Photosynthetic Algae, Thalassiosira weissflogii, Response to CO2 Change

Glossary

The major theme of this summer’s Woodrow Wilson Institute was The Human Impact on Our Environment. As part of the program individuals were divided into mentor groups studying various aspects of the earth’s environment. A significant "driving force" of the earth’s environment are the biogeochemical cycles which mediate virtually all biological chemical reactions. The most familiar and perhaps the most significant to all life on our planet involves the cycling of the elements carbon (C) and oxygen (O) via photosynthesis and respiration.

Our mentor group focused on the ability of a marine diatom Thalassiosira weissflogii to fix Carbon Dioxide during photosynthesis. Marine or ocean phytoplankton include about 30,000 species divided among seven divisions. Some of these divisions include Chlorophytes, Pyrrophytes (dinoflagelates), Bacillariophytes (diatoms), and Coccolithophores. These tiny phytoplankton constitute about 1 percent of the entire planet’s biomass, but account for over 50% of the planet’s photosynthesis. The general equation for photosynthesis is:

Light

CO₂ +H₂0 ß à (CH₂0) + 0₂

Chlorophyll

These reactions occur in a cell organelle called the chloroplast which contains photosynthetic pigments called chlorophyll. In the light reactions of photosynthesis, light energy is used to split water thereby obtaining H+ ions and high energy electrons and releasing oxygen. Carbon dioxide is then fixed (reduced) by the addition of hydrogen to form glucose, a simple sugar. A key player in the carbon fixation reaction of photosynthesis—the Calvin Cycle—is the enzyme RubisCO.

Our investigation also focused on the following equilibrium chemical reaction:

                                                                             CA

CO₂ +H₂0 ß à H⁺ + HC0₃^- (bicarbonate)

Uncatalyzed, this reaction reaches a constant in about one minute. But with the enzyme, carbonic anhydrase, the reaction speeds up to 10^-6 seconds.

_{Cells can easily take-up uncharged CO2 through their cell membranes, but cannot easily take-up the charged HCO3^- ion. In the Calvin Cycle, RubisCO requires CO2 and is unable to utilize bicarbonate directly. In seawater, 1% of the inorganic carbon is in the form of CO2 while the remaining 99% is in the form of bicarbonate HC03^-. Therefore, when carbon dioxide is limited, it would give the cells an evolutionary advantage to be able to convert bicarbonate into useable carbon dioxide. This mechanism is the enzyme carbonic anhydrase.}

In conclusion, studying diatoms and their physiological response to changes in CO₂ concentrations will help scientists estimate carbon fluxes in the global biogeochemical cycle of carbon.