Photophysiological Tolerance of Algal Symbionts of Archaias Angulatus to Temperature Extremes

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 (F _v /F _m ) 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 [F _v /F _m ], photoefficiency [α], maximum electron transport rate [ETR _max ], and minimum saturating irradiance [E _k ]) were similar between in hospite and extracted symbionts. With the exception of E _k , 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 O ₂ and CO ₂ production or consumption, with ETR _max at ∼200 µ mol photon m ⁻² s ⁻¹ and photoinhibition greater than 500 µ mol photon m ⁻² s ⁻¹ . Photoinhibition occurred at temperatures greater than 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 .