Bacteria live almost everywhere: in soil, freshwater, oceans, food, the human body, and environments that seem too hot, cold, acidic, salty, or oxygen-poor for most organisms. Their success comes partly from an extraordinary ability to obtain energy and nutrients in many different ways.
Like all living cells, bacteria need energy to power processes such as growth, movement, repair, and reproduction. They also need nutrients, including sources of carbon, nitrogen, phosphorus, sulfur, and various minerals. But bacteria do not all obtain these resources from the same sources. Some use sunlight, some consume organic compounds, and others obtain energy by carrying out chemical reactions involving inorganic substances.
Understanding how bacteria acquire energy and nutrients begins with two related questions: What supplies the cell with energy, and where does the cell get the raw materials needed to build itself?
Energy and nutrients are not the same thing
Energy and nutrients serve different roles in a bacterial cell.
Energy is needed to drive cellular work. Bacteria use it to make cellular components, transport substances across membranes, move their flagella, repair damage, and carry out many chemical reactions that would not happen spontaneously.
Nutrients, by contrast, provide the atoms and molecules from which the cell is built. Carbon is especially important because it forms the backbone of proteins, carbohydrates, lipids, nucleic acids, and many other biological molecules. Nitrogen is needed for proteins and nucleic acids, while phosphorus is a component of nucleic acids and cell membranes. Sulfur occurs in certain amino acids and other cellular compounds.
A substance can sometimes serve both purposes. For example, when a bacterium breaks down a sugar, it obtains chemical energy from the reactions while also using carbon compounds as raw material for building cellular components.
How bacteria obtain energy
Bacteria can be broadly divided according to the source of energy they use.
Phototrophs use light
Phototrophs obtain energy from light. They contain specialized pigments and molecular systems that capture light energy and convert it into a form the cell can use.
Photosynthetic bacteria are not all alike. Some carry out forms of photosynthesis that release oxygen, while others use different electron donors and do not produce oxygen. Cyanobacteria, for example, perform oxygen-producing photosynthesis and use water as an electron source, releasing oxygen as a byproduct.
Other photosynthetic bacteria use compounds such as hydrogen sulfide or certain organic molecules instead of water. Their photosynthetic machinery therefore works differently from that of plants and cyanobacteria.
Light provides the initial energy input, but the cell must convert that energy into usable chemical forms before it can support most cellular work.
Chemotrophs obtain energy from chemical reactions
Chemotrophs obtain energy from chemical substances rather than light. Their metabolism extracts energy by transferring electrons between molecules in controlled chemical reactions.
Some bacteria use organic compounds such as sugars, fatty acids, or amino acids. These organisms are often called chemoorganotrophs. Humans and many other animals rely heavily on the same general principle: extracting energy from organic molecules.
Other bacteria obtain energy from inorganic substances. These chemolithotrophs can use compounds containing hydrogen, sulfur, iron, nitrogen, or other elements. For example, some bacteria gain energy by oxidizing reduced sulfur compounds, while others oxidize ammonia or nitrite.
This ability allows bacteria to live in places where organic food is scarce. A microbial community around a mineral-rich environment, for instance, may be supported by chemical reactions involving inorganic compounds rather than by sunlight.
Cellular respiration turns chemical energy into usable energy
When bacteria break down energy-rich compounds, they do not simply release all of the energy as heat. Instead, they capture part of it through a series of controlled reactions.
A central molecule in this process is ATP (adenosine triphosphate), which acts as a readily usable energy carrier inside cells. Bacteria use ATP to power many energy-requiring processes.
In cellular respiration, electrons are removed from an energy-rich molecule and passed through a sequence of carriers. This process can establish a proton gradient across the cell membrane. A proton gradient is a difference in the concentration of hydrogen ions across the membrane, and it stores potential energy.
The enzyme ATP synthase uses that gradient to make ATP.
In aerobic respiration, oxygen serves as the final electron acceptor. But oxygen is not essential for all forms of bacterial energy metabolism. Some bacteria use other electron acceptors, including nitrate, sulfate, or carbon dioxide, depending on their metabolic capabilities and environmental conditions.
Fermentation works without an external electron acceptor
Some bacteria obtain energy through fermentation, a process that does not use an external electron acceptor such as oxygen.
During fermentation, organic molecules are partially broken down, producing a relatively small amount of ATP compared with efficient respiratory pathways. The cell also converts metabolic intermediates into products that allow essential electron-transfer reactions to continue.
Different bacteria produce different fermentation products. Depending on the species and conditions, these can include acids, alcohols, gases, or other compounds.
Fermentation is therefore not simply a backup version of respiration. It is a distinct metabolic strategy that can be advantageous when suitable respiratory electron acceptors are unavailable.
Where bacteria get their carbon
Energy is only part of the problem. A bacterium also needs a source of carbon, the fundamental building material of its cellular structures.
Bacteria that obtain most of their cellular carbon from carbon dioxide are called autotrophs. They use carbon dioxide to build organic molecules through carbon-fixation pathways. The energy required for this process can come from light or from chemical reactions.
Bacteria that obtain their carbon from preexisting organic compounds are called heterotrophs. They may consume sugars, organic acids, amino acids, lipids, and other compounds released by living or dead organisms.
The distinction between energy source and carbon source is important because they are independent characteristics. A bacterium can, for example, use light for energy while obtaining carbon from organic compounds, or it can obtain energy from inorganic chemical reactions while fixing carbon dioxide.
Bacteria acquire nitrogen, phosphorus, sulfur, and minerals
Carbon is not enough to build a cell. Bacteria also require several other elements.
Nitrogen is essential for amino acids and nucleotides. Some bacteria can use ammonium or nitrate as nitrogen sources, while others depend on organic nitrogen compounds. Certain bacteria can also carry out nitrogen fixation, converting atmospheric nitrogen gas into biologically usable nitrogen compounds. This process requires substantial energy and specialized enzymes.
Phosphorus is needed for nucleic acids, phospholipids, and energy-related molecules such as ATP. Bacteria commonly acquire it as inorganic phosphate or from phosphorus-containing organic compounds.
Sulfur is required for particular amino acids and other cellular molecules. Depending on the organism, sulfur may come from sulfate, sulfide, sulfur-containing organic compounds, or other sulfur compounds.
Bacteria also need smaller amounts of elements such as iron, magnesium, potassium, calcium, and trace metals. These substances can serve structural roles, participate in enzyme activity, or help maintain cellular chemistry.
Nutrients often have to be broken down before bacteria can use them
Bacteria frequently encounter nutrients in forms too large or chemically inaccessible to take directly into the cell. Many species solve this problem by releasing extracellular enzymes into their surroundings.
These enzymes break large molecules into smaller compounds. For example, bacteria can secrete enzymes that digest proteins into smaller peptides and amino acids or break complex carbohydrates into simpler sugars.
The resulting molecules can then be transported across the bacterial cell membrane.
This strategy is particularly important for bacteria living on dead organic matter. Instead of swallowing food, as animals do, many bacteria digest material outside the cell and absorb the resulting small molecules.
Transport systems control what enters the cell
Once usable molecules are available, bacteria must bring them across the cell envelope.
Some small molecules can cross membranes relatively easily, but many nutrients require specialized transport proteins. These proteins can provide channels or carriers that selectively move substances into or out of the cell.
Bacteria often use active transport when nutrients are scarce. Active transport requires energy and allows a cell to concentrate substances inside itself even when their concentration is higher outside the cell.
Some transport systems are highly specialized, while others can move related groups of compounds. Their activity can also be regulated so that bacteria do not spend energy importing nutrients they do not need.
Bacterial metabolism changes with the environment
A bacterium’s metabolism is not necessarily fixed. Environmental conditions can determine which pathways are useful.
Oxygen is a major example. An organism capable of aerobic respiration may switch to fermentation or another respiratory pathway when oxygen becomes unavailable. Other bacteria are specialized for oxygen-free environments and may be harmed by oxygen.
The availability of particular nutrients also matters. If a preferred carbon source disappears, a bacterium may activate genes that allow it to use another compound. This metabolic flexibility helps bacteria survive environments in which food sources fluctuate.
Some bacteria can also enter slower-growing or dormant states when conditions become unfavorable. In those states, their energy requirements decrease, allowing them to persist until resources become available again.
Bacteria occupy very different nutritional niches
Because bacteria can use such a wide range of energy and nutrient sources, they play fundamentally different roles in ecosystems.
Some bacteria are decomposers, breaking down organic material and returning nutrients to the environment. Others participate in cycles of nitrogen, sulfur, carbon, and other elements. Photosynthetic bacteria can contribute organic matter to ecosystems, while bacteria that oxidize inorganic compounds can support communities in environments with little or no sunlight.
Bacteria also live in close association with plants and animals. Some obtain nutrients from their hosts or from materials in the surrounding environment. Others provide useful metabolic products or transform compounds in ways that affect their host.
The result is a vast network of microbial metabolisms in which one organism’s waste product can become another organism’s energy source or nutrient.
The key idea: bacteria match metabolism to opportunity
There is no single way that bacteria get food or energy. Their diversity comes from the enormous range of chemical reactions they can exploit.
At the simplest level, bacterial nutrition can be understood by separating three questions:
- Where does the energy come from? Light or chemical reactions.
- Where does the carbon come from? Carbon dioxide or organic compounds.
- Which other elements are required? Nitrogen, phosphorus, sulfur, minerals, and trace nutrients.
A bacterium’s particular combination of these strategies determines much of where it can live and what role it plays there. Some bacteria harvest sunlight, some consume organic matter, and others obtain energy from chemical reactions involving substances such as sulfur, ammonia, hydrogen, or iron. Through these varied metabolic strategies, bacteria can turn resources from their surroundings into both usable cellular energy and the raw materials needed to build a living cell.
