Nanoplanktonic siliceous cysts from Antarctica are algae

Siliceous cysts, in the size range 2.0 to 5.5 m are an important component of the nanoplankton of the Antarctic seas and the sub-arctic Pacific Ocean. It has been suggested that these organisms may be cysts in the life cycle of choanoflagellates (Acanthoecidae, Protozoa). We conclude that these organisms are algae, based on finding that living cells from Prydz Bay, Antarctica, autofluoresce red, indicating the presence of chlorophyll and that transmission electron microscopy of sectioned cells shows the presence of chloroplasts.