Leucothrix mucor is a common marine filamentous bacterium that grows as an epiphyte on algae. Because it shares morphological features with nitrogen-fixing cyanobacteria like Calothrix, researchers hypothesized it might be capable of nitrogen fixation despite lacking heterocysts. Mague and Lewin tested this using the acetylene reduction assay, a standard method for detecting nitrogenase activity. Pure cultures of Leucothrix were grown in media with and without combined nitrogen sources, some pre-treated with nitrogen gas. Across 72 samples tested under various oxygen conditions (aerobic, microaerophilic, and anaerobic), no ethylene production was detected, indicating no nitrogenase activity. In contrast, positive control samples of Calothrix showed clear nitrogen-fixing activity. The severely limited growth of Leucothrix in nitrogen-free medium corroborated the biochemical results. The authors conclude that Leucothrix lacks the ability to fix atmospheric nitrogen, likely depending instead on fixed nitrogen compounds from algal exudates in its natural marine habitat.

Key findings

• Leucothrix mucor showed no demonstrable nitrogenase activity in 72 samples tested under aerobic, microaerophilic, and anaerobic conditions
• The organism exhibited severely restricted growth in nitrogen-free medium, supporting the absence of nitrogen fixation capability
• Despite morphological similarities to nitrogen-fixing cyanobacteria, Leucothrix lacks the N2-fixing enzyme complex
• Positive controls using Calothrix confirmed the acetylene reduction assay was functioning properly