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Water temperature affects the physiology of large benthic foraminifers (LBFs) with algal symbionts dwelling in coral reef environments. However, the detailed physiological responses of LBF holobionts to temperature ranges occurring in their habitats are not known. We report net oxygen (O2) production and respiration rates of three LBF holobionts (Baculogypsina sphaerulata and Calcarina gaudichaudii hosting diatom symbionts, and Amphisorus kudakajimensis hosting dinoflagellate symbionts) measured in the laboratory at water temperatures ranging from 5°C to 45°C in 2.5°C or 5°C intervals and with light saturation levels of ∼500 µmol m−2 s−1. In addition, the recovery of net O2 production and respiration rates after exposure to temperature stress was assessed. The net O2 production and respiration rates of the three LBF holobionts peaked at ∼30°C, indicating their optimal temperature for a short exposure period. At extreme high temperatures (≥40°C), the net O2 production rates of all three LBF holobionts declined to less than zero and the respiration rates slightly decreased, indicating that photosynthesis of algal symbionts was inactivated. At extreme low temperatures (≤10°C for two calcarinid species and ≤5°C for A. kudakajimensis), the net O2 production and respiration rates were near zero, indicating a weakening of holobiont activity. After exposure to extreme high or low temperature, the net O2 production rates did not recover until the following day, whereas the respiration rates recovered rapidly, suggesting that a longer time (days) is required for recovery from damage to the photosystem by temperature stress compared to the respiration system. These results indicate that the oxygen metabolism of LBF holobionts can generally cope well with conditions that fluctuate diurnally and seasonally in their habitats. However, temporal heat and cold stresses with high light levels may induce severe damage to algal symbionts and also damage to host foraminifers.

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25Low-lying, small reef islands on Pacific atolls are vulnerable to coastal erosion 26 and flooding, mainly due to rises in sea level, as well as local stresses associated with 27 overpopulation. Because reef island sediments on Pacific atolls are composed mostly of 28 shells of large benthic foraminifers (LBFs), their future stability depends on 29 understanding foraminiferal shell sources and their production rates around the 30 islands. Here, we report on the distribution and population density of LBFs around 31 Fongafale Island, the main populated island of Tuvalu in the South Pacific, where 32 coastal erosion and inundation have occurred. We discuss their controlling factors, in 33 particular, anthropogenic influences on foraminiferal distribution. The results based on 34 multivariate analyses (canonical correlation analysis and multiple regression analysis) 35 demonstrated that Baculogypsina sphaerulata and Amphistegina lobifera were more 36 common on the high-energy, intertidal ocean reef flats (≤ 10 6 and ≤ 10 4 individuals m -2 , 37 respectively), and B. sphaerulata was less common around the populated area of the 38 island. On the relatively low-energy subtidal lagoon reef flats, Amphistegina lessonii 39 was the most common species, particularly in the dry (leeward) season, with an 40increasing density in deeper offshore zones (≤ 10 4 individuals m -2 ). Sorites orbiculus 41 was less common than the other species, but was found on both sides of the reef flats (≤ 42 10 3 individuals m -2 ). Macroalgal δ 15 N values indicated that upwelling nutrient-rich 43 deep water could be the source of nitrogen for oceanic waters, whereas nutrients in 44 lagoon seawater could be partly derived from groundwater mixing with domestic 45 wastewater, especially near the populated area. These results suggest that the 46 distribution and population density of LBFs in the study area are influenced by spatial 47 differences in water energy and water quality, and that B. sphaerulata is less tolerant 48 3 of nutrient loading than other species. Comparison of distributions between living 49 individuals and empty shells indicated that ocean reef flats are the primary source of 50 foraminiferal shells for reef islands and lagoon beaches. 51 52