A photoheterotroph is an organism that uses a unique metabolic strategy, combining two distinct ways of obtaining nourishment. These organisms use light as their source of energy but must consume organic compounds from their environment to obtain the carbon necessary for growth and building cellular structures.
Photoheterotrophs are typically microscopic, primarily consisting of certain types of bacteria and archaea, though some protists also exhibit this lifestyle. They thrive in environments that offer both sunlight and a steady supply of dissolved organic carbon, such as surface waters, soil, and even the human gut.
Deconstructing the Photoheterotroph
The name “photoheterotroph” is derived from three Greek roots: photo, hetero, and troph. The prefix photo refers to light, indicating that these organisms capture light energy to power their cellular processes. They utilize light-harvesting pigments, such as bacteriochlorophyll or bacteriorhodopsin, to generate adenosine triphosphate (ATP), the cell’s energy currency, through photophosphorylation.
The second part, hetero, means “other” or “different,” and is paired with troph, meaning “nourishment” or “feeding”. This signifies that, unlike plants, photoheterotrophs cannot use carbon dioxide (\(\text{CO}_2\)) as their sole carbon source. Instead, they must absorb pre-formed organic compounds, such as carbohydrates, fatty acids, and alcohols, from their surroundings.
This distinction between energy and carbon sources is the defining feature of the photoheterotrophic lifestyle. They use light only for energy production. They rely on consuming organic matter for carbon. This dual dependence allows them to conserve energy by avoiding the complex, resource-intensive machinery required to fix inorganic carbon into organic matter.
Placing Photoheterotrophs in the Trophic Spectrum
The metabolic world is divided into four primary trophic modes based on an organism’s source of energy and its source of carbon. The energy source is either light (photo-) or chemical compounds (chemo-), and the carbon source is either inorganic \(\text{CO}_2\) (autotroph) or organic molecules (heterotroph).
Photoautotrophs are the most familiar light-users, including plants and algae, which use light for energy and fix \(\text{CO}_2\) for carbon. They are the foundation of most food webs. Chemoautotrophs obtain energy by oxidizing inorganic chemicals, such as hydrogen sulfide or iron compounds, and also fix \(\text{CO}_2\) for carbon. These organisms are often found in extreme environments, like deep-sea vents.
Chemoheterotrophs represent the largest group of organisms, which includes all animals, fungi, and many bacteria. They derive both their energy and carbon from consuming pre-formed organic compounds. The photoheterotroph stands apart by uniquely combining the energy source of a photoautotroph (light) with the carbon source of a chemoheterotroph (organic matter).
The photoheterotrophic strategy is metabolically flexible. They use light to generate ATP, allowing them to save the energy that chemoheterotrophs would spend breaking down organic matter solely for energy. This efficiency gives them a competitive advantage in environments where both light and organic nutrients are available, but where inorganic carbon is scarce.
Real-World Examples and Ecological Function
Photoheterotrophs are predominantly found in aquatic and soil environments that are rich in decaying organic material. A prominent example is the group known as purple non-sulfur bacteria, such as Rhodobacter sphaeroides. These bacteria perform anoxygenic photosynthesis (not producing oxygen as a byproduct) and can switch to chemoheterotrophic metabolism when light is absent.
Another important example is the green non-sulfur bacteria, like Chloroflexus aurantiacus, which can be found in microbial mats in hot springs. These organisms, along with others like the heliobacteria in soil, are environmental recyclers. They consume dissolved organic material, effectively linking the light-driven energy cycle with the chemical-driven carbon cycle.
Their ecological function is significant in the biogeochemical cycling of carbon and other nutrients. By utilizing light to power the assimilation of organic carbon, photoheterotrophs contribute to the clean-up and detoxification of certain environments, such as wastewater. They can also be harnessed for industrial applications, including the production of biohydrogen.