Southern Ocean GasEx Blog

Dispatches from the Southern Ocean Gas Exchange Experiment

Day in the life of a phytoplankton

Posted by sogasex on March 1, 2008

By Bob Vaillancourt, LDEO

Our team (see picture below) is tasked with measuring the photosynthetic sink for CO2 as part of the broader Southern Ocean GasEx objective of understanding ocean-atmospheric coupling. Our approach is simply to capture water samples from the upper, sun-lit layer of the ocean and then incubate these samples, which contain millions of tiny single-celled plants called phytoplankton, under simulated light conditions, and measure the amount of CO2 that is incorporated into their little bodies. In the process of photosynthesis, this CO2 is momentarily removed from the gaseous, inorganic pool, and into the particulate organic pool. Thus the CO2 that was once floating around in our atmosphere has now been moved into the oceanic food web. This carbon may next end up on the ocean bottom, where it will be effectively removed from the atmosphere for thousands of years, or may just as easily be rapidly recycled back into CO2 and spit back to the atmosphere.

We make measurements of CO2 uptake by phytoplankton using two basic approaches. One way is to simulate ‘a day in the life’ of the phytoplankton by placing newly captured phytoplankton into incubators (see picture below) that simulate the physical conditions of temperature and light intensity at which the plants were residing in nature. Veronica Lance from Lamont-Doherty performs these measurements, along with colleague Pete Strutton from Oregon State University. The plants are allowed to grow under these conditions for one complete day then they are harvested and the amount of carbon incorporation is measured using tracers of CO2. This is termed ‘daily integrated production.’ When coupled with measurements of nitrogen uptake made by Pete, we can understand what portion of this plant growth represents carbon that is removed from the atmosphere on a long term basis, and what portion will be recycled back to the atmosphere.

The second method we employ is the photosynthesis-irradiance experiment. Here, instead of growing the plants under simulated natural conditions, they are exposed to a wide range of light intensities (at constant ambient temperatures) in an incubator called a ‘radial photosynthetron’ (see picture below). This is like putting the tiny plants on a treadmill and telling them to show us what they can do–what is the maximum speed you can run, or what is the minimum speed you can run, and how fast can you accelerate? From this experiment we understand the potential for carbon uptake. The potential for growth gives us important insight into how healthy the plants are, and whether they are starving for nutrients, or being burnt by the sun. Using simultaneous measurement of the rate of light absorption by their pigments, made by colleague Bruce Hargreaves from Lehigh University, we understand how efficiently they are using the photons they absorb.

We are moments away from pulling away from the dock now and entering the Straits of Magellan. Soon we will be out into the Southern Ocean and our measurements will begin. Check in periodically to see how things are going.

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Bob Vaillancourt (LDEO), Bruce Hargreaves (Lehigh) and Veronica Lance (LDEO) making vitamin D

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The on-deck incubators are for simulated in-situ experiments. They are shaded with blue sheet filters to simulate the bluish color of submarine light

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The radial photosynthetron (with cooling provided by a circulating water chiller) is the plant-world equivalent of the treadmill

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