Photosynthesis is the fundamental biological process that converts light energy into chemical energy, stored in carbohydrate molecules synthesized from carbon dioxide and water. This process forms the foundation of almost all food webs on Earth. The reaction often releases oxygen as a byproduct, producing and maintaining the oxygen content of the planet’s atmosphere. The ability to perform this function is distributed across a wide range of life forms, extending far beyond the familiar green plants.
Terrestrial Photosynthetic Eukaryotes
The most recognizable group of photosynthetic organisms belongs to the Kingdom Plantae, which dominates the terrestrial landscape. Land plants are eukaryotes, meaning their cells contain membrane-bound organelles, including chloroplasts, which are the specialized structures where photosynthesis occurs. These organisms use chlorophyll \(a\) and \(b\) pigments to capture light energy, employing a process known as oxygenic photosynthesis that splits water molecules and releases molecular oxygen.
The diversity of land plants is immense, encompassing towering trees, low-lying shrubs, and the many species of grasses that cover the continents. Even less conspicuous organisms, such as mosses and ferns, play a role in this global energy conversion. All these forms are united by their cellular machinery that fixes atmospheric carbon dioxide into organic compounds, forming the base of most terrestrial ecosystems. Their collective biomass represents over 80% of the total biomass of all life on Earth.
Aquatic Algae and Other Protists
In aquatic environments, a different group of eukaryotes, collectively known as algae and other protists, shoulder the primary responsibility for photosynthesis. This group is highly diverse, ranging from macroscopic seaweeds to single-celled organisms. Seaweeds, or macro-algae, include species like kelp and rockweed, which anchor themselves to the seafloor and can grow to significant sizes, forming underwater forests in coastal zones.
The most ecologically significant organisms in the aquatic realm are the microscopic forms called phytoplankton. These single-celled organisms, such as diatoms, coccolithophores, and dinoflagellates, drift in the sunlit upper layers of the ocean and freshwater bodies. Phytoplankton are estimated to produce roughly half of the world’s oxygen supply, making their photosynthetic output comparable to that of all terrestrial plants combined. They form the base of the marine food web, supporting virtually all ocean life.
Beyond the main algal groups, other eukaryotic protists also perform photosynthesis. The genus Euglena, for example, consists of flagellated organisms that can function as photoautotrophs, synthesizing their own food using light. These various aquatic eukaryotes highlight that the majority of global oxygen production originates not from land plants, but from the oceans.
Photosynthetic Bacteria and Archaea
The earliest forms of photosynthesis evolved in the prokaryotic domain, which includes bacteria and archaea. Cyanobacteria are bacteria that perform oxygenic photosynthesis, the same oxygen-producing process used by plants and algae. These organisms were responsible for the “Great Oxidation Event” over two billion years ago, fundamentally changing the Earth’s atmosphere by introducing free oxygen. Their internal structures, called thylakoids, contain chlorophyll and function as the photosynthetic machinery, making them the evolutionary ancestors of plant chloroplasts.
Other groups of bacteria perform a different type of process called anoxygenic photosynthesis, which does not produce oxygen. These bacteria typically use compounds other than water as electron donors, such as hydrogen sulfide, hydrogen gas, or organic molecules. Examples include Purple Sulfur Bacteria and Green Sulfur Bacteria, which utilize a pigment called bacteriochlorophyll to absorb light at wavelengths inaccessible to most oxygenic photosynthesizers. Instead of oxygen, the byproduct of anoxygenic photosynthesis is often elemental sulfur or sulfate.
While bacteria are widely distributed photosynthesizers, the other major domain of prokaryotic life, Archaea, does not contain organisms that perform carbon-fixing photosynthesis. Some archaea, like the halophilic Halobacteria, do harness light energy through a protein called bacteriorhodopsin, which creates a proton gradient to generate energy without fixing carbon dioxide into sugar. True photosynthesis, which produces biomass from carbon dioxide, is exclusively carried out by various groups of bacteria, algae, and plants.