Future Challenges

Algae: An Alternative to Gasoline

The burning of fossil fuels appears to be a major contributor to global warming and a major source of pollutants in the environment. Accordingly, scientists have been searching for alternative sources of energy. Hydrogen is one such energy source, since when it burns water is its only by-product. One promising source of hydrogen comes from a green alga, Chlamydomonas reinhardtii, found around the world as the green pond scum. This alga potentially could produce plenty of hydrogen, because it can directly split water into hydrogen and oxygen using an enzyme, hydrogenase. It appears that this alga evolved to take advantage of radically different environments. When this alga lives in ordinary air and sunlight, it converts sunlight, water, and carbon dioxide into oxygen and the other products the alga needs for growth. If, however, this alga is deprived of an essential nutrient such as sulfur or forced to live in an anaerobic environment, it switches to another mechanism of metabolism and produces hydrogen instead. The problem of getting this alga to produce large amounts of hydrogen arises when there is oxygen in the environment. The alga then shuts off its hydrogen-producing enzyme and produces only trace amounts. Since this alga is photosynthetic, it normally produces oxygen as a by-product of photosynthesis.

Scientists have found that when they deprive the alga of essential sulfate salts, the alga no longer maintains the protein complex needed for producing oxygen photosynthetically and instead goes into its hydrogen-producing metabolic pathway. Two major problems need to be solved, however, before large quantities of hydrogen could be produced by this alga. The first problem is that the alga cannot grow in this sulfur-deprived mode for very long before the organism needs to revert to the oxygen-producing mode. The alga must replenish the proteins that are burned up in the course of metabolism in order to survive. One laboratory reported that they could grow the alga for 4 days before they needed to add sulfate so that the organism could return to its normal metabolic processes. During the period of sulfate deprivation, the alga produced 3 ml of hydrogen for each liter of growing culture. Although this amount of hydrogen is not large, it is an improvement over the amount recovered in the past.

A second problem is that when oxygen is available, the alga produces very small amounts of hydrogen. To solve this problem, workers in another laboratory were able to put nitrogen gas above the bottled algae so that the oxygen leaving the water did not stop the reaction. When the oxygen built up in the air above the water, they could flush it out with more nitrogen. This method of producing hydrogen was sustained for 58 days. Scientists hope that they will be able to develop oxygen-tolerant algae and thereby increase hydrogen production. These research efforts offer a future challenge to scientists as they explore alternative sources of energy for the world.

320 Chapter 12 The Eukaryotic Members of the Microbial World

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