Going Deep With Photosynthesis
February 2, 2011 
6 Comments
This morning was Feb. 2 for us–Groundhog Day. I looked out the window but saw no groundhogs. I didn’t even see any ground. The really odd part was that in the afternoon we crossed the International Dateline and the date turned back to February 1st. That means that when I wake up tomorrow morning, it’s going to be Groundhog Day again. I feel like I’m in a movie.
That feeling of déjà vu persisted as I followed Dr. Angelicque White, Katie Watkins-Brandt, and Dr. Allen Milligan of Oregon State University around. Each of them told me one part of how they measure how fast phytoplankton conduct photosynthesis.
It seems like a simple question: how much carbon do the tiny plants produce, and how much light do they use to do it? But it took most of the afternoon to get it through my head. I toured a succession of labs crammed with sensitive analytical equipment, hooked together with rubber and plastic tubes of every size, propped up with pieces of wood, and held fast to the counters with bungee cords and bits of string. Oceanographers are like hermit crabs—constantly moving in to new spaces where their belongings don’t quite fit.
As far as I can tell, measuring photosynthesis involves filtering small things like phytoplankton cells from the water, shining expensive lights on them, and measuring how much light comes back. Then, to correct for some imperfections in this method, you filter out some even smaller parts of the cells and shine a different expensive light on them.
This happens about three times, and then you take the answers from each of those operations, multiply some of them and divide by the rest, and you get the answer. It’s actually quite fascinating, and it all makes sense as long as you have someone like Dr. White or Dr. Milligan to explain it to you. Read on through the slideshow to see what I learned:
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- Dr. White loves looking through microscopes. She has an adapter for her camera that she uses to take photos and video of plankton. Here she places a slide under a blue fluorescent light to make the photosynthetic parts glow red (see Jan 26 post). To start my lesson on measuring photosynthesis, she reminded me of the basic process: plants use the energy of sunlight to turn carbon dioxide into organic carbon (in the form of sugar), giving off oxygen as a byproduct. Put a bit more simply, Sunlight + Carbon Dioxide = Organic Carbon + Oxygen.
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- The black cylinder on the left is an ACS, or absorbance-attenuation spectral instrument. That mass of filters, tubes, and pumps takes seawater from outside the ship, ‘de-bubbles’ it, and feeds it into the ACS. This allows us to measure how many particles are in the water as we steam by, without having to stop. It and other instruments in this room use the basic process described in the last slide to deduce information. The ACS tells Dr. White how much particulate carbon is in the water. Since that’s what plants make, it indicates roughly how much photosynthesis has happened. Another sensor measures the amount of carbon dioxide in the water. Since carbon dioxide is an ingredient in photosynthesis, if its levels in the seawater drop, that also means more photosynthesis has happened.
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- But to know how well phytoplankton are growing you can’t just count cells. That would be like counting the number of tomato plants in your garden but not paying attention to how many tomatoes they had or whether they were dying or not. What Dr. Milligan, Katie, and Dr. White want to know is how much light the phytoplankton use up and how much carbon they make into sugar. They measure these amounts very carefully—light in terms of the number of photons, and carbon in terms of number of atoms. The first step is to grow phytoplankton in a seawater sample in blue light for 24 hours, as Katie is doing here. The room looks like it could be in any lab anywhere, but it’s out on the back deck of the Palmer. See the bright-orange float coat hanging behind her?
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- What’s so special about the light box (or ‘photosynthetron’ as the scientists call it)? It shines a constant blue light on the samples. Smoky gray filters at the bottom of each tube allow Katie to select different amounts of that light for each sample, simulating different depths in the ocean. When she prepares the samples, she adds radioactive carbon-14. The phytoplankton will use this type of carbon in photosynthesis, and the radioactive atoms will act like a tracer. After 24 hours Katie will put the samples in a machine that counts how much carbon-14 the plants used. I asked Katie how she felt about working with radioactive material, but she wasn’t worried. ‘It’s just beta particles,’ she said. ‘It’s not like crazy scary gamma rays.’ And what about her mom? ‘She’s been worrying about me for 26 years of my life, so when it came to carbon-14 her reaction was pretty mild.’
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- After I left Katie’s ‘rad van’ I thought I knew how photosynthesis was measured. But when I went down to Dr. Milligan’s lab bench I found him filtering Katie’s phytoplankton to do some more measurements. There were still a couple of numbers missing from their calculation. They knew how many carbon atoms had been produced, but they didn’t yet know the amount of light well enough. They couldn’t just measure the amount of blue light that they had shone on the phytoplankton. They needed to know how much of that light the phytoplankton had absorbed, versus how much had gone streaming right past the cells.
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- Dr. Milligan is soft spoken and laughs a lot when he talks. He’s a surfer, a free-diver who can hold his breath for several minutes, and a biologist of very small things. He’s interested in what happens deep inside of cells, far inside their chloroplasts—how proteins and pigments capture photons of sunlight and transfer that energy to electrons. ‘This is quantum mechanics,’ he told me. ‘These excitons, this antenna of proteins, they’re not anything. We call them ‘excitons’ because we don’t know what else to call them.’ Then he laughed and continued loading this machine. It’s a spectrophotometer, and it measures how much light is captured by the phytoplankton’s chlorophyll and other pigments. And those results give the final numbers that let the team calculate the photosynthetic rates of the phytoplankton.
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- This evening at 7:00 we had a science meeting to review what we’ve found so far and plan the next two weeks of our trip. But there are so many possibilities—and so many interesting questions—that several of the scientists gathered before the meeting to review the options. Here, Dr. Josh Kohut points to Pennell Bank, where we were yesterday and where glider RU06 is now. After 2 weeks, this is the part of the Ross Sea we know best. Dr. Adam Kustka, Bruce Huber, Julius Busecke, Dr. Phoebe Lam, and Dan Ohnemus are trying to decide whether it’s better to explore east (left) of where Dr. Kohut is pointing, or west. Finally, after a 2-hour meeting, they made the decision like the classic hard-working scientists they are: they’re going to study both.
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- So after making that decision, we finished up our CTD rosette at station 29 and set course for a patch of productive water that’s just outside the shallow part of the Ross Sea. This area is deep—about 1,500 meters—but it shows consistent phytoplankton blooms year after year. The scientists want to know how the surface water gets enough iron for the phytoplankton to grow. We should be there early tomorrow morning—just in time for Groundhog Day to begin.
Read more in these related posts:
- A Little Iron Goes a Long Way (Jan 26)
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