Several foraminifers found in warm and low-nutrient ocean surface water have photosynthetic algae as endosymbionts
(photosymbiosis). To understand the trophic interactions, we studied Globigerinoides sacculifer, a spinose planktic foraminifer
that has a dinoflagellate endosymbiont. We controlled two nutritional factors, feeding and inorganic nutrients in the seawater. The
growth of the host and the symbionts and the photophysiological parameters were monitored under four experimental conditions.
The results demonstrated that the holobionts primarily relied on phagotrophy for growth. The foraminifers grew considerably,
and the chlorophyll a content per foraminifer, which is an indicator of the symbiont population, increased in the fed groups, but
not in the unfed groups. The nutrient-rich seawater used for some of the cultures made no difference in either the growth or
photophysiology of the holobionts. These observations indicated that the symbionts mainly utilized metabolites from the hosts
for photosynthesis rather than inorganic nutrients in the seawater. Additionally, we observed that the symbionts in the starved
hosts maintained their photosynthetic capability for at least 12 days, and that the hosts maintained at least some symbionts until
gametogenesis was achieved. This suggests that the hosts have to retain the symbionts as an energy source for reproduction. The
symbionts may also play an indispensable role in the metabolic activities of the hosts including waste transport or essential
compound synthesis. Overall, our results revealed a novel mode of photosymbiosis in planktic foraminifers which contrasts with
that found in benthic photosymbiotic foraminifers and corals.

• Effect of nutritional condition on photosymbiotic consortium of cultured Globigerinoides sacculifer (Rhizaria, Foraminifera)