Photosynthetic plasticity of endosymbionts in larger benthic coral reef Foraminifera

Abstract

Larger benthic Foraminifera (LBF) are ecologically important coral reef protists that harbour a large diversity of symbionts from at least four algal phyla. In this study the photosynthetic plasticity of different endosymbiontic algae found within LBF was investigated using pulse amplitude modulated (PAM) fluorometry. Maximum quantum efficiencies of photosystem II (Fv/Fm) obtained from foraminiferal specimens directly after field collection indicated several pronounced differences between species containing endosymbionts from different algal phyla and, to a lesser extent, also varied between species that contain the same phyla of endosymbiontic algae. Foraminiferal species retaining functional chloroplasts and rhodophyte-bearing species had distinctly lower Fv/Fm, than LBF with dinoflagellates, diatoms or chlorophytes. A laboratory experiment was conducted over 48h exposing species occurring in high- (photophilic), medium- and low-light (sciaphilic) environments to three manipulated light levels. Photophysiological responses were monitored by measuring Fv/Fm at regular intervals and rapid light curves (RLCs) at the end of the experiment. This experiment demonstrated oscillation of maximum quantum efficiencies according to the light-dark cycle. Changes in the shape of the RLCs (e.g., higher α and lower Ek under low light conditions) indicated that photosynthetic plasticity allows LBF to acclimatise rapidly (<48h) to different light conditions.