Photosynthesis at Sea
Photosynthesis is so common that many of us take it for granted. We might value trees and other land plants for their wood, food, shade, or beauty, but we often fail to appreciate that these attributes are really just byproducts of the plants' true business—turning sunlight into energy for their own growth and reproduction. And the photosynthetic byproduct that we think about least (it is after all a colorless, odorless, and tasteless gas), is in truth the one that's most important to us. For without the oxygen that plants give off, none of us would have the luxury of appreciating the strength of an oak beam, the taste of maple syrup, or the color of autumn leaves.
We appreciate the importance of marine photosynthesis even less. Photosynthesis at sea isn't accomplished by towering redwoods or stately pines, but largely by tiny single-celled plants that we can hardly see, even when we take the trouble to look. Yet scientists estimate the marine plankton generate about half of Earth's oxygen, even though their biomass is orders of magnitude less than that of terrestrial plants. Most of the biomass of land plants lies in their support structures: trunks and branches to expose leaves to the sun, roots to gather water and nutrients and buttress the plant against the wind. Marine plankton require none of that. They are essentially all leaf; tiny floating photochemical factories that use seawater for both support and nurture. And they reproduce much more quickly than land plants. On average, marine plankton produce a new generation every few days, land plants take a decade or two.
One of our main goals in IVARS is to better understand how different groups of phytoplankton influence the cycling of carbon and other elements in the Ross Sea. We want to know which phytoplankton groups are here, how they're distributed in space and time, and how their differing photosynthetic activities influence biogeochemical cycles. Measuring the biochemistry and byproducts of photosynthesis is thus a key part of our work.
We use several different tests for analyzing the photosynthetic machinery and its products. A recap of photosynthetic chemistry helps explain them. In general, photosynthesis can be described by a chemical equation in which plants use light-sensitive pigments to capture the energy of sunlight. They use this energy to convert water and carbon dioxide into carbohydrates and oxygen:
H2O + CO2 + sunlight --pigments--> CH2O + O2
Scientists have identified about 30 different kinds of photosynthetic pigments in marine phytoplankton. In addition to chlorophylls (green pigments), there are xanthophylls (yellow pigments), phycoerythrins (red pigments), and phycocyanins (blue pigments). Because each pigment is most sensitive at a particular intensity and wavelength of light, different groups of phytoplankton use different combinations and proportions of pigments to best harvest the light where they live. In effect, each phytoplankton species or group has a pigment "fingerprint."
We take advantage of this fingerprint to identify the types of phytoplankton in the IVARS samples, using a device called an "HPLC" (for High-Pressure Liquid Chromatograph). After capturing phytoplankton cells on a filter, we soak the filter in acetone and shake the resulting solution rapidly using a "sonicator." The shaking ruptures the cell walls, releasing the photosynthetic pigments. We then run these pigments through a column inside the HPLC. Small pigment molecules travel through the column quickly, larger molecules move more slowly. Thus the time required for a pigment to travel through the HPLC can be used to identify its molecular weight and composition. HPLC allows us to identify pigments and plankton quickly, which enables us to study lots of samples and thereby determine how phytoplankton communities differ from place to place and with depth. We also view a few samples with a microscope to identify plankton visually. This method is more exact but slower.
Another test uses a device called a fluorometer to measure the amount of chlorophyll in a sample. Because plankton have a relatively fixed ratio between the chlorophyll and carbon in their cells, knowing the chlorophyll concentration allows us to estimate the carbon value. This in turn gives us biomass—the size and number of phytoplankton in an area—which corresponds to the plants' primary productivity. This is the total amount of carbon and nitrogen that the plants were able to extract from seawater and incorporate into their tissues through photosynthesis and growth.
By comparing productivity values from each IVARS station against other measured parameters such as taxonomic composition, salinity, temperature, and nutrient concentration, we'll be better able to understand the biological, chemical, and physical factors that control changes in productivity from place to place and year to year in the Ross Sea.