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The Difference Between Red & Green Algae

Red algae and green algae share a name, but little else. The phylum Rhodophyta includes all red algae species including those that produce the agar and carrageenan that adds texture to certain foods and beverages. Scientists classify organisms in the phylum Chlorophyta as green algae, but the term "green algae" also colloquially applies to other species; while all Chlorophyta are green algae, not all green algae are Chlorophyta.

Color
- Chlorophyta form the basis of numerous aquatic and terrestrial food webs.
  
  Although both red algae and green algae are autotrophs -- organisms that produce their own food -- they use different metabolic pathways to photosynthesize their nutrients. The green algae of the phylum Chlorophyta use chlorophylls a and b, while Rhodophyta use only chlorophyll a. Strong reddish pigments, such as phycobilin and phycoerythrin, obscure the fainter green hue of the single type of chlorophyll these algae contain, giving them an overall reddish hue. Chlorophyta, by contrast, exhibit a vivid spring green color.
Structure
- Taxonomists and botanists describe some varieties of red algae as coralline algae because of their ability to deposit carbonates around themselves for protection or stability. These Rhodophyta help construct reefs, contributing as much as corals do to the infrastructure of some marine reefs. While green algae do not construct reefs, some varieties contain calcium carbonate in their cell walls. This mineral also comprises the shells of marine mollusks and gives white sand beaches their pure white hue.
Genetics
- A 2010 Rutgers University study published in the February 2011 issue of Current Biology describes an analysis of the genomes of red algae and green algae. Scientists found evidence that the two shared a common ancestry at one time, but that time must have been distant as the two varieties of algae share about half of their genetic material. Organisms that have fewer genes in common diverged a longer time ago; humans and fruit flies, for example, share more than 50 percent of their genetic makeup.
Economic Applications
- Rhodophyta species include edible algae, such as varieties of nori, the seaweed sheets that hold some sushi rolls together. Other types produce materials that become gels and colloids for food, scientific research and industrial processes. Researchers are examining green algae species for their utility as biofuels. Other species are edible as varieties of sea lettuce. Green algae filter sewage and water in aquarium tanks.
Location
- Red algae grow almost exclusively in marine environments. Green algae species have colonized freshwater aquatic environments and even land in the form of arctic organisms that thrive on snow and species that form symbiotic relationships with fungi in lichens. Green algae lives in seawater as well, but the thousands of Chlorophyta species dwell in all but the most arid environs.

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