Ontogenetic dynamics of photosymbiosis in cultured planktic foraminifers revealed by fast repetition rate fluorometry

Photosymbiosis is an important ecological strategy that allows the host to thrive in oligotrophic environments, and the photosymbiosis of planktic foraminifers is no exception. Here, we present ontogenetic information about the photosymbiosis of planktic foraminifers that we obtained via analyses of the chlorophyll fluorescence [fast repetition rate (FRR) fluorometry] of symbiotic algae within the host. We cultured two symbiont-bearing planktic foraminifers (Globigerinoides sacculifer and Globigerinella siphonifera Type II) until their natural death, and conducted FRR measurements on individual host-algal consortia through the culture study. Time-series FRR analyses revealed no clear temporal trend in photophysiology but did reveale species specificity. The light-absorption efficiency of the photosynthetic system was significantly higher in Gn. siphonifera than in Gs. sacculifer indicating higher potential to acclimate to low-light environment for Gn. siphonifera. In contrast to the physiology, the chlorophyll a content of foraminifer, a metric of the quantity of symbionts, showed conspicuous ontogenetic changes; the chlorophyll a content, initially less than 30 ng, reached a maximum of more than 140 ng, then it was all digested or lysed at the end of the host's ontogeny. The changes of symbiont biomass and relatively invariant photophysiology indicate dynamic rise and fall of potential photosynthesis of symbiont population during the host's life processes, not just the progressive increase of photosynthesis. Because photosynthesis of symbionts can alter the geochemical composition of the foraminiferal calcifying microenvironment, our results will also contribute to better understanding of the effect of photosynthesis on the foraminiferal tests that are important for paleoecological and paleoceanographic works.