Description: Monosiphonous filaments creeping endophytically in Polysiphonia, forming small microscopic spots.

Habitat: Very rare. Endophytic in Polysiphonia.

Distribution: Widespread in the British Isles. Europe: Portugal, Spain, France, Baltic Sea, Norway and Iceland. Greenland. Atlantic coast of North America: Canada.

Similar Species: There are a number of other similar microscopic algae, all are difficult to identify.

Distribution Map from NBN: Mikrosyphar polysiphoniae at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Mikrosyphar polysiphoniae at iNaturalist World Species Observations database.

WoRMS: Mikrosyphar polysiphoniae at World Register of Marine Species.