High temperature inhibits cnidarian-dinoflagellate symbiosis establishment through nitric oxide signaling

Abstract

16Coral reef ecosystems depend on a functional symbiosis between corals and 17 photosynthetic dinoflagellate symbionts (Symbiodiniaceae), which reside inside the 18 coral cells. Symbionts transfer nutrients essential for the corals' survival, and loss of 19 symbionts ('coral bleaching') can result in coral death. Temperature stress is one factor 20 that can induce bleaching and is associated with the molecule nitric oxide (NO). 21 Likewise, symbiont acquisition by aposymbiotic hosts is sensitive to elevated 22 temperatures, but to date the role of NO signaling in symbiosis establishment is 23 unknown. To address this, we use symbiosis establishment assays in aposymbiotic 24 larvae of the anemone model Exaiptasia pallida (Aiptasia). We show that elevated 25 temperature (32°C) enhances NO production in cultured symbionts but not in 26 aposymbiotic larvae. Additionally, we find that symbiosis establishment is impaired at 27 32°C, and this same impairment is observed at control temperature (26ºC) in the 28 presence of a specific NO donor (GSNO). Conversely, the specific NO scavenger 29 (cPTIO) restores symbiosis establishment at 32ºC; however, reduction in NO levels at 30 26°C reduces the efficiency of symbiont acquisition. Our findings indicate that explicit 31 NO levels are crucial for symbiosis establishment, highlighting the complexity of 32 molecular signaling between partners and the adverse implications of temperature stress 33 on coral reefs. 34Introduction: 36 The endosymbiotic relationship between photosynthetic dinoflagellates from the family 37 Symbiodiniaceae and marine invertebrates plays a crucial role in coral reefs [1, 2, 3, 4, 38 3 5], which are ecosystems of immense ecological and economic importance [3, 4, 6]. 39 Particularly, reef-building corals depend on dinoflagellate symbionts because the 40 translocation of photosynthetically fixed carbon, such as glucose, glycerol, and amino 41 acids, is capable of satisfying up to 95% of the hosts' energy requirements in an 42 otherwise nutrient-poor environment [7, 8]. Accordingly, intracellular symbionts are 43 essential for host nutrition, tissue growth, and biomineralization to create the iconic reef 44 structures, which are home to more than 25% of all marine species [9, 10]. 45Coral reef decline as a result of bleaching is threatening reefs worldwide. Typically, 46 coral bleaching is triggered by biotic and abiotic stress [11, 12, 13, 14] including 47 diseases, increased seawater temperature, acidification, salinity [13, 14, 15, 16, 17], UV 48 radiation [18, 19], and pollution [14]. Failure to re-acquire symbionts within a relatively 49 short time frame leads to coral death [20]. Thus, it is imperative to better understand the 50 underlying mechanisms of cnidarian-dinoflagellate symbiosis in health and disease to 51 protect these ecosystems. 52 The family of Symbiodiniaceae comprises several genera including Symbiodinium 53 (formerly Clade A), Breviolum (formerly Clade B), Cladocopium (formely Clade C), 54 Durusdinium (formerly...