Plankton as a Source of Food


THERE are several points which should be raised in connexion with the communication from Dr. Nicholas Polunin1 in which the value of phytoplankton as plant manure is discussed. He suggests that the development of a rich growth of algæ in an open bucket or pan of water increases the value of this water as plant manure. It is, however, obvious that the only increase in total matter in the water is the carbon absorbed by the algæ. This carbon, incorporated in the algal cells, is of very doubtful value as plant manure. Growth of algæ in the water is dependent on the presence of dissolved nutrients, notably nitrogen and phosphorus salts, and the transformation of these into organic compounds does not increase their amount, or their value as plant manure, but rather the reverse, since the complex organic compounds must be decomposed by bacteria before the nutrients become available to the plants manured. For the purpose suggested, that is, the watering of vegetables, which are short–period crops, it would seem more important to add the nutrients in readily available form than to build up a reserve of organic matter in the soil. Furthermore, it is probable that there is a loss of nitrogen from the water during the development of the plankton, owing to the escape of gaseous nitrogen by denitrification or by the reaction between amino–nitrogen and nitrite. It is unlikely that any of the algæ capable of fixing nitrogen, (that is, members of the Cyanophyceæ) would grow in such tanks or buckets. It would therefore appear that the bucket of water constitutes a more valuable plant manure in its original state than it does after the development of a phytoplankton.

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PENNINGTON, W. Plankton as a Source of Food. Nature 148, 314 (1941).

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