Reduced Expression of the Rate-Limiting Carbon Fixation Enzyme RuBisCO in the Benthic Foraminifer Baculogypsina sphaerulata holobiont in Response to Heat Shock

Baculogypsina sphaerulata (Parker and Jones, 1860) is a common large benthic foraminifer (LBF) and is an important calcifier in coral reef ecosystems. As there are concerns that global increasing temperatures may compromise the survival of this species, which forms a symbiotic relationship with the diatom Nitzschia sp., we investigated the response of the B. sphaerulata holobiont from the intertidal algal flats of Xiao Liu Chiu Island, Taiwan to heat shock. B. sphaerulata specimens were incubated at 26 (ambient), 28, 30, 32, or 34 °C for 5 h designed to simulate short pulses of elevated temperature that occur in situ from subaerial exposure at low tide.  To assess the molecular-level response,we measured the expression of the ribulose 1-5-bisphosphate carboxylase/ oxygenase (RuBisCO) protein in the diatom symbiont. There was a significant decrease in expression of this rate-limiting carbon fixation enzyme in B. sphaerulata holobionts incubated at 34 °C/8 °C above ambient (~50% decline relative to controls), but expression was resilient to levels of warming up to 6 °C above ambient. This suggests that exposure to high temperatures occasionally experienced in nature may diminish the capacity for carbon fixation of the diatom symbiont. Given the importance of photosynthesis and carbon fixation in these marine calcifiers, these data suggest that climate-driven ocean warming may exert deleterious effects on the B. sphaerulata holobiont.