Algae in Association with Heterotrophic or Holozoic Organisms

The association of an alga capable of photosynthesis with another organism dependent on a supply of organic food, be it another alga, a fungus, or an invertebrate animal, is a very widespread phenomenon. The algae concerned are usually unicellular or simple filamentous forms, and, apart from the Zooxanthellae inhabiting marine invertebrates, are for the most part members of the Chlorophyceae or Myxophyceae. My theme is a very wide one, and it will therefore be necessary to confine my remarks to certain selected examples. The combination of alga and fungus to constitute a lichen has been the subject of repeated investigation and has been variously interpreted. Whilst in the past penetration of the algal cells, or gonidia as they are commonly called, by fungal haustoria had only been rarely reported and was regarded by many as something quite unusual, if not abnormal, the more recent work of Fry (1928), and especially that of Geitler (1933, 1934, 1937, 1938) and of his pupil Tschermak (1941, 1943), has shown that it is comparatively common. In some lichens the haustoria penetrate deeply into the algal cells, seemingly occupying an invagination of the protoplast, but not necessarily leading to its destruction. In heteromerous lichens it is more usual to find what have been called intramembranous haustoria which only penetrate through the cell wall up to the protoplast of the algal cell. Haustoria of this type are formed during the summer months and may be discoverable only for a comparatively brief period.