Abstract
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.
Highlights
Climate change, in particular global warming and ocean acidification, adversely affect coral reefs and reef calcifying organisms [1,2]
This study revealed the effect of fluctuating water temperatures, such as those that occur in the field, on net O2 production and respiration in large benthic foraminifers (LBFs) holobionts
Their recovery following short-term exposures to temperature stress was assessed. 30uC was found to be the optimal temperature for net photosynthesis and respiration of the studied LBF holobionts. This result is consistent with a previous culture experiment on dinoflagellate-bearing Marginopora vertebralis, which showed that net O2 production rates remained high between 28uC and 32uC and declined at 34uC under the same seawater pH and high light levels (300 mmol photons m–2 s–1) [17]
Summary
In particular global warming and ocean acidification, adversely affect coral reefs and reef calcifying organisms [1,2]. Rising water temperatures caused by global warming induce bleaching (defined as the loss of photosynthetic microalgae and/or their photopigments) of corals and other reef organisms with symbiotic algae [4]. Mass bleaching events have occurred in association with episodic elevated water temperatures in recent decades [5], and bleaching events are likely to become a chronic phenomenon in many reef areas in the coming decades [6]. The rate of excitation (light capture by reaction centers) exceeds the rate of light utilization (photochemistry). This excess energy results in a buildup of reactive oxygen species (ROS) [10,11]. The overproduction of ROS causes damage to the photosynthetic apparatus of symbionts as well as to host cells [12,13]
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