Autotrophs derive energy from one of two possible nonliving sources: sunlight and chemical reactions involving simple chemicals.
Photoautotrophs and Photosynthesis
Photoautotrophs are photosynthetic; that is, they capture the energy of light rays and transform it into chemical energy that can be used in cell metabolism. In general, photosynthesis relies on special pigments to collect the light and uses the energy to convert CO2 into simple organic compounds. There are two major variations in the mechanisms of photosynthesis.
Oxygenic (oxygen-producing) photosynthesis can be summed up by the equation:
in which (CH2O)n is shorthand for a carbohydrate. This type of photosynthesis occurs in plants, algae, and cyanobacteria and uses chlorophyll as the primary pigment. Carbohydrates formed by the reaction can be used by the cell to synthesize other cell components. This topic is covered in greater depth in chapter 8. Because these organisms are the primary producers in most ecosystems, they constitute the basis of food chains by providing nutrition for heterotrophs. This type of photosynthesis is also responsible for maintaining the level of oxygen gas in the atmosphere that is so vital to many organisms.
The other form of photosynthesis is termed anoxygenic (no oxygen produced). It may be summarized by the equation:
Note that this type of photosynthesis is different in several respects: (1) It uses a unique pigment, bacteriochlorophyll; (2) its hydrogen source is hydrogen sulfide gas; (3) it gives off elemental sulfur as one product; and (4) the reactions all occur in the absence of oxygen. Common groups of photosynthetic bacteria are the purple and green sulfur bacteria that live in various aquatic habitats, often in mixtures with other photosynthetic microbes.
Chemoautotrophy—A Radical Existence
Compared to common, familiar organisms, chemoautotrophs have adapted to the most stringent nutritional strategy on earth. All chemoautotrophs are bacteria or archaea that survive totally on in organic substances such as minerals and gases. They require neither light nor organic nutrients in any form, and they derive energy in diverse and sometimes surprising ways. In very simple terms, they remove electrons from inorganic substrates such as hydrogen gas, hydrogen sulfide, sulfur, or iron and combine them with other inorganic substances such as carbon dioxide, oxygen, and hydrogen. These reactions release simple organic molecules and a modest amount of energy to drive the synthetic processes of the cell. Figure 1a provides an example of an unusual bacterium that inhabits hot springs in New Zealand. Chemoautotrophs play an important part in recycling inorganic nutrients and elements. For an example of chemoautotrophy and its importance to deep-sea communities, see 7.1 Making Connections.
Fig1. Examples of chemotrophy. (a) Venenivibrio, an extremophilic bacterium that lives in acidic hot springs and derives energy by combining hydrogen gas with oxygen to form water and hydrogen peroxide. (b) Methanocaldococcus jannaschii is an archaeon and methanogen that inhabits hot vents in the seafloor. It also metabolizes hydrogen gas, but its product is methane gas (180,000×). (a): Dr. Adrian Hetzer; (b): Electron Microscope Lab, UC Berkeley
The Methanogen World
Methanogens are a unique type of chemoautotroph widely distributed in the earth’s habitats. All known methanogens are archaea, and many of them are found in extreme habitats, ranging from hot springs and vents to the deepest, coldest parts of the ocean (figure 1b). Others are common in soil, swamps, and even the intestines of humans and other animals.
Methanogens’ metabolism is adapted to producing methane gas (CH4 or “swamp gas”) by reducing carbon dioxide, using hydrogen gas under anaerobic conditions, summarized as:
Researchers sampling deep underneath the seafloor have uncovered massive deposits of methanogens. The immensity of this community has led one group of scientists to estimate that it comprises nearly one-third of all life on the planet! Evidence points to its extreme age: It has been embedded in the earth’s crust for billions of years. Placed under tremendous pressures, the methane it releases becomes frozen into crystals. Some of it serves as a nutrient source for other extreme archaea, and some of it escapes into the ocean. From this position, it may be a significant factor in the development of the earth’s climate and atmosphere. Some microbial ecologists suggest that rises in sea temperature could increase the melting of these methane deposits. Because methane is an important “greenhouse gas,” this process could contribute considerably to ongoing global climate change.
