Physiological Studies with the Nitrogen-Fixing Alga, Nostoc muscorum

Abstract

1. These pure culture studies show the conditions most favorable for growth and nitrogen fixation by the common blue-green alga, Nostoc muscorum Ag., previously isolated from soil, freed from other organisms, and its nitrogen-fixing ability demonstrated. A detailed morphological description of the organism is given. 2. The organism readily obtains both its nitrogen and carbon from the air. Its rate of growth and nitrogen fixation is ten- to twenty-fold greater than that previously reported for other nitrogen-fixing blue-green algae. The quantity of nitrogen fixed is as high as 10 mg. N in 45 days and 18 mg. N in 85 days per 100 cc. of a medium containing no carbohydrate. 3. When kept in the dark in a medium containing glucose, normal appearing chlorophyll forms and growth and nitrogen fixation proceed slowly. A fixation of 10 to 12 mg. N per gram of glucose consumed is common under these conditions. 4. The organism thrives in a variety of media, provided the pH is satisfactory. The lower and upper limits of growth are approximately 5.7 and 9.0, respectively, while the optimum covers the range of about 7.0 to 8.5. 5. A medium containing the usual inorganic salts, without carbonates and sugar, and with K2HPO4 as a buffer, is satisfactory for use in unaerated cultures. Growth and nitrogen fixation are much more rapid if the medium is aerated, preferably with air containing about 1 per cent CO2. The addition of sugars increases the rate of growth of both aerated and unaerated cultures, especially the latter; likewise combined nitrogen, such as nitrates, ammonia, and asparagin, is usually used somewhat more rapidly than is free nitrogen gas. 6. In unaerated liquid cultures containing an abundance of carbonates and exposed to a high light intensity, the pH rises to about 9.6 and most of the cells die. This results from the utilization of the carbonates as a source of CO2 when neither adequate CO2 gas nor sugar is supplied. 7. A light intensity of 175 to 350 foot candles from Mazda lamps gives satisfactory results. A 12-hour day is somewhat preferable to continuous illumination except at the lower light intensities. 8. Neither calcium nor strontium, at least in concentrations greater than traces, is necessary for growth in the presence of combined nitrogen. In a nitrogen-free medium nitrogen fixation markedly decreases in their absence, however, suggesting that they play a catalytic role in nitrogen fixation, as in the case of Azotobacter. Natural humic acid increases growth and nitrogen fixation, probably due chiefly to its content of available iron. Boron and manganese have no appreciable beneficial effect. 9. These studies indicate that Nostoc muscorum is an organism of considerable importance in soils and possibly in fresh ponds and lakes, adding both nitrogen and organic matter to its growth medium. A brief general discussion of the economic importance of the Myxophyceae in soils is given.