Quarterly Journal of Microscopical Science, Vol s3-93, 71-96, Copyright © 1952 by Company of Biologists

On the Nutrition of Ochromonas

¹ Botany School, University of Cambridge

A species of the chrysophycean genus Ochromonas from England was grown in pure culture and compared with similar strains isolated in the United States. Although these organisms possess chromatophores with chlorophyll, they need organic substances not only as a source of nitrogen compounds, but as independent carbon and energy sources; they also require certain growth factors. Peptones are better as nitrogen sources than single amino-acids, few of which give satisfactory results. The utilization of inorganic nitrogen could not be demonstrated. As carbon sources sugars, alcohols, and fats, and, to a lesser degree, acetate, are suitable. Growth factors contained in liver extract, milk and yeast autolysate, and in smaller amounts in most peptones, are indispensable. Enzymes hydrolyzing starch, sucrose, fat, and protein are excreted.

Ochromonas ingests and digests starch grains, casein, oil droplets, and small organisms. Neither bacteria nor yeast nor small algae were found able to support growth of any of the strains, even in the light. An additional carbon compound and possibly special vitamins are required. The hypothesis is put forward that phagotrophy chiefly serves for obtaining chemical compounds which the cell cannot synthesize itself and which have the character of vitamins. Phagotrophy therefore supplements the other means of nutrition, in the main quantitatively, while sugar, &; c., supplement it quantitatively and are indeed indispensable.

The same media which support ample growth in the light do so also in the dark. This shows that photosynthesis is of little importance for the growth of the organisms, although it keeps them alive for long periods, while in the dark they soon die.

Correlated with the manifold ways of acquiring food, there is also a remarkable variability in the size and shape of the cells, and still more in the dimensions and pigmentation of the chromatophores, and in the reserves stored as leucosin, volutin, and oil. These features which now prove to depend so much on conditions have previously been widely used in the diagnosis of species. The basis of classification should therefore be revised. Apart from the stigma, which is specific, use ought to be made only of those characters whose variation with conditions is reasonably well known.

The extraordinary nutritional versatility of Ochromonas is considered to be a primitive character. From ancestors with a mixed nutritional habit, phototrophic, phagotrophic, and saprotrophic flagellates seem to have evolved, each with only one of the three modes of nutrition that are combined in Ochromonas.