Diatoms are unicellular algae of great ecological importance.So far,very little is known about the regulation of carbon fixation in these algae; however,there are strong indications that in diatom plastids,the ferredoxin/thioredoxin system might play a minor role in redox regulation of the photosynthetic reactions compared to land plants.Until now,it is nknown whether there are fewer or other target enzymes of thioredoxins in diatoms.Only a single potential target enzyme for thioredoxin,the plastidic fructose-l,6-bisphosphatase,has yet been identified.Nevertheless,during the an-notation of the genome of the diatom Phaeodactylum tricoutum,we identified several genes encoding different thi-oredoxins.Utilizing in vivo expression of GFP:presequence fusion proteins in P.tricoutum,we were able to show that these thioredoxins are targeted either into plastids,mitochondria,or remain in the cytosol.Surprisingly,two of the three usually cytosolic thioredoxin h proteins are apparently plastid associated and,together with a thioredoxin reductase,putatively located in the periplastidic compartment.This is one of the few indications for so far unknown enzymatic reac-tions in the space between the two pairs of diatom plastid envelope membranes.