All About Photosynthetic Organisms

Horse chestnut tree and sun
Horse chestnut tree and sun. Frank Krahmer / Getty Images

Some organisms are capable of capturing the energy from sunlight and using it to produce organic compounds. This process, known as photosynthesis, is essential to life as it provides energy for both producers and consumers. Photosynthetic organisms, also known as photoautotrophs, are organisms that are capable of photosynthesis. Some of these organisms include higher plants, some protists (algae and euglena), and bacteria.

Photosynthesis

Diatom
Diatoms are single-celled photosynthetic algae, of which there are about 100,000 species. They have mineralized cell walls (frustules) that contain silica and provide protection and support. STEVE GSCHMEISSNER/Getty Images

In photosynthesis, light energy is converted to chemical energy, which is stored in the form of glucose (sugar). Inorganic compounds (carbon dioxide, water, and sunlight) are used to produce glucose, oxygen, and water. Photosynthetic organisms use carbon to generate organic molecules (carbohydrates, lipids, and proteins) and build biological mass. The oxygen produced as a bi-product of photosynthesis is used by many organisms, including plants and animals, for cellular respiration. Most organisms rely on photosynthesis, either directly or indirectly, for nourishment. Heterotrophic (hetero-, -trophic) organisms, such as animals, most bacteria, and fungi, are not capable of photosynthesis or of producing biological compounds from inorganic sources. As such, they must consume photosynthetic organisms and other autotrophs (auto-, -trophs) in order to obtain these substances.

Photosynthetic Organisms

  • Plants
  • Algae (Diatoms, Phytoplankton, Green Algae)
  • Euglena
  • Bacteria - Cyanobacteria and Anoxygenic Photosynthetic Bacteria

Photosynthesis in Plants

Chloroplasts
This is a colored transmission electron micrograph (TEM) of two chloroplasts seen in the leaf of a pea plant Pisum sativum. Light and carbon dioxide are converted into carbohydrates by the chloroplast. Large sites of starch produced during photosynthesis are seen as dark circles within each chloroplast. DR KARI LOUNATMAA/Getty Images

Photosynthesis in plants occurs in specialized organelles called chloroplasts. Chloroplasts are found in plant leaves and contain the pigment chlorophyll. This green pigment absorbs light energy needed for photosynthesis to occur. Chloroplasts contain an internal membrane system consisting of structures called thylakoids that serve as the sites of conversion of light energy to chemical energy. Carbon dioxide is converted to carbohydrates in a process known as carbon fixation or the Calvin cycle. The carbohydrates can be stored in the form of starch, used during respiration, or used in the production of cellulose. Oxygen that is produced in the process is released into the atmosphere through pores in the plant leaves known as stomata.

Plants and the Cycle of Nutrients

Plants play an important role in the cycle of nutrients, specifically carbon and oxygen. Aquatic plants and land plants (flowering plants, mosses, and ferns) help to regulate atmospheric carbon by removing carbon dioxide from the air. Plants are also important for the production of oxygen, which is released into the air as a valuable by-product of photosynthesis.

Photosynthetic Algae

Green Algae
These are Netrium desmid, an order of unicellular green algae that grow in long, filamentous colonies. They are mostly found in freshwater, but they can also grow in saltwater and even snow. They have a characteristically symmetrical structure, and a homogeneous cell wall. Credit: Marek Mis/Science Photo Library/Getty Images

Algae are eukaryotic organisms that have characteristics of both plants and animals. Like animals, algae are capable of feeding on organic material in their environment. Some algae also contain organelles and structures found in animals cells, such as flagella and centrioles. Like plants, algae contain photosynthetic organelles called chloroplasts. Chloroplasts contain chlorophyll, a green pigment which absorbs light energy for photosynthesis. Algae also contain other photosynthetic pigments such as carotenoids and phycobilins.

Algae can be unicellular or can exist as large multicellular species. They live in various habitats including salt and freshwater aquatic environments, wet soil, or on moist rocks. Photosynthetic algae known as phytoplankton are found in both marine and freshwater environments. Most marine phytoplankton are composed of diatoms and dinoflagellates. Most freshwater phytoplankton are composed of green algae and cyanobacteria. Phytoplankton float near the surface of the water in order to have better access to sunlight needed for photosynthesis. Photosynthetic algae are vital to the global cycle of nutrients such as carbon and oxygen. They remove carbon dioxide from the atmosphere and generate over half of the global oxygen supply.

Euglena

Euglena are unicellular protists in the genus Euglena. These organisms were classified in the phylum Euglenophyta with algae due to their photosynthetic ability. Scientists now believe that they are not algae but have gained their photosynthetic capabilities through an endosymbiotic relationship with green algae. As such, Euglena have been placed in the phylum Euglenozoa.

Photosynthetic Bacteria

Cyanobacteria
The genus name for this cyanobacterium (Oscillatoria cyanobacteria) comes from the movement it makes as it orientates itself to the brightest light source available, from which it gains energy by photosynthesis. The red coloration is caused by autofluorescence of several photosynthetic pigments and light-harvesting proteins. SINCLAIR STAMMERS/Getty Images

Cyanobacteria

Cyanobacteria are oxygenic photosynthetic bacteria. They harvest the sun's energy, absorb carbon dioxide, and emit oxygen. Like plants and algae, cyanobacteria contain chlorophyll and convert carbon dioxide to sugar through carbon fixation. Unlike eukaryotic plants and algae, cyanobacteria are prokaryotic organisms. They lack a membrane bound nucleus, chloroplasts, and other organelles found in plants and algae. Instead, cyanobacteria have a double outer cell membrane and folded inner thylakoid membranes that are used in photosynthesis. Cyanobacteria are also capable of nitrogen fixation, a process by which atmospheric nitrogen is converted to ammonia, nitrite, and nitrate. These substances are absorbed by plants to synthesis biological compounds.

Cyanobacteria are found in various land biomes and aquatic environments. Some are considered extremophiles because they live in extremely harsh environments such as hotsprings and hypersaline bays. Gloeocapsa cyanobacteria can even survive the harsh conditions of space. Cyanobacteria also exist as phytoplankton and can live within other organisms such as fungi (lichen), protists, and plants. Cyanobacteria contain the pigments phycoerythrin and phycocyanin, which are responsible for their blue-green color. Due to their appearance, these bacteria are sometimes called blue-green algae, although they are not algae at all.

Anoxygenic Photosynthetic Bacteria

Anoxygenic photosynthetic bacteria are photoautotrophs (synthesize food using sunlight) that don't produce oxygen. Unlike cyanobacteria, plants, and algae, these bacteria don't use water as an electron donor in the electron transport chain during the production of ATP. Instead, they use hydrogen, hydrogen sulfide, or sulfur as electron donors. Anoxygenic photosynthetic bacteria also differ from cyanobaceria in that they do not have chlorophyll to absorb light. They contain bacteriochlorophyll, which is capable of absorbing shorter wavelengths of light than chlorophyll. As such, bacteria with bacteriochlorophyll tend to be found in deep aquatic zones where shorter wavelengths of light are able to penetrate.

Examples of anoxygenic photosynthetic bacteria include purple bacteria and green bacteria. Purple bacterial cells come in a variety of shapes (spherical, rod, spiral) and these cells may be motile or non-motile. Purple sulfur bacteria are commonly found in aquatic environments and sulfur springs where hydrogen sulfide is present and oxygen is absent. Purple non-sulfur bacteria utilize lower concentrations of sulfide than purple sulfur bacteria and deposit sulfur outside their cells instead of inside their cells. Green bacterial cells are typically spherical or rod-shaped and the cells are primarily non-motile. Green sulfur bacteria utilize sulfide or sulfur for photosynthesis and can not survive in the presence of oxygen. They deposit sulfur outside of their cells. Green bacteria thrive in sulfide-rich aquatic habitats and sometimes form greenish or brown blooms.