March 14, 2002
New molecular tools make study of tiny phytoplankton more possible
It’s the most prolific plant in the world’s oceans, explains assistant professor Ginger Armbrust.
Behind her the screen is filled with the image of a single cell, one of the diatoms that flourishes in our coastal waters. It’s far too small to see with the naked eye but projected here it’s 12 feet long.
However, Armbrust isn’t talking about Ditylum brightwelli, resplendent on the screen in tones of red, yellow and green. Rather it’s just to the left, there, where she has to use an arrow to draw our attention to what’s only a black dot even at this tremendous magnification.
At 0.6 microns, a hundred of these cells could fit end-to-end across the width of a human hair.
“They are the most abundant photosynthetic organism in the ocean,” she says. But studying something that we can’t identify with powerful microscopes, much less track how they behave, has been almost impossible.
“Thanks to the development of new molecular tools that allow us to study species at the DNA level, biological oceanography has entered into an exciting period of discovery,” says Armbrust, a faculty member in the UW’s School of Oceanography who was the final speaker in the Ocean to Stars Lecture Series sponsored by the school and the College of Ocean and Fishery Sciences.
Phytoplankton are single-celled algae that generate about half the oxygen we breathe, form the base of the food web in the seas and remove the greenhouse gas carbon dioxide from the atmosphere.
Scientists such as Armbrust are interested in phytoplankton communities, how they interact with what’s around them and how they respond to change. For example:
Global climate change affects sea temperatures and the amount of light penetrating into the ocean, two things that cause phytoplankton to take up very different amounts of carbon dioxide.
Sewage effluent and agricultural runoff entering the ocean act as fertilizers, causing huge blooms of phytoplankton that can’t be sustained and, when they die, rob the water of oxygen and kill most other life. Sometimes spanning an area the size of New Jersey, the largest such U.S. “dead zone” is in the Gulf of Mexico.
The number of toxic algae outbreaks — which can be hazardous to fish, other marine organisms and sometimes humans — are on the rise probably because of pollution and other growing pressures from human activities in coastal waters.
Sequencing cellular DNA help scientists, first, with the perplexing task of identifying and classifying phytoplankton that defy the traditional approach of visually examining and grouping organisms using common morphological characteristics.
It turns out that Puget Sound, for instance, has a community of phytoplankton distinct from those in the close-by waters of the Strait of Juan de Fuca. Individuals within a single species all look the same to us, Armbrust says, but students and faculty working in UW’s Marine Molecular Biotechnology Laboratory have been able to measure the genetic difference and discern the diversity of species and subspecies.
DNA sequencing has uses beyond simple identification. Molecular tools can monitor phytoplankton interacting with what’s in the ocean around them. For instance, scientists had thought certain kinds of phytoplankton favored different depths of the ocean because they were taking advantage of high concentrations of nutrients such as nitrate and nitrite.
This assumption proved erroneous. Armbrust’s colleague in the School of Oceanography, Gabrielle Rocap, found one of the phytoplankton in question didn’t have the gene to utilize nitrate and the other didn’t have the genes to use either nitrate or nitrite. Something else in the ecosystem is driving those phytoplankton to prosper where they are.
As far as monitoring how phytoplankton interact with other microbes such as bacteria, Armbrust’s group is working the gene that is expressed when phytoplankton get too much sunlight, become stressed and need to excrete organic compounds.
“When diatoms give off organic carbon, it’s like ringing the dinner bell for bacteria,” Armbrust says. “We can watch the bacteria respond to the presence of specific organic compounds by watching them turn on genes required to utilize those compounds.”
Research by Armbrust and her graduate students is funded by the National Science Foundation, Office of Naval Research and Department of Energy, with some of her undergraduate students supported through the UW’s Mary Gates Fellowship program.
Knowing more about phytoplankton, even as tiny as they are, is important because they are the most dynamic photosynthetic communities on our planet, Armbrust says. Phytoplankton in the ocean have only 0.2 percent of the biomass of the plants on land yet they absorb carbon dioxide and produce the same 50 billion tons of organic carbon a year that land plants generate.
“Phytoplankton are the sentinel organisms telling us about the health of our oceans,” she says.
