We studied metabolic activity of the symbiont-bearing large benthic foraminiferHeterostegina depressaunder different light conditions. Besides the overall photosynthetic performance of the photosymbionts estimated by means of variable fluorescence, the isotope uptake (13C and15N) of the specimens (= holobionts) was measured.Heterostegina depressawas either incubated in darkness over a period of 15 days or exposed to an 16:8 h light:dark cycle mimicking natural light conditions. We found photosynthetic performance to be highly related to light supply. The photosymbionts, however, survived prolonged darkness and could be reactivated after 15 days of darkness. The same pattern was found in the isotope uptake of the holobionts. Based on these results, we propose that13C-carbonate and15N-nitrate assimilation is mainly controlled by the photosymbionts, whereas15N-ammonium and13C-glucose utilization is regulated by both, the symbiont and the host cells.