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

Abstract

Both global and local factors affect coral reefs worldwide, sometimes simultaneously. An interplay of these factors can lead to phase shifts from hard coral dominance to algae or other invertebrates, particularly soft corals. However, most studies have targeted the effects of single factors, leaving pronounced knowledge gaps regarding the effects of combined factors on soft corals. Here, we investigated the single and combined effects of phosphate enrichment (1, 2, and 8 μM) and seawater temperature increase (26 to 32 °C) on the soft coral Xenia umbellata by quantifying oxygen fluxes, protein content, and stable isotope signatures in a 5-week laboratory experiment. Findings revealed no significant effects of temperature increase, phosphate enrichment, and the combination of both factors on oxygen fluxes. However, regardless of the phosphate treatment, total protein content and carbon stable isotope ratios decreased significantly by 62% and 7% under temperature increase, respectively, suggesting an increased assimilation of their energy reserves. Therefore, we hypothesize that heterotrophic feeding may be important for X. umbellata to sustain their energy reserves under temperature increase, highlighting the advantages of a mixotrophic strategy. Overall, X. umbellata shows a high tolerance towards changes in global and local factors, which may explain their competitive advantage observed at many Indo-Pacific reef locations.

Figure 1

Experimental design with a 1st experimental phase of pure PO₄ enrichment, followed by a 2nd experimental phase during which temperature was increased stepwise from 26 to 32 °C. Sampling of oxygen fluxes took place after each week (W0-5), while corals for protein content as well as elemental and stable isotope analysis were collected at the beginning, and at the end of each experimental phase.

Figure 2

Gross- (P_gross), net photosynthesis (P_net), and respiration (R) of Xenia umbellata under experimental conditions: only temperature increase without PO₄ addition (control), 1 µM PO₄ + temperature increased from week 3 onwards (1 µM PO₄), 2 µM PO₄ + temperature increased (2 µM PO₄), and 8 µM PO₄ + temperature increased (8 µM PO₄). Letters indicate significant differences for P_gross and R between weeks and asterisks indicate significant differences between treatments per time (p < 0.05, pairwise comparison t-test, Bonferroni adjustment). Error bars represent standard deviations. Dots represent the individually measured data points. The vertical line indicates the start of the temperature treatment and the average temperature for all tanks for the different time points is given on top of the graph.

Figure 3

Protein content of Xenia umbellata colonies under experimental conditions: only temperature increase without PO₄ addition (control), 1 µM PO₄ + temperature increased from week 3 onwards (1 µM PO₄), 2 µM PO₄ + temperature increased (2 µM PO₄), and 8 µM PO₄ + temperature increased (8 µM PO₄). Letters above bars indicate significant differences between weeks and letters within bars indicate significant differences within the treatment over time (p < 0.05, pairwise comparison t-test, Bonferroni adjustment). Error bars represent standard deviations. Dots represent the individually measured data points. The vertical line indicates the start of the temperature treatment and the average temperature for all tanks for the different time points is given on top of the graph.

Figure 4

Carbon (a), and Nitrogen (b) stable isotope ratio, percent carbon (c), percent nitrogen (d), and carbon to nitrogen ratio (e) of Xenia umbellata colonies under experimental phosphate conditions: only temperature increase without PO₄ addition (control), 1 µM PO₄ + temperature increased from week 3 onwards (1 µM PO₄), 2 µM PO₄ + temperature increased (2 µM PO₄), and 8 µM PO₄ + temperature increased (8 µM PO₄). Letters indicate significant differences between weeks (p < 0.05, pairwise comparison t-test, Bonferroni adjustment). Error bars represent standard errors. Dots represent the individually measured data points. The vertical line indicates the start of the temperature treatment and the average temperature for all tanks for the different time points is given on top of the graph.