Algal-foraminiferal symbiosis in the planktonic foraminifer Globigerinella aequilateralia; I, Occurrence and stability of two mutually exclusive chrysophyte endosymbionts and their ultrastructure

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Abstract
We have observed two ultrastructurally distinct endosymbiotic algae in the cytoplasm of the planktonic foraminifer Globigerinella aequilateralis. Based on their fine structure we classify both algal types as Chrysophycophytes. Both species of algae formed highly stable, mutually exclusive associations, which persisted in laboratory culture throughout the vegetative growth period of the foraminifer. A daily photoperiod was required, however, to prevent symbiont digestion by the host. Among foraminifera placed in a normal light/dark cycle and offered particulate food (brine shrimp nauplii), symbiont density increased in the foraminiferal cytoplasm as foraminiferal size increased. Starved specimens kept in the light digested some of their symbionts, but many algae were still present in the cytoplasm after six days of starvation. In contrast, forminifera kept in continuous darkness rapidly digested their symbionts over the six-day period, even when offered particulate food. We conclude that symbiont productivity is essential for maintaining the stability of the algal-foraminiferal association. This is the first study to report more than one species of algae as suitable endosymbionts in a single species of planktonic foraminifera.

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