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Here's an Arcella amoeba. It reminds us how far soil biology has yet to go.

This week we came across something unusual, and of course it got us all geeky. It was an amoeba we found in an aged batch of inoculum material.

But it wasn’t just any amoeba. It was a member of the genus Arcella, which dwarfs most other amoebae. You can see the size difference in the photo. The egg-shaped object, just below and to the right of the round Arcella (of which we found several), is a roughly average-looking testate amoeba. Arcella eat amoebae of this size, along with many other soil-dwelling denizens.

For all the time we’ve been aware of Arcella, we know remarkably little about them. The genus was named way back in 1832, but hasn’t been widely studied. Over the years Arcella species have been reported mostly in freshwater aquatic habitats and mosses, and in soils only rarely.

This is the first Arcella we can recall having ever seen, but several of our colleagues report finding them with some regularity. There’s a prevailing view among students of the soil ecologist Dr. Elaine Ingham (and professionals, like us, who employ her techniques) that the notion that Arcella are rare in soils is mistaken.

Arcella tests tend to form in shapes like hats, or like bent and contorted discs. Generally there’s a hole in the center from which the amoeba stretches its pseudopods (extensions that ooze out like arms) for locomotion and feeding; you can see the off-center holes in our Arcella. In the right-hand specimen, you can also see the amoeba itself, apparently pressed toward the lower-right side of its test.

The fact that we haven’t encountered many Arcella before doesn’t mean they’re necessarily rare in soils. In our work, we don’t deal with taxonomy, but instead with broad organism categories that we can identify by shared morphologies. There are many, many genera and species of amoeba, most of them similar in appearance. Are Arcella any more scarce than the average taxa among those others? Hard to say, but probably not. They just look distinctive, so you know when you see one.

It may seem odd that we so lack awareness of this distinct and very noticeable organism in our soils. But not if you keep in mind the reality of 20th Century scientific inquiry, which mostly ignored the soil microbiome. Before Dr. Ingham and her colleagues published their landmark work in the 1980s, we possessed little understanding of how soil microbes form communities and interact with plants and other microbes.

But now we have techniques (thanks again to Dr. Ingham) for employing simple microscopy that allows us to correlate microbial populations with plant behavior. As we use this knowledge to solve the problems of modern agriculture, we continue developing a greater awareness of all the diverse microbes out there, and the roles they each play in the below-ground universe.

Which is, of course, a big reason to always be looking. If you keep looking at your soil, you’ll soon find something interesting and cool -- maybe even something new.

As always, stay curious.

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