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Floridean starch is a type of storage glucan found in and in red algae (or rhodophytes), in which it is usually the primary sink for carbon fixation from photosynthesis. It is found in grains or granules in the cell's cytoplasm and is composed of an alpha glucan glucose polymer with a degree of branching intermediate between amylopectin and glycogen, though more similar to the former. The polymers that make up floridean starch are sometimes referred to as "semi-amylopectin".
| Organisms | Red algae, | Green algae, | Some bacteria, some archaea, Fungus, |
| Composition | Semi-amylopectin; classically without amylose, though some examples exist with amylose present | Amylopectin and amylose | Glycogen |
| Storage location | In the cytosol | Inside | In the cytosol |
| Building block | UDP-glucose | ADP-glucose | Eukaryotes: UDP-glucose Bacteria: ADP-glucose |
| Branching | Intermediate level of branching | Amylopectin: Branches are relatively rare and occur in clusters Amylose: Almost entirely linear | Branches are relatively frequent and evenly distributed |
| Genes required for maintenance | Fewer than 12 | 30–40 | 6–12 |
Historically, floridean starch has been described as lacking amylose. However, amylose has been identified as a component of floridean starch granules in some cases, particularly in unicellular red algae.
Evidence indicates that both ancestors would have had established mechanisms for carbon storage. Based on review of the genetic complement of modern , the last common ancestor of the Archaeplastida is hypothesized to have possessed a cytosolic storage mechanism and to have lost most of the endosymbiotic cyanobacterium's corresponding genes. According to this hypothesis, the rhodophytes and glaucophytes retained the ancestral eukaryote's cytosolic starch deposition. Starch synthesis and degradation in green algae and plants is much more complex – but significantly, many of the enzymes that perform these metabolic functions in the interior of modern plastids are identifiably of eukaryotic rather than bacterial origin. (2025). 9784431554943, Springer Japan. ISBN 9784431554943
In a few cases, red algae have been found to use cytosolic glycogen rather than floridean starch as a storage polymer; examples such as Galdieria sulphuraria are found in the Cyanidiales, which are unicellular .
Other organisms whose evolutionary history suggests secondary endosymbiosis of a red alga also use storage polymers similar to floridean starch, for example, and cryptomonad. The presence of floridean starch-like storage in some apicomplexan parasites is one piece of evidence supporting a red alga ancestry for the apicoplast, a non-photosynthetic organelle.
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