Day 1

I've done this hundreds of times. Stooping at the edge of a road-side ditch, I dip up a jar of water, making sure to include some algae, and a glob of decomposing leaves. I know this simple collecting technique is guaranteed to conjure up some amazing small organisms, but I never know what's really in store. I'm gambling there's a good topic for a micronaturalist exploration. Back in the lab, the jar goes on an east-facing window sill.

Three days later a distinctive odor has begun to permeate the lab, coming from the jar of decaying leaves. (Why can’t I find a suitable lid?) A bacterial film covers the water, and just below the surface are whitish clouds of tiny particles, barely visible against a dark background. They seem to undulate slowly beneath the scummy surface. Using a finely drawn-out glass pipette, I suck up a sample and transfer the material to a slide. I carefully ease on a coverglass, and the moving specks disperse evenly throughout the liquid inside the glass sandwich.

Distracted by a phone solicitor for 30 seconds, I return to look again at the slide on the microscope stage and, to my surprise, the specks are no longer evenly distributed, but have formed several clumps. Scanning the preparation using the 4X objective (with its 5mm field of view) shows small, smoothly moving cells clustering around bits of detritus picked up with the sample. The detritus seems to be attracting free-swimming cells from the surrounding fluid.

A wide, dark-field view of Colpidium	A dark-field view of Colpidium reveals its bluish macronucleus.

The cells swim in a spiral, rotating around their long axis, but they stop this "free swim" as they near their tightly packed sisters. Centering on a growing cluster of cells, I change to the 10X objective and adjust the iris to create higher contrast image. A quick scan of the drawings in Jahn, How to Know the Protozoa, convinces me that this is Colpidium, a bacteria feeding ciliate, belonging to one of the main branches of ciliate evolution, the holotrichs. If Paramecium is a deer and Didinium is a wolf, then Colpidium must be a rabbit. All of the clusters are surrounding bits of detritus shimmering with bacteria.

A view of Colpidium showing the formation of food vacuoles. On the right is my notebook illustration	Colpidium Notebook Illustration

The high power objective (40X) shows that the cells are covered by cilia. There is an indented region near the front end, leading into a cytostome (cell mouth). The mouth opens into a short gullet that terminates in a membrane bubble, rapidly filling with bacteria. The food vacuole breaks free, moving toward the posterior end of the cell where it joins several other bacteria-filled vacuoles. At the same time, a new food vacuole begins forming at the end of the gullet. This type of rapid feeding by ciliates consumes up to 40% of the cell°s plasma membrane per hour, converting it into food vacuoles. Following digestion, the vacuole migrates to the cell surface, ejects some undigested material and the vacuole membrane merges back into the plasma membrane.

To see details on the cell surface, I use a scrap of paper towel to draw water from under the coverglass, pressing the cells just enough to slow them down. Centering a cluster of colpidia, I apply a drop of immersion oil and carefully immerse the oil immersion lens. Under this extreme magnification one Colpidium fills the field of view. Tuning the differential interference lighting system shows the rows of individual cilia. It’s a stunning sight. The cilia are perfectly aligned and their beat forms a flowing wave that travels along the row. Differential interference contrast lighting (DIC) creates very thin optical sections, and with a slight change of focus I make another discovery. Just below the cell surface there are mitochondria lining each of the ciliary rows. Of course, that’s exactly where you would expect to find them. Mitochondria are the power-houses that supply ATP, the molecule that provides the energy that drives the ciliary beat.

Bands of cilia as seen with DIC lighting

DIC lighting reveals mitochondrialined up beneath the bands of cilia

Later

A few days later, I look at the stinking brew jar again only to find the clouds of Colpidium have disappeared, replaced by some barely visible specks. The microscope shows the specks to be Euplotes, a large ciliate with an appetite for cells in the size range of Colpidium. I’m guessing that these large predators wiped out the Colpidium population, but there could have been other factors at work. Had I followed events more closely, I might have learned a good deal more. Think I’ll culture up another batch of Colpidium, introduce a few Euplotes, and start making daily observations. Sounds like a future Notebook entry!