Quantum efficiency of chloroplasts retained from food by the mixotrophic ciliate <i>Strombidium rassoulzadegani</i>

Abstract Strombidium rassoulzadegani is an oligotrich ciliate from coastal waters that retains functional chloroplasts from its algal diet. Using a Fluorescence Induction and Relaxation fluorometer, we measured the maximum quantum efficiency (QE) of Photosystem II in two algal foods, one chlorophyte and one cryptophyte, and in the chloroplasts retained by the ciliate. Within the ciliate, QE of the chloroplasts was about half the value (c. 0.3) of that in exponentially-growing, nutrient-replete algal cultures (0.5–0.6). Under starvation, when the ciliates were not able to replace aging chloroplasts, QE declined further, to values of 0.1–0.2. The rate of decline was similar at irradiances from 30–100 μmol m−2 s−1 and slightly faster in total darkness. Exposure of the ciliates to the Photosystem II inhibitor DCMU decreased growth rates much more strongly in chlorophyte-fed ciliates, compared to those fed the cryptophyte. The decline in the chlorophyte-fed ciliates was much greater than expected, given the small subsidy to growth provided by the retained chloroplasts, suggesting that their role in the ciliate’s metabolism may be more than just a photosynthetic boost.