Nitrogen fixation by blue-green algae in Yellowstone thermal areas

Potential nitrogen-fixing blue-green algae were common in three hot spring streams studied in Yellowstone National Park. In two streams, where the dominant nitrogen-fixing algae were species of Calothrix, nitrogen fixation, as measured by uptake of 15N2, was detected in situ in the temperature range 28–46 C. At higher temperatures nitrogen fixation was not detected, although the algae may have received fixed nitrogen from a growth of Calothrix, Nostoc, and unicellular algae which occurred at lower temperatures on the sides of the streams. In the third stream, where Mastigocladus was abundant, nitrogen fixation was detected at temperatures up to 54 C, although the optimum for fixation was near 42.5 C. The overall data imply that in situ nitrogen fixation contributes to the productivity of Yellowstone hot spring regions and that Mastigocladus and Calothrix are the most important nitrogen-fixing blue-green algae.