Manipulative temperature experiments with the foraminifer Sorites orbiculus using inoculated Symbiodiniaceae symbionts

The presence of algal symbionts in Large Benthic Foraminifera (LBF) facilitates the success of the group as important carbonate producers in the ocean. However, the symbiosis makes the holobiont more susceptible to heat stress. Modulating the foraminiferal host-symbiont relationship is one approach that could serve as an adaptation mechanism to elevated temperatures. Recently, a menthol-DCMU (3-(3,4-dichlorophenyl)-1,1-dimethylurea) bleaching method made investigations of host-symbiont combinations in foraminifera possible. Here, we performed a manipulative temperature experiment at three temperatures (25°C, 28°C, and 31°C) on the dinoflagellate-bearing Sorites orbiculus for two weeks. Before the experiment, specimens were menthol-DCMU bleached and inoculated with strains of Symbiodiniaceae (ITS2 strains: F2 sensu stricto, CCMP2467, and KB8) previously isolated from cnidarian hosts. Three controls, including untreated specimens, menthol-DCMU bleached specimens continuously treated with DCMU, and menthol-DCMU bleached specimens recovering in artificial seawater, were used. To assess the physiological impact of the treatments on the specimens, the survivorship and growth of the hosts, and the efficiency of photosystem II (F_v:F_m) of the symbionts were measured. Survivorship was between 75-100% based on PAM fluorescence values and light microscopy. Inoculated specimens with strain KB8 exhibited similar growth to the controls at 31°C. Contrastingly, strain CCMP2467 had lower growth than the controls at each temperature. Growth did not differ between the controls. PAM fluorometry revealed that photosynthetic yields (F_v:F_m) between the 25°C and 31°C treatments were not different between strains, while in the 28°C treatment, strain CCMP2467 showed low photosynthetic activity, indicating stress in the photosystems. Contrary to our expectations, menthol-DCMU bleached individuals continuously treated with DCMU, exhibited similar growth rates as untreated holobionts. The results suggest that S. orbiculus can sustain growth between a temperature range of 25°C to at least 31°C independently of a functional symbiosis. Further investigations are needed to gain insights into the host-symbiont relationship, the potential of its modulation as an adaptation mechanism to elevated temperatures, and the role of symbionts in the growth and calcification of foraminifera.