A photoautotroph is an organism that can synthesize its own food source via sunlight and carbon dioxide. Photoautotrophs utilize energy captured from photons to convert inorganic carbon products in the environment into organic molecules that they use as an energy source.
The word “photoautotroph” is a combination of two words “phototroph” and “autotroph.” Phototrophs are organisms that use energy from sunlight to drive their metabolisms. “Autotrophs” are organisms that can construct organic matter from inorganic materials. Thus, a “photoautotroph” is an organism that can make its own organic nutrients using energy from light.
The most common kinds of photoautotroph are plants. All green plants use a process called photosynthesis to use energy gathered from sunlight to convert atmospheric carbon dioxide into glucose. One of the byproducts of this process is oxygen, which the plants expel back into the atmosphere. Almost all oxygen on earth comes from plant photosynthesis; approximately 25% from land plants, and over 70% from phytoplankton in the Earth’s oceans.
Several bacteria are also known to be photoautotrophs. Photosynthetic activity from cyanobacteria in the oceans are credited with turning early Earth’s oxygen-poor atmosphere into an oxygen-rich one. The change to an oxygen dominant atmosphere made possible high energy organic oxidizing reactions, which allowed the development of multicellular complex life.
Autotrophs, Heterotrophs, Phototrophs And Chemotrophs
First, a bit on the specific terminology of “photoautotroph.” The word “photoautotroph” refers to just one specific kind of metabolic organization. Organisms can be classified based on their source of organic material and the kind of energy they use to drive biological reactions. In general, all organisms can be divided into the two broad categories of autotroph and heterotroph. Autotrophs are capable of producing their own organic nutrients from inorganic substances in the environment. Heterotrophs, on the other hand, cannot produce their own carbon products, so they must get it from elsewhere. Virtually all animals, humans included, are heterotrophs.
Further, phototrophs are organisms that use the energy from photons in light to drive their metabolic processes. Chemotrophs are organisms that use energy from oxidation chemical reactions. These four terms, autotroph, heterotroph, phototroph, and chemotroph and be combined with one another to describe different kinds of metabolic organizations. Chemoheterotrophs, for example, would be organisms that gain organic carbon from external sources and use chemical reactions to produce energy for biological processes. Chemoautotrophs would then be defined as organisms that can create their own sources of organic matter and use energy from chemical reactions to do so. Likewise, photoheterotrophs are organisms that acquire organic carbon from the environment and use photon energy for biosynthesis. Lastly, photoautotrophs would then be creatures that can fix their own source of organic carbon, and use energy from light to synthesize nutrients.
Under these 4 divisions are further classifications, depending on the kind of chemical reactions used, or the exact manner in which energy is derived from photons. Lithotrophs, for example, are chemoheterotrophs that rely exclusively on inorganic chemical reactions to derive their energy. Most organisms can be classified according to this schema, but a few do not fit neatly in. Some fungi and some bacteria, for instance, are radiotrophs in that they get their primary energy for biosynthesis from gamma radiation.
Examples of Photoautotrophs
Most known photoautotrophs are ordinary terrestrial plants. All green plants engage in oxygenic photosynthesis and produce molecular oxygen. The chlorophyll in plant cells that allows them to convert light energy into chemical energy. It is also what gives plants their color. Chlorophyll absorbs all light wavelengths except green light, so plants appear green to us. A few plants do not naturally produce chlorophyll, so they have evolved to parasitize other plants and fungi. These parasitic plants are one of the few instances of a non-photoautotrophic plant.
Plants provide the majority of the atmosphere’s oxygen content. Terrestrial plants account for about a fourth, while phytoplankton in the oceans account for the remaining three fourths. Their activity also removes carbon dioxide from the atmosphere. A typical tree produces about 260 pounds (117.93 kg) of oxygen a year. One acre of trees is capable of producing enough oxygen to sustain about 18 people annually.
Most of the other known photoautotrophs are bacteria. The most well-known photoautotrophic bacteria are cyanobacteria. Cyanobacteria are the only prokaryotes that perform oxygenic photosynthesis. They can do this because they have cellular organelles very close in structure to plant chloroplasts. In fact, it is believed that the first plant chloroplasts actually came from cyanobacteria that were engulfed by larger cells.
Cyanobacteria are also credited with kick-starting the development of complex multicellular life on Earth. 4 billion years ago, Earth’s atmosphere was mostly made of hydrogen, nitrogen, and reducing agents like methane. Most organisms living at this time used hydrogen reactions to drive biological processes. Over time, the photosynthetic activity of cyanobacteria pumped molecular oxygen into the atmosphere. About 600 million years ago, the oxygen content of the atmosphere became significant, changing it from a reducing environment to an oxidizing one. The introduction of oxygen into the atmosphere allowed for new kinds of redox reactions that were capable of generating the amounts of energy necessary to sustain complex multicellular life.
Photoautotrophs And Photosynthesis
Photosynthesis refers to the process by which photoautotrophs capture energy from photons. Most photosynthetic organisms use energy from light to convert inorganic carbon compounds into complex organic molecules. The byproduct of this reaction is molecular oxygen, so it is sometimes called oxygenic photosynthesis. The general chemical equation for oxygenic photosynthesis is:
CO2 + H2O + photons → [CH2O] + O2
In this equation, the formula [CH2O] is the empirical formula for many complex organic molecules like glucose. Essentially, during oxygenic photosynthesis, organisms use energy from light to knock hydrogen off of water and attach them to the carbon base to construct organic compounds. The leftover oxygen from the water molecule is then released into the atmosphere.
All existing green plants and many bacteria use photosynthesis to capture and use energy from photons. Plant cells contain special organelles called chloroplasts. Each plant cell contains roughly 10-100 chloroplast, with higher concentrations on anatomical regions that receive more direct sunlight. Chloroplasts are composed of a dual-membranes that enclose an inter-membrane space. Populating this space are little compartments called thylakoids.
The thylakoids are the locus of the photon interactions that generate energy. Thylakoids use energy from light to make ATP. Light entering the plant cell strikes the thylakoids and is absorbed by chlorophyll and other pigments. The photons from light knock electrons off of the absorbing pigments, which are transported down an electron transport train. The energy from this transport train is used to create ATP and to oxidize 2 molecules of water to create a diatomic oxygen molecule and four hydrogen ions. Chemical energy from ATP is then used to fuse hydrogen ions with inorganic carbon compounds to form organic hydrocarbons. The molecular oxygen is then expelled back into the air.
Other photoautotrophs engage in a form of anoxygenic photosynthesis. Anoxygenic photosynthesis is very similar to oxygenic photosynthesis, except takes place in the absence of oxygen. Most anoxygenic photosynthates are bacteria that live in low oxygen environments. Anoxygenic photosynthesis typically uses other hydrogen chalcogenides such as hydrogen sulfide (H2S), as a reducing agent instead of water. As a result, anoxygenic photosynthates usually produce sulfur as a byproduct instead of molecular oxygen.
The exact wavelength of light that stimulates photosynthetic activity varies depending on the type of photosynthesis. Terrestrial plants that use chlorophyll tend to be tuned to light in short frequency (violet-blue) and long frequencies (yellow-red). Photosynthetic organisms in the ocean tend to have receptors attuned to mid-range wavelength (blue-green). Lastly, some bacteria respond to light in non-visible spectra.
Ecological Role Of Photoautotrophs
Photoautotrophs are of fundamental importance to all ecosystems in the world. Photoautotrophs provide a constant source of the basic building block required for life. Photoautotrophs use energy from sunlight to construct the basic organic molecules that all other heterotropic life relies on. Heterotrophs gain this energy by consuming autotrophs and breaking down their organic matter with chemical reactions. Even carnivores rely on autotrophs, as the prey they eat gets their energy from autotroph consumption.
On land, plants are a major source of food for heterotrophic life. In fact, for humans, plant consumption is vastly more energy efficient than animal consumption. Livestock fed plants absorb only about 20% of the biochemical energy contained in the plants. When humans eat the livestock, they gain only about 20% of the energy in the animal. This means that only about 5% of the original plant energy ends up making it to our plates. In contrast, plant-based diets are more energy efficient as we can get that energy directly from plants instead of it passing through livestock first.
In aquatic environments, photoautotrophs like plankton, algae, bacteria, and various single-celled eukaryotes are the main food source for heterotrophs. Phytoplankton in the ocean is also responsible for the majority of oxygen in the Earth’s atmosphere. These maritime organisms also play the dual role of regulating the amount of carbon dioxide in the atmosphere via their photosynthetic activity.