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

Abstract

Hard corals are in decline as a result of the simultaneous occurrence of global (e.g., ocean warming) and local (e.g., inorganic eutrophication) factors, facilitating phase shifts towards soft coral dominated reefs. Yet, related knowledge about soft coral responses to anthropogenic factors remains scarce. We thus investigated the ecophysiological response of the pulsating soft coral Xenia umbellata to individual and combined effects of phosphate enrichment (1, 2, and 8 μM) and ocean warming (26 to 32°C) over 35 days. Throughout the experiment, we assessed pulsation, mortality, Symbiodiniaceae density, and cellular chlorophyll a content. Simulated ocean warming up to 30°C led to a significant increase in polyp pulsation and by the end of the experiment to a significant increase in Symbiodiniaceae density, whereas cellular chlorophyll a content significantly decreased with warming, regardless of the phosphate treatment. The combination of phosphate enrichment and simulated ocean warming increased pulsation significantly by 41 – 44%. Warming alone and phosphate enrichment alone did not affect any of the investigated response parameters. Overall, X. umbellata displayed a high resilience towards ocean warming with no mortality in all treatments. Phosphate enrichment enabled soft corals to significantly increase their pulsation under increasing temperatures which may enhance their resilience towards ocean warming. This, in turn, could further facilitate their dominance over hard corals on future reefs.

The widely distributed soft coral Xenia umbellata exhibits high resistance against phosphate enrichment and temperature increase

“…In combination with retaining their photosynthates for themselves, these conditions may allow the Symbiodiniaceae to grow, leaving the host with less organic C, as demonstrated for the hard coral species Stylophora pistillata 27 . This hypothesis is supported by the parallel study of Klinke et al 100 , who showed increased algal cell densities under higher temperatures. However, a recent study by Rädecker et al 101 shows that diazotrophs derived N may not be utilized by the Symbiodiniaceae under heat stress.…”

Species-Specific Response of Corals to Imbalanced Ratios of Inorganic Nutrients

“…This is because nutrient enrichment can lead to the disruption of the symbiosis between corals and their associated microbiota [ 47 , 48 , 49 ]. In contrast to scleractinian corals, studies have shown that organic and inorganic contaminants do not always have detrimental effects on soft corals or even increase their resistance to thermal stress [ 22 , 50 ], which may facilitate their ecological dominance over scleractinian corals. Therefore, further studies comparing the responses of soft corals and scleractinian corals to nutrient enrichment are needed to better predict future reef development.…”

Short-term ocean acidification decreases pulsation and growth of the widespread soft coral Xenia umbellata

“…Future scenarios for corals will likely include multiple global and local factors, with ocean warming as the most urgent threat. In some of the previously mentioned studies, the addition of a local factor mitigated the effects of ocean warming [39,53,76], while others exacerbated the effects of ocean warming [38]. Future studies should also investigate combined effects of multiple global factors that are expected to happen simultaneously, i.e., ocean warming and acidification [78].…”

“…Xenia umbellata has been subject to experiments assessing the (combined) effects of global and/or local factors; e.g., dissolved organic C eutrophication and warming [39,74,75], nitrate eutrophication and warming [38], phosphate eutrophication and warming [53,76], water flow and food availability [41], and in-situ eutrophication [54,77]. Future scenarios for corals will likely include multiple global and local factors, with ocean warming as the most urgent threat.…”