In each pinch of soil are scores of little shelled amoebae, crawling around like microscopic mollusks bearing their homes on their backs, their tentacles grasping for prey.
Such is one of the many wild kingdoms that humans never see because it exists beyond our vision, but this one is particularly obscure and unappreciated. One group to which these creatures belong, the Cercozoa, has probably never been heard of by anyone who is not a protistologist, a person who studies the microbes that have DNA storage compartments called nuclei, like ourselves.
Yet these little creatures are not only delightful to observe, they are important ecological movers and due to their diet. They prey on bacteria and archaea, the most microscopic and abundant life on Earth (unless you count viruses).
Now a team of scientists from the University of Copenhagen and the Natural History Museum in London has conducted a molecular screen of soil in a nearby forest that reveals, conservatively, that there are at least four times as many “species” of Cercozoa in the soil than have been previously described, and likely more than that. In four samples of just a half a gram of soil, they found more than 1000 species each.They described their findings as evidence of a “large, hitherto unknown diversity”.
And I think in honor of that finding, it’s worth taking a little tour of the world of Cercozoa. But before we do, if you are a connoisseur of biodiversity like myself, after you finish this post you must look through the Cercozoa and other amoeba galleries found at this web pages of the Penard Labs, a private protistology lab in Berlin whose work I draw on in the links below. These little creatures and many others on their site are jaw-dropping in their intricacy, diversity of forms, and beauty.
The Cercozoa is a group of mostly single-celled organisms bearing long tentacles. These they use to ensnare bacteria in much the same way jellyfish harvest plankton. The tentacles are called “filopods” (a portmanteau of filose and pseudopods), and sometimes the filopods are connected to each other to form a network -- a web of trip wires waiting to trap bacteria (You can see many beautiful examples of these filamentous traps surrounded by sausage-shaped bacteria on this page.)
Some are armed with spines. Many but not all have shells. Some build their shells out of precision-crafted silica scales or buttons and look they jumped out of the pages of a Scandanavian design magazine. One even covers itself in layers of perforated siliceous pearls. To see what I mean, peruse this page. If you only look at one set of photos linked to this post, this is the one.
Other cercozoans lack shells entirely. The vampyrellids, as their name implies, pierce the cells of algal hosts and drain their vital fluids. They turn green and then orange from their steady diet of chloroplasts.
Another cercozoan called Leptophrys swallows prey like diatoms whole. With this done, Leptophrys converts its full body into a digestive cyst. These cysts are often mirror images of the food contained inside, like a snake that’s swallowed a rabbit. New Leptophrys amoebae later escape from these cysts via holes they drill in the side (see bottom eight photos here).
Some cercozoans are heliozoans, a purely descriptive term for any protist bearing filopods stiffened by rods called microtubules. This gives them the appearance of a radiant sun, hence the name “sun animal”. Some of the heliozoans are further – and incredibly beautifully -- encased in ornamented shells borne on long stalks (see the top three photos on this page.)
Finally, Cercozoans also include naked little amoebae evocatively called Viridiraptor (algae raptor). True to their name, they invade much larger algal cells and eat the owner out of house and home just as parasitic wasp larvae do to insects (you really must see these images, particularly the algal cell being cleaned out by a bunch of cerozoid bank robbers at top, and the Spirogyra algae that has been invaded by not one but two of these pernicious pests at bottom.)
Though unquestionably fascinating to look at, these little guys are also important to soil biology. Just by looking through soil scientists have surmised that the Cercozoa is one of the most dominant groups of free-living protists in temperate soils, but molecular studies have confirmed this.
A 2013 molecular soil screen found that Cercozoans made up about 30% of protists in desert or semi-desert soils and about 15% in wetter soil. A 2015 study of the genes being turned into proteins in forest and grassland soil found that 40-60% of these were produced by Cercozoa. In the ocean, too, they play an important role. Their DNA was found in something from a tenth to a quarter of all sequences found in ocean floor sediments in a 2011 study.
In this study of cercomonads, the most abundant grouop, representing more than 40% of the sequences they detected in their soil samples, were small cercozoans called Glissomonads, which include the rapacious Viridiraptor I just described. The single most abundant genus was Trinema, one of the large shelled amoebas clad in mid-century mod silica scales.
Yet even with all the diversity I just described, it’s likely there is much more out there. I’ll close with some revealing and refreshingly frank words from their paper:
It is commonplace that [DNA] sequencing studies find a high unknown diversity of microorganisms in almost all habitats. Hence, we were not surprised to find a high level of unknown cercozoan diversity. However, we find it remarkable that even with our highly conservative data treatment, the number of [functional species] on our single [biological marker] still exceeds by a factor of more than four the number of Cercozoa that have been assigned a name based on their morphology.
We don’t have to leave Earth to find gaping holes in the biological map filled with exciting new continents awaiting discovery.
Harder, Christoffer Bugge, Regin Rønn, Asker Brejnrod, David Bass, Waleed Abu Al-Soud, and Flemming Ekelund. "Local diversity of heathland Cercozoa explored by in-depth sequencing." The ISME journal (2016).