Bacterial glycogen and plant starch metabolism both require the presence of malto-oligosaccharide assimilation enzymes. In Escherichia coil maltotetraose is generated through debranching of the glycogen limit dextrin produced by glycogen phosphorylase. This maltotetraose if further metabolised through the combined action of amylomaltase (an α-1, 4 glucanotransferase) and maltodextrin phosphorylase. In the starch accumulating alga Chlamydomonas reinhardtii we show that a deficiency in D-enzyme (the plant α-1, 4 glucanotransferase) leads to a severe decrease in starch content and a modification in amylopectin structure as well as a modification in amylose content. We further show that there are 2 distinct plastidial phosphorylases in Chlamydomonas. Kinetic and genetic studies suggest these forms may be related to the maltodextrin and glycogen-type of phosphorylases from bacteria.