Photosynthesis is the fundamental biological process that converts light energy into chemical energy, primarily in the form of sugars. This conversion uses water and carbon dioxide, producing organic compounds that fuel the organism’s metabolism. The process generates nearly all the atmospheric oxygen on Earth, making it a foundational mechanism for life. All organisms that perform this reaction are considered photoautotrophs, forming the base of almost every global food web. This ability is distributed across a diverse range of life forms, spanning complex eukaryotic cells and simpler prokaryotic cells.
Terrestrial Eukaryotes: Plants
Terrestrial plants represent the most familiar group of photosynthetic organisms, encompassing everything from giant redwood trees and flowering plants to small mosses and ferns. As eukaryotes, their cells contain a membrane-bound nucleus and specialized organelles, including chloroplasts. The chloroplast is the site of photosynthesis, and its internal membranes house the chlorophyll pigments that absorb light energy.
Chlorophyll efficiently captures light in the blue and red regions of the visible spectrum, reflecting the green light that gives plants their characteristic color. The energy captured drives the conversion of water and atmospheric carbon dioxide into glucose and oxygen. Plants regulate carbon dioxide intake through tiny pores on their leaves called stomata, which manage gas exchange and prevent excessive water loss.
The dominance of plants in terrestrial ecosystems establishes them as the primary producers on land, supporting herbivores and the entire food chain. All terrestrial plants perform oxygenic photosynthesis, using water as the electron donor and releasing molecular oxygen as a byproduct.
Aquatic Eukaryotes: Algae and Photosynthetic Protists
Photosynthetic eukaryotes are not confined to land; the majority of global oxygen production originates from aquatic organisms. This group, collectively referred to as algae and photosynthetic protists, dominates marine and freshwater environments. These organisms range from massive, multicellular brown algae, such as kelp, to microscopic, single-celled organisms known as phytoplankton.
Phytoplankton, including organisms like diatoms and dinoflagellates, are single-celled eukaryotes that drift in the upper layer of the ocean. Despite their minute size, they are estimated to be responsible for approximately 50% of the world’s oxygen production. They form the base of the marine food web, supporting all higher trophic levels.
Algae and protists exhibit a remarkable diversity in their light-harvesting pigments, allowing them to thrive at various water depths.
- Green algae contain chlorophyll a and b, similar to land plants, and tend to live closer to the water’s surface.
- Brown algae, such as kelp, possess the accessory pigment fucoxanthin, which helps them capture blue-green light that penetrates deeper water.
- Red algae, which can live at the greatest depths, utilize phycoerythrin, a pigment that absorbs shorter-wavelength blue light, effectively transferring that energy to the chlorophyll reaction center.
Prokaryotes: Photosynthetic Bacteria
The ability to photosynthesize exists in the prokaryotic domain, which includes bacteria lacking a nucleus and membrane-bound organelles. This group contains the oldest forms of photosynthetic life on Earth, divided into two distinct types: oxygenic and anoxygenic.
Oxygenic Photosynthesis
Cyanobacteria, often called blue-green algae, are the most prominent oxygenic photosynthetic bacteria, performing the same water-splitting reaction as plants and algae. These organisms are of immense evolutionary significance, as they were the first to evolve oxygenic photosynthesis over three billion years ago. The oxygen they released gradually accumulated in the atmosphere, leading to the Great Oxygenation Event and paving the way for oxygen-dependent life forms. Furthermore, chloroplasts within plant and algal cells originated from an ancient endosymbiotic relationship with a cyanobacterium.
Anoxygenic Photosynthesis
Other bacteria perform anoxygenic photosynthesis, meaning their process does not produce oxygen as a byproduct. Instead of using water as the electron donor, these bacteria rely on compounds such as hydrogen sulfide, hydrogen gas, or organic molecules. Examples include Purple Sulfur Bacteria and Green Sulfur Bacteria, which thrive in anaerobic environments like deep-water sediments or hot springs.
These anoxygenic phototrophs use unique pigments called bacteriochlorophylls, specialized to absorb light in the far-red and infrared spectrum. When Purple Sulfur Bacteria use hydrogen sulfide as an electron source, they release elemental sulfur granules instead of oxygen. This ancient, non-oxygen-producing form of photosynthesis represents a distinct biochemical pathway from that used by plants and cyanobacteria.