Return to Diatom Alley

Some days back, we talked about the start of Colin’s diatom growth experiment. Colin has collected diatoms from the Southern Ocean and has been growing them in Diatom Alley. You can read all about Colin’s motivation and goals for the growth experiment here.

Since he began the experiment, setting the diatoms up with all they need to thrive and reproduce—ample light, nutrients, mixing of the water, and an optimal temperature—Colin has monitored their growth.

Diatom growth is exponential. Assuming each diatom survives, think of it like this: a single diatom splits into two to become two diatoms. Each of those diatoms splits in two, and now you have four. Each of those four splits in two, and now you have eight, and after splitting again sixteen, then thirty-two, then sixty-four, and so on.

Exponential diatom growth over time. [Diagram by Marlo Garnsworthy]

To monitor how they’re growing, Colin has been using two methods. One is to take a sample of water from each of the three carboys in which the diatoms are growing and test its fluorescence, which is related to the amount of chlorophyll within the sample. (Remember, chlorophyll is the green pigment within plants that they use to photosynthesize.) An increase in fluorescence shows the diatoms are growing and reproducing because more chlorophyll roughly means a greater number of diatoms.

Of course, since the experiment began with seawater, there are likely phytoplankton other than diatoms in each sample, as well as a very small proportion of zooplankton (tiny plankton drifting through the oceans, but animals not plants, many of which eat phytoplankton). But when conditions are optimal—meaning there is ample light, an abundance of the right nutrients, the right temperature, and mixing—diatoms tend to dominate over other kinds of phytoplankton. The effects of any zooplankton on the sample (meaning, predation) are likely to be negligible and therefore not affect this experiment in any significant way.

So far, Colin has seen a steady exponential increase in fluorescence, indicating that the diatoms love the conditions in Diatom Alley and are growing and reproducing well—just as expected.

Colin’s second method of monitoring what is happening in the three carboys is to test the amount of nitrate in the water. Along with silica, iron, some vitamins, and other nutrients, nitrate is one of the nutrients he supplied to the diatoms.

To measure nitrate, he takes a small sample of water from each of the three carboys and runs it through a machine called an UV-Vis IC (Ultraviolet- Visible (as in wavelength of light) ion chromatograph), which is designed to test the amount of nitrate in a sample (as well as some other elements/compounds when so desired). Colin is looking for a decrease in the amount of nitrate, and the amount remaining in each sample indicates how quickly the diatoms in each carboy are using up the available nutrients.

Colin runs a sample of water through the IC to analyze its nitrate content. (Note how the IC, like everything else aboard the ship, is tied down!)

At first, Colin needed to perform these measurements only once a day, but because diatom growth is exponential, the frequency of measurement must increase. Once the diatoms were to thriving, he measured fluorescence and nitrate twice a day. Now the diatoms have used up nearly 50% of the available nutrients—the point at which the experiment will be complete—he is measuring fluorescence and nitrate much more often, every six hours.

Last night, I was excited to see a visible change in the water in the carboys. Colin says he saw a huge change just from the morning when they were just starting to look greenish. They look decidedly green now!

Once the diatoms have used up 50% of their nutrients, it’s time to complete this part of the experiment. Colin will filter the diatoms out of the water, package them for transport, and freeze them. Back in the lab in Rhode Island, he will perform more analyses of what he has collected.

He will soon start the begin this experiment all over again with water samples from 61° S. He has chosen this area because it’s in the polar frontal zone where a very different diatom community lives. But first, it’s time to filter, which I’ll be exploring on the blog in the next day or so.

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