Floridean starch is a type of storageglucan found inglaucophytes and inred algae (or rhodophytes), in which it is usually the primary sink forfixed carbon fromphotosynthesis. It is found in grains or granules in the cell'scytoplasm and is composed of anα-linkedglucosepolymer with a degree of branching intermediate betweenamylopectin andglycogen, though more similar to the former. The polymers that make up floridean starch are sometimes referred to as "semi-amylopectin".[1]
Floridean starch consists of apolymer ofglucose molecules connected primarily by α(1,4) linkages, with occasional branch points using α(1,6) linkages. It differs from other common α-linked glucose polymers in the frequency and position of the branches, which gives rise to different physical properties. The structure of floridean starch polymers is most similar toamylopectin and is sometimes described as "semi-amylopectin". Floridean starch is often described in contrast tostarch (a mixture ofamylopectin andamylose) andglycogen:[1]
| Floridean starch | Starch | Glycogen | |
|---|---|---|---|
| Organisms | Red algae,glaucophytes | Green algae,plants | Somebacteria, somearchaea,fungi,animals |
| Composition | Semi-amylopectin; classically without amylose, though some examples exist with amylose present | Amylopectin andamylose | Glycogen |
| Storage location | In thecytosol | Insideplastids | 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 lackingamylose. However, amylose has been identified as a component of floridean starch granules in some cases, particularly in unicellular red algae.[2][3]
Features such as UDP-glucose building blocks and cytosolic storage differentiate theArchaeplastida into two groups: the rhodophytes and glaucophytes, which use floridean starch, and the green algae and plants (Chloroplastida), which use amylopectin and amylose. There is strongphylogenomic evidence that the Archaeplastida aremonophyletic and originate from a single primaryendosymbiosis event involving aheterotrophic eukaryote and aphotosyntheticcyanobacterium.[1][4]
Evidence indicates that both ancestors would have had established mechanisms for carbon storage. Based on review of the genetic complement of modernplastid genomes, thelast 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.[1][5] 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.[1][2]
In a few cases, red algae have been found to use cytosolic glycogen rather than floridean starch as a storage polymer; examples such asGaldieria sulphuraria are found in theCyanidiales, which are unicellularextremophiles.[6][7]
Other organisms whose evolutionary history suggestssecondary endosymbiosis of a red alga also use storage polymers similar to floridean starch, for example,dinoflagellates andcryptophytes. The presence of floridean starch-like storage in someapicomplexan parasites is one piece of evidence supporting a red alga ancestry for theapicoplast, a non-photosynthetic organelle.[8]
Floridean starch is named for a class of red algae, theFlorideae (now usually termedFlorideophyceae).[9] It was first identified in the mid-19th century and extensively studied bybiochemists in the mid-20th century.[10]