Micro-algae (c. 2.5 to 3.5 mum wide x3.8 to 4.5 mum long) occur abundantly within perialgal vacuoles in the intracapsular cytoplasm of the radiolarian Dictyocoryne truncatum, a triangular-shaped, spongiose skeletal radiolarian. The fine structure of the microalgae resembles that of yellow-brown pigmented symbionts observed in larger spongiose skeletal radiolaria of the spongodrymid type. The density of microalgae in a typical ultrathin section is c. 4/100 mum2. Bacteroids (0.2x0.5 mum) are present throughout the intracapsular cytoplasm. There is no evidence of a vacuolar membrane enclosing the bacteroids, but each is surrounded by an electron lucent zone. The central capsule of radiolaria contains the nucleus and cytoplasmic organelles and is bounded by a capsular wall. Previously, algae associated with radiolaria have been observed in the extracapsulum. The occurrence of intracapsular microalgae in D. truncatum is of interest since this indicates that the intracapsular cytoplasm, previously thought to be largely specialized for metabolism, storage of reserve substances and production of reproductive swarmers, can also be a site for host-algal interactions.
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