The pulsating soft coral Xenia umbellata shows high resistance to warming when nitrate concentrations are low

Abstract

The resistance of hard corals to warming can be negatively affected by nitrate eutrophication, but related knowledge for soft corals is scarce. We thus investigated the ecophysiological response of the pulsating soft coral Xenia umbellata to different levels of nitrate eutrophication (control = 0.6, medium = 6, high = 37 μM nitrate) in a laboratory experiment, with additional warming (27.7 to 32.8 °C) from days 17 to 37. High nitrate eutrophication enhanced cellular chlorophyll a content of Symbiodiniaceae by 168%, while it reduced gross photosynthesis by 56%. After additional warming, polyp pulsation rate was reduced by 100% in both nitrate eutrophication treatments, and additional polyp loss of 7% d−1 and total fragment mortality of 26% was observed in the high nitrate eutrophication treatment. Warming alone did not affect any of the investigated response parameters. These results suggest that X. umbellata exhibits resistance to warming, which may facilitate ecological dominance over some hard corals as ocean temperatures warm, though a clear negative physiological response occurs when combined with nitrate eutrophication. This study thus confirms the importance of investigating combinations of global and local factors to understand and manage changing coral reefs.

Phosphate enrichment increases the resilience of the pulsating soft coral Xenia umbellata to warming

“…Wiedenmann et al (2013), for instance, observed that corals previously incubated at high N:P ratios (< 5 weeks) displayed signs of bleaching within 10 days when exposed to a stepwise increase in heat (up to 30°C) and light (up to 160 mmol m ¬2 s ¬1 ) stress due to phosphate starvation. Supporting results were observed by Thobor et al (2022), who found a higher susceptibility of X. umbellata to ocean warming when previously exposed to nitrate enrichment (37 µM) for two weeks followed by a combined treatment of nitrate enrichment and a stepwise temperature increase from 26 to 32°C in three weeks. However, X. umbellata colonies in the present study were not exposed to high N:P ratios and thus should not suffer from phosphate starvation.…”

“…Recently, Vollstedt et al (2020) observed that the pulsating soft coral Xenia umbellata benefits from organic eutrophication (in form of glucose), resulting in an increased resilience to warming. In contrast, inorganic eutrophication (in form of nitrate) decreased the resilience of X. umbellata to warming (Thobor et al, 2022) supporting the finding of Wiedenmann et al (2013): high N:P ratios can decrease the resilience of corals to warming, possibly due to a destabilized symbiosis between the coral host and its Symbiodiniaceae following phosphate starvation. Phosphate generally plays an important role during thermal stress in corals by maintaining Symbiodiniaceae density and photosynthetic rates (Ezzat et al, 2016).…”

“…Afterwards, the tower system was disconnected from the main system and tanks were separated at the start of the experiment (day 0) to ensure equal starting water conditions across treatments. Further, the temperature in each tank was increased to ~26°C (1°C per day) to commensurate experimental conditions of previous studies (Vollstedt et al, 2020;Thobor et al, 2022). 10% of the artificial seawater was replaced daily to maintain a high water renewal rate in each tank.…”

“…This procedure was repeated until the final water temperature of 32°C was reached. The selected temperature treatment was based on the experimental design of previous studies investigating the combined effect of organic/ inorganic eutrophication and ocean warming on X. umbellata (Vollstedt et al, 2020;Thobor et al, 2022).…”

“…The pulsating soft coral X. umbellata was used as a study organism because it is a common species in the Indo-Pacific region (Benayahu, 1990;Fabricius and Klumpp, 1995;Fox et al, 2003;Janes, 2013). We hypothesize that phosphate, in contrast to nitrate, will increase the resilience of X. umbellata to warming because its Symbiodiniaceae will not suffer from phosphate starvation (Wiedenmann et al, 2013;Rädecker et al, 2015;Thobor et al, 2022). We tested our hypothesis in a five-week laboratory experiment with two phases.…”