NOAA/WDS Paleoclimatology - Symbiont photosynthetic competency and its effect on boron proxies in planktic foraminifera, Puerto Rico, Spring 2010 and 2015, and Santa Catalina Island, Summer 2013

Boron proxies in the calcium carbonate shells of planktic foraminifera are sensitive to seawater acidity, but B/Ca ratios and isotopic composition (i.e. d11B) recorded by different foraminifer species grown under identical environmental conditions differ significantly and systematically. Specifically, the symbiont-bearing Globigerinoides ruber records higher B/Ca and d11B than Trilobatus sacculifer and Orbulina universa, and it has been hypothesized that these differences are caused by species-specific rates of symbiont photosynthesis and habitat depth. According to this hypothesis, greater symbiont photosynthesis elevates the microenvironmental pH of G. ruber to a greater degree than in T. sacculifer and O. universa. Here we test this hypothesis by applying fast repetition rate fluorometry (FRRF), Chlorophyll a quantification, and symbiont counts in laboratory grown specimens of G. ruber, T. sacculifer and O. universa to study species-specific differences in symbiont photosynthetic competency. In addition, we report B/Ca shell profiles measured by laser ablation on the same specimens previously monitored by FRRF, and d11B data of discrete populations of all three species grown under high and low light conditions in the laboratory. While the light experiments document that symbiont photosynthesis elevates pH and/or d11B in the calcifying microenvironment of all three foraminifer species, the FRRF, Chl. a and symbiont abundance data are relatively uniform among the three species and do not scale consistently with intra shell B/Ca, or with observed species-specific offsets in B/Ca or d11B. Implications of these findings for foraminiferal physiology and biomineralization processes are discussed.