Larger benthic foraminifers that host algal symbionts are abundant in the meiobenthos of coral-reef environments in tropical and subtropical regions worldwide. This study investigated photophysiological responses of chlorophyte endosymbionts of Archaias angulatus, both in hospite and in cytoplasm extracted from the host shell, across a range of temperature conditions (4.4–33.9°C) using pulse amplitude modulated chlorophyll fluorometry and chlorophyll a measurements. Typical mean yields (Fv/Fm) for symbionts in hospite were 0.6–0.7, and for extracted symbionts were 0.5–0.6. The trends in all of the photophysiological parameters measured or calculated (yield [Fv/Fm], photoefficiency [α], maximum electron transport rate [ETRmax], and minimum saturating irradiance [Ek]) were similar between in hospite and extracted symbionts. With the exception of Ek, the in hospite values were somewhat higher than those for the extracted symbionts. Though chlorophyll a negatively correlated with temperature, individual variability was high. Light curves based on median relative Electron Transport Rate (rETR) and associated photophysiologically-derived parameters were consistent with results from previous studies of the A. angulatus holobiont based on O2 and CO2 production or consumption, with ETRmax at ∼200 µmol photon m−2 s−1 and photoinhibition >500 µmol photon m−2 s−1. Photoinhibition occurred at temperatures >31.0°C. We observed survival of the holobiont and minimal reduction in symbiont photoefficiency at 4.4°C, revealing a lower thermal-tolerance range than had been previously reported for A. angulatus.

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