Of the 379 known historical populations, only about 50 remain intact today and many of these are in rough shape. Agricultural and industrial runoff are exacting a significant toll on its long term survival. To make matters worse, sexual reproduction in the waterwheel is a rare event. Most often this plant reproduces vegetatively, reducing genetic diversity. What's more, natural dispersal into new habitats is extremely limited. 

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Oddly enough, populations of this plant have popped up in a few locations in eastern North America. These introductions were not a mistake either. Carnivorous plant enthusiasts concerned with the plight of this species in its native habitat began introducing it into water ways in New Jersey, New York, and Virginia where it is now established. Oddly enough, these introductions have performed far better than any of the reintroduction attempts made in its native range in Europe. Of course, this is always cause for concern. Endangered or not, the introduction of a species into new habitat is always risky. Still, there is hope yet for this species. Its popularity among plant growers has led to an increase in numbers in cultivation. At least folks have learned how to cultivate it until more comprehensive and effective conservation measures can be put into place. 

Photo Credits: [1] [2] [3] [4]

Further Reading: [1] [2] [3] [4] [5]

The Bladderwort Microbiome Revealed

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The bladderworts (Utricularia spp.) are among the most cosmopolitan groups of carnivorous plants on this planet. Despite their popularity, their carnivorous habits have been subject to some debate. Close observation reveals that prey capture rates are surprisingly low for most species. This has led some to suggest that the bladderworts may be benefiting from more passive forms of nutrient acquisition. To better understand how these plants utilize their traps, a team of researchers decided to take a closer look at the microbiome living within. 

The team analyzed the trap fluid of a handful of floating aquatic bladderwort species - U. vulgaris, U. australis, and U reflexa. In doing so, they uncovered a bewildering variety of microorganisms perfectly at home within the bladderwort traps. Thanks to sophisticated genetic tools, they were able to classify these microbes in order to investigate what exactly they might be doing inside the traps. 

Their findings were quite astonishing to say the least. The traps of these plants harbor extremely rich microbial communities, far richer than the microbial diversity of other carnivorous plant traps. In fact, the richness of these microbial communities were more akin to the richness seen in the rooting zone of terrestrial plants or the gut of a cow. In terms of the species present, the microbial communities of bladderwort traps most closely resembled that of the pitchers of Sarracenia species as well as the guts of herbivorous iguanas.

The similarities with herbivore guts is quite remarkable. Its not just coincidental either. The types of microbes they found weren't new to science but their function was a bit of a surprise. A large percentage of the bacteria living within the fluid are famously known for producing enzymes that digest complex plant tissues. Similarly, the team found related microbe groups that specialize on anaerobic fermentation. These types of microbes in particular are largely responsible for the breakdown of plant materials in the rumen of cattle.

As it turns out, the microbes living within the traps of these bladderworts are serving a very important purpose for the plant - they are breaking down plant and algae cells that find their way into the traps each time they open and close. In doing so, they give off valuable nutrients that the bladderworts can then absorb and utilize. Let me say that again, the bacteria living in bladderwort traps are digesting algae and other plant materials that these carnivorous plants can then absorb.

Now these bacteria are also responsible for producing a lot of methane in the process. Interestingly enough, the team was not able to detect measurable levels of methane leaving the traps. This would be odd if it wasn't for the community of methane-feeding microbes also discovered living within the traps. The team believes that these organisms metabolize all of the methane being produced before it can escape the traps. 

As remarkable as these findings are, I don't want to give the impression that these carnivorous plants have taken up a strict vegetarian lifestyle. The team also found myriad other microorganisms within the bladder traps, many of them being carnivores themselves. The team also found a rich protist community. A majority of these were euglenids and ciliates. 

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These sorts of protists are important microbial predators and the numbers recorded within the traps suggest that they are a rather significant component of these trap communities. As they chase down and consume bacteria and other protists, they release valuable nutrients that the plants can absorb and utilize. Numbers of these predatory protists were much higher in older traps, which have had much more time to accumulate a diverse microbiome. Astonishingly, it is estimated that the protist communities can cycle the entire contents of the bladderwort traps upwards of 4 or 5 times in a 24 hour period. That is some serious turnover of nutrients!

The protists weren't the only predators found within the traps either. There are also a considerable amount of bacterial predators living there as well. These not only cycle nutrients in similar ways to the protist community, it is likely they also exhibit strong controls on the biodiversity within this miniature ecosystem. In other words, they are considered keystone predators of these microcosms.

Also present within the traps were large amounts of fungal DNA. None of the species they found are thought to actually live within the traps. Rather, it is thought that they are taken up as spores blown in from the surrounding environment. Exactly how these organisms find themselves living inside bladderwort traps is something worth considering. The plants themselves are known for being covered in biomfilms. It is likely that many of the organisms living within the traps were those found living on the plants originally. 

Taken together, the remarkable discovery of such complex microbial communities living on and within these carnivorous plants shows just how complex the ecology of such systems really are. Far from the active predators we like to think of them as, the bladderworts nonetheless rely on a mixture of symbiotic orgnaisms to provide them with the nutrients that they need. The fact that these plants are in large part digesting plant and algae materials is what I find most astonishing.

Essentially, one can almost think of bladderworts as plants adorned with tiny, complex cow stomachs, each utilizing their microbial community to gain as much nutrients as they can from their living environment. The bladderworts gain access to nutrients and the microbes get a place to live. The bladderworts really do seem to be cultivating a favorable habitat for these organisms as well. Analysis of the bladder fluid demonstrated that the plants actively regulate the pH of the fluid to maintain their living community of digestive assistants. In doing so, they are able to offset the relative rarity of prey capture. Keep in mind that this research was performed on only three species of bladderwort originating from similar habitats. Imagine what we will find in the traps of the multitude of other Utricularia species.

Photo Credits: [1] [2] [3] [4]

Further Reading: [1]