The Diurnal Pattern of Dinitrogen Fixation by Cyanobacteria in Situ

S ummary The diurnal patterns of cyanobacterial acetylene reduction (nitrogen fixation) and seasonal variations in these patterns were investigated in situ on a salt marsh and in a limestone cave. In the field, the non‐heterocystous species fixed N 2 mainly at night, while heterocystous species fixed N 2 mainly during the day. These basic patterns could be influenced by environmental conditions such as O 2 availability and temperature. No acetylene reduction was observed in December, January or February.