Lab Notes

Rearing a Plankton Menagerie

by Shawn Carlson

Amateurs can easily rear both marine and freshwater plankton for examination, for feeding larger aquatic animals or for use in more advanced research projects. Ocean enthusiasts should go to their local aquarium store and purchase a kit to make 50 gallons of seawater (for about $15) as well as a simple salinity tester. You'll need to order the plankton from Aquaculture Supply or call 352-567-0226. Make sure they also sell you a copy of Plankton Culture Manual, by Frank H. Hoff and Terry W. Snell (florida Aqua Farms, 1999; $26.50)--the bible of plankton cultivation. I recently grew up a batch of Nannochloropsis (catalogue no. AA-NCP, $8.50) and Tetraselmis (AA-TET, $11), both green algae that can live in either freshwater or salt water. And I raised a little saltwater rotifer known as Brachionus plicatilis (AB-R1S, $10). You may also want to grow diatoms‹a type of algae that strengthens its cell walls with fantastically beautiful silica structures. If so, a good choice might be Chaetoceros (AA-CHA, $11).

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Clear plastic soda bottles in the two-liter size make ideal culture flasks. To prevent yours from being taken over by bacteria, you'll need to sterilize everything before you begin. So go to a store that sells pool supplies and purchase granular chlorine. Dissolve as much of the solid as possible into 30 milliliters (about an ounce) of warm water. Then prepare a 10-to-1 dilution by mixing five milliliters (one teaspoon) of the concentrated chlorine solution into 45 milliliters of distilled water. Be careful you don't transfer any undissolved crystals into the sterilizing solution you are preparing.

Next, fill your two-liter containers nearly to the top with either distilled water or seawater and add five drops of the sterilizing solution to each. Wait two hours for the chlorine to do its work. Chlorine evaporates quickly from solution, so you'll have to make up a fresh batch of sterilizing fluid every time you need some. In this sense, evaporation is a nuisance, but you can take advantage of it to remove the chlorine in the flasks by bubbling air through the water for about 24 hours. A few drops of bottled dechlorinating agent from a tropical-fish store will do the job in seconds. Either way, don't introduce your plankton until you've verified, using a kit for testing home pools, that no chlorine is detectable.

A single pump for a 10-gallon aquarium can easily aerate 10 culture flasks. Use a multiport manifold (a common piece of aquarium plumbing with one input and many outputs) to distribute the air to the different cultures. Some stiff plastic tubing (also available at the aquarium store) will allow you to inject the air at the bottom of each flask. But you should pump it through a filter with 0.5-micron openings, such as SLFH05010 from Millipore ($79 for a 10-pack; 800-645-5476), to keep bacteria from invading your sterilized containers [see illustration on opposite below].

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Now enrich each flask with the appropriate nutrients. Aquaculture Supply sells Micro Algae Grow (catalogue no. FA-MIS, $4.20) for cultivating most kinds of green algae and Liquid Silicate Solution (FA-SS6, $3.50) for culturing diatoms. Directions come with the packages.

The plankton samples arrive in the mail growing in small plastic dishes filled with gelatin. To remove the living cells, submerge the gel beneath a thin layer of your growing solution and allow it to soak for 12 hours. The microorganisms will then easily rub off the gel under the gentle pressure of a sterile cotton swab. Inoculate each flask with about 10 milliliters (two teaspoons) of the resulting solution. Make sure at every step that all your instruments are germ-free by carefully washing them with detergent and sterilizing solution and then rinsing them with distilled water.

Ideally, your culture should be incubated at 19 degrees Celsius (about 66 degrees Fahrenheit), but I had no problems just letting mine sit at room temperature. Avoid exposure to direct sunlight, because the sun's rays can quickly heat your flasks to lethal levels. Instead place the flasks in front of a bright fluorescent lamp for 18 hours a day. A standard bulb of at least 2,500 lumens works fine, but some aquarists recommend "grow-lights," which produce more of the energetic blue photons used in photosynthesis.

Once you start things going, you should keep aerating the water constantly. In about a week, your container should attain a deep green hue, which indicates that the culture is mature and ready to feed to other aquatic creatures. In as few as 10 days, the cellular population explosion can generate enough waste to poison itself, so don't wait too long. If you extract 10 milliliters of mature culture to start a new batch, you'll never need to purchase another starter gel.

The professionals grow larger quantities of algae in 20-liter (five-gallon) containers called carboys. Some scientific supply companies charge $100 for these transparent plastic bottles, but you could just as well use a discarded five-gallon jug from a watercooler. Aquarists usually install a special arrangement of tubing into their carboys to pass the air through without risking contamination. I used a hot-air gun to bend a stiff plastic aquarium tube and achieved the same result [see illustration at left].

Want to grow a lot of plankton? Fill an empty water jug with distilled water or salt water and add five milliliters of fresh sterilizing solution. As before, let things stand for two hours, then dechlorinate the water and test it. Add the necessary nutrients and inoculate the jug with the contents of one complete flask of mature culture. Connect the air pump and make sure the container gets plenty of fluorescent light.

You can track the rate of growth with a special dipstick sold by Aquaculture Supply (AC-DM9, $7.75). Just submerge the stick into the jug until the greenish water obscures the black ring on the bottom, then read the depth off the scale on the side. For each species, you can gauge the density of cells using a table supplied with the stick. After about a week, my water jug had more than 10 million cells living in each milliliter‹some 200 billion cells in all.