Metabolism: Making a Living
Peering into a hot, acidic spring, it may seem that the daily life of a thermoacidophile is pretty far removed from your own life. Our environments and lifestyles maybe drastically different but the most basic requirements for life are shared by every organism on earth. One of these requirements is the ability to harness energy to drive the cellular processes of growth and reproduction. In essence, all life “runs” on the energy of chemical bonds. In addition, cells require a carbon source for building organic molecules. The carbohydrates, fats, proteins and nucleic acids that carry out vital functions within cells are all built around carbon’s ability to form complex structures.
Metabolism, in the broadest sense, is the sum of all of the chemical reactions taking place within the cell. Specifically, we can speak of the mode by which an organism “profits” from chemical reactions to power its cellular functions. The living world displays great diversity in modes of metabolism but all are variations on the same theme: an organism must be able collect molecules from its environment that will act as electron donors. These have a potential to give their electrons to an electron acceptor with a net profit in energy that can be stored in a usable form within the cell.
Imagine you were given an expensive watch each morning. If you want to pay your bills and buy food, you need to trade the watch for less expensive items and conserve the difference in value in the form of money. This is much like the process of metabolism. An organism takes up high value substances, trades down to less energetically valuable compounds and keeps the difference!
To take part in this metabolic marketplace all you need to do is eat and breathe. The organic compounds that you consume from plant and animal tissues act as electron donors, with the oxygen that we breathe acting as an electron acceptor. The byproducts of our metabolism are carbon dioxide and water, plus the energy to go about our lives. This mode of metabolism is called aerobic respiration because it can occur only in the presence of oxygen. The process yields a great deal of energy as organic compounds have a high potential to donate electrons and oxygen is a first-rate electron acceptor.
Plants, of course, must employ an additional strategy, as they do not consume organic compounds. In order to obtain carbon for energetic and structural needs, they must first fix carbon from atmospheric carbon dioxide and incorporate it into useful organic compounds. The energy for this process comes directly from the sunlight and so we call the technique photosynthesis. You may get a sense of how important plants are when you reflect on the byproducts of photosynthesis, carbon compounds and oxygen. Without these primary producers converting carbon dioxide into organic molecules and releasing oxygen into the atmosphere, aerobic respiration like ours could not exist!
Metabolic Diversity
If the carbon cycle that plants and animals depend on seems like a remarkable system, the microbial world provides a dazzling array of nutrient cycles. Microbes are the masters of metabolic inventiveness. In order to exploit habitats that may lack oxygen, some organisms can use an alternate electron acceptor such as nitrate or carbonate in the process of anaerobic respiration. Trading down with these substances does not yield as much energy as employing oxygen does but anaerobic respiration does allow organisms to live in oxygen-free environments such as deep soil and bogs. Furthermore, many prokaryotes have energetic processes that do not rely on carbon, either. Picture the deep ocean trenches where fissures in the sea floor spew superheated gases into the surrounding water. In this environment, organisms, such as the archean Pyrodictium, may use hydrogen as an electron donor and sulfur as an electron acceptor. In human terms, it is as if Pyrodictium “eats” hydrogen and “breathes” sulfur! We find bacteria that live on nitrogen compounds, some that take energy from iron, and others that live in petroleum storage tanks, happily subsisting on hydrocarbons!
Just as plants and animals form an interdependent nutrient cycle, microbes too exist in ecological relationships to each other. In the microscale habitat of soil particles, we find prokaryotes that produce methane as a byproduct of metabolism living in close relationship to others that consume it for their energetic needs. We can very well say that one microbe’s trash is another’s treasure!
To see how metabolism ties organisms together in an interdependent network, lets examine the microbial marketplace within a typical acidic thermal spring. Because plants are physiologically excluded from such severe environs (see next section), photosynthetic primary production falls to photosynthetic algae and cyanobacteria. By converting CO2 into organic compounds using solar energy, these organisms feed a host of decomposers, such as Sulfolobus and Thermoproteus, and form the foundations of the carbon cycle. Through cellular respiration, decomposers in turn breakdown organic molecules and release the very CO2 required by photosynthesizers.
So far the story sounds familiar. But hot springs microbes take part in nutrient cycles besides that of carbon, as well. Fundamentally different from any ecosystem where energy ultimately comes from the sun, these metabolic networks rely on the chemical energy of inorganic ions. One of the most important biogeochemical cycles in volcanic regions is the sulfur cycle. Geothermal regions contain abundant sulfur compounds carried out of the underlying rock by heat-expanded water (see section two for an explanation). These compounds provide the energetic basis for a uniquely prokaryotic community. For example, the ever-accommodating Sulfolobus uses elemental sulfur (S0) precipitated out of ground water as an electron donor, combines it with oxygen and releases sulfate (SO42-). This sulfur oxidation nets Sulfolobus a tidy energetic yield but the sulfur cycle does not stop there. Sulfur reducers, such as the bacterium Thermodesulfobacteria, can then take sulfate as an electron acceptor and combine it with hydrogen to form sulfide (H2S). Hot spring habitats host microbes that participate in still other cycles, including those of hydrogen, nitrogen and iron. Each cycle carries its own roster of players with unique metabolic pathways, but the fundamental strategy remains the same: convert available energy into useful cellular activity.
If we could follow the flow of energy through a hydrothermal ecosystem, we would see lines drawn from the sun and the from earth’s interior, passing in turn through the cells of every resident and uniting each into a network of metabolic interdependence. This biological dance, which seems familiar in the context of rainforests or desert, is played out even in the forbidding pools of volcanic hot springs. We can now see how residents take part in this flow of energy, but you may rightly wonder how organisms even survive the boiling cauldron of a thermal pool. The subsequent section will examine the adaptations that it takes to call such a hostile realm your home.
Our tour of life in Lassen Volcanic National Park continues with a look at: