Organisms may be broadly divided into how they acquire food. Heterotrophs, such as ourselves, obtain food from other organisms. Autotrophs are able to manufacture their own food from inorganic sources. Many autotrophs make their own food from light energy, carbon dioxide and water through a process known as photosynthesis. These organisms constitute the foundation of many ecosystems, providing the nutrients and energy heterotrophs depend upon.
Prokaryotic vs. Eukaryotic Photosynthesis
Photosynthetic eukaryotes -- plants and algae -- possess specialized structures within their cells called chloroplasts, where photosynthesis occurs. Photosynthetic prokaryotes -- bacteria -- lack such specialized structures, instead conducting photosynthesis on folds of their plasma membrane. Chloroplasts possess some characteristics of prokaryotes and may have arisen from a photosynthetic prokaryote entering into a symbiotic relationship with a eukaryotic organism.
The most visible photosynthetic organisms are plants. They are characterized as sedentary, multicellular, photosynthetic eukaryotes with cells surrounded by a cell wall of cellulose. This group provides the fruits, nuts and vegetables in our diet, the lumber for our homes and medicines for our illnesses. Plants include and are typically divided into mosses, ferns, conifers and flowering plants.
Algae often appear macroscopically very similar to plants but lack the sophisticated organ structure found in plants. Algae may be unicellular like the Euglena, colonial as in Volvox or multicellular as exemplified by the brown algae that form the underwater kelp forests. A dark night on the ocean may reveal a beautiful glow in the water from the bioluminescence of dinoflagellates, but this same group can cause devastating red tides.
The microscopic size leads many observers to overlook the significant role played by photosynthetic bacteria in ecosystems. Cyanobacteria, formerly referred to as blue-green algae, are aquatic and a likely candidate for the organism that became the chloroplast found in plants and algae. Other photosynthetic bacteria display a diversity in the pigments and processes for photosynthesis, including an ability to use molecules other than water as the electron donor, resulting in no oxygen being produced.