Possible Adaptive Value of Endosymbionts to Their Protozoan Hosts1,2

ABSTRACT. It is generally accepted that in symbiotic systems involving algal species as cellular endobionts there is some positive benefit to the host organisms. In this paper special consideration is given to the larger foraminifera, protozoa that serve as very useful model systems for the study of aspects of inter/intracellular integration and adaptation—living, as they do, in nutrient-limited but well illuminated shallow tropical seas and containing endosymbiotic algae in abundance. A considerable amount of information is now available on physiological as well as morphological adaptations of the host species to pigmented protists representing diverse algal divisions (phyla). Brief mention is also made of bacterial endosymbionts of certain ciliates.