Carnivorous Plants Reveal Hidden Microbial Diversity
Some carnivorous plants, such as pitcher plants and carnivorous bromeliads (Brocchina and Catopsis) hold ‘pools’ of liquid within the plant consisting of rainwater and secreted substances such as sugars, used to lure and trap insect prey. The pols act as pitfall traps which are designed in such a way that make them incredibly difficult to escape from, with downward pointing hairs and slippery sides of the pitchers (see article 'The Diversity and Ecology of Carnivorous Plants'). Unsurprisingly, several species have evolved to take advantage of these traps and consume the dead insect bodies, such as the crab spider Synema obscuripes and the diver ant Camponotus schmitzi, the latter of which habitually lives on pitcher plants, descending into the pitchers of Nepenthes species to retrieve insect prey.
These nutrient rich pools are also home to an array of single celled organisms called protozoa, which feed on bacteria and organic matter (see article 'Cryptic Biodiversity: The Microbes That Make Our Ecosystems Work'). The pools represent a complex microecosystem sustained by the decomposition of insects caught by the plant. As the insect bodies begin to decompose, the organic matter and other nutrients from the bodies are released and taken up by he digestive enzymes of the plant, however single celled algae and bacteria are also able to take up these nutrients for growth and metabolism.
The pools provide a habitat for a wide variety of tiny organisms, some visible to the naked eye but many microscopic, less than 1 mm in size. Many species are similar to those found in ponds and lakes, and like other freshwater , the foundations of the pool food networks are bacteria and algae, which are in turn food for protozoa.
Two main groups of protozoa have been found in these pools: flagellates and ciliates. Flagellates are smaller in size, and feed on bacteria and algae. They are distinguishable by their tail-like flagellum which pulls the cell through the water. Some species can have two or more flagella, and some will only have one.
Ciliates in general are far larger than flagellates, and feed on bacteria, algae, flagellates and even other ciliates. One of the most commonly occurring and abundant ciliates which has been found in these pools is called Colpoda steini. Colpoda is very common in a variety of habitats, including soil, freshwater and even blood, as although it is a free living species, under certain environmental pressures it can become parasitic. It is very sensitive to environmental changes though, and is notoriously difficult to study in the lab as after a couple of days will become dormant and form a cyst (a protective 'shell' surrounding the cell).
Euplotes sp. is another ciliate which has been found in the pools of carnivorous plants, and is also common in freshwater and marine ecosystems. Euplotes is a medium-sized ciliate, at around 100 microns in size, and feeds on bacteria, algae and flagellated protozoa.
In addition to sustaining microbial diversity, the pools of carnivorous plants also provide a habitat for small animals, such as mosquito larvae and copepods, tiny crustaceans distantly related to crabs. These tiny animals play a crucial role in the pools as predators of bacteria and other microbes in the pools. As such, a tiny ecosystem is sustained within the pool, with bacteria and photosynthetic algae providing food for microbial grazers including ciliates and flagellates, which are in turn food for small invertebrates including mosquito larvae and copepods.
In larger aquatic ecosystems such as rivers, lakes and oceans, larger invertebrates and small fish would feed on these invertebrates (which would be part of the zooplankton assemblage), thus transferring energy to larger organisms. In these pools, these tiny invertebrates are the top feeders in the microecosystem, however larger invertebrates such as the diver ant and crab spider (mentioned above) are known to scavenge on the dead bodies of other insects which are separate from this community.
Buch, F., Rott, M., Rottloff, S., Paetz, C., Hilke, I., Raessler, M., and Mithofer, A., 2013. Secreted pitfall-trap fluid of carnivorous Nepenthes plants is unsuitable for microbial growth. Annals of Botany, 111 (3), 375-383.
Kneitel, J. M.and Miller, T. E., 2002. Resource and top-predator regulation in the pitcher plant (Sarracenia purpurea) inquiline community. Ecology, 83 (3), 680-688.