Nitrogen fixation in a non-heterocystous cyanobacterial mat from a mountain river

<jats:title>Abstract</jats:title><jats:p><jats:italic>In situ</jats:italic> nitrogen fixation was investigated in a cyanobacterial mat growing on the bed of rocks of the Muga River, Spain. The filamentous non-heterocystous cyanobacterium <jats:italic>Schizothrix</jats:italic> dominated the mat, showing nitrogenase activity in the light at similar rates to those found in nearby heterocystous <jats:italic>Rivularia</jats:italic> colonies. N<jats:sub>2</jats:sub> fixation in the light was significantly increased by an inhibitor of PSII and oxygen evolution, DCMU (3-[3,4-dichlorophenyl]-1,1-dimethylurea), and anaerobic conditions. However, no nitrogenase activity was found in the dark. Addition of fructose as a respiratory substrate induced nitrogenase activity in samples incubated under aerobic conditions in the dark but not in anaerobic conditions. Microelectrode oxygen profiles showed internal microaerobic microzones where nitrogen fixation might concentrate. Analyses of the 16S rRNA gene revealed only the presence of sequences belonging to filamentous non-heterocystous cyanobacteria. <jats:italic>nifH</jats:italic> gene diversity showed that the major phylotypes also belonged to this group. One of the three strains isolated from the <jats:italic>Schizothrix</jats:italic> mat was capable of fixing N<jats:sub>2</jats:sub> and growing in the absence of combined N. This was consistent with the <jats:italic>nifH</jats:italic> gene analysis. These results suggest a relevant contribution of non-heterocystous cyanobacteria to nitrogen fixation in these mats.</jats:p>