Photosynthesis and respiration of the soft coral Xenia umbellata respond to warming but not to organic carbon eutrophication

Abstract

Eutrophication with dissolved organic carbon (DOC) as a far under-investigated stressor, and ocean warming, can strongly affect coral reefs and hard corals as major reefs ecosystem engineers. However, no previous studies have investigated the metabolic responses of soft corals to DOC eutrophication, or its interaction with ocean warming. Thus, we investigated respiration and photosynthesis response of Xenia umbellata, a common mixotrophic soft coral from the Indo-pacific, to (1) three levels of DOC eutrophication simulated by glucose addition over the first 21 days of experiment and (2) ocean warming scenarios where the temperature was gradually increased from 26 °C (control condition) to 32 °C over another 24 days in an aquarium experiment. We found no significant difference in response to DOC treatments and all corals survived regardless of the DOC concentrations, whilst subsequent exposure to simulated ocean warming significantly decreased gross photosynthesis by approximately 50% at 30 °C, and 65% at 32 °C, net photosynthesis by 75% at 30 °C and 79% at 32 °C, and respiration by a maximum of 75% at 30 °C; with a slight increase at 32 °C of 25%. The ratio between gross photosynthesis and respiration decreased by the end of the warming period but remained similar between controls and colonies previously exposed to DOC. Our findings suggest that soft corals may be more resistant than hard corals to DOC eutrophication and in consequence, may potentially experiment in less magnitude the negative effects of increased temperature or subsequently both stressors. The results of this study may contribute to explain the successful role of soft corals in phase shifts as reported from many coral reefs. Where predicted declines in reef ecosystems health due to increased eutrophication levels can be exacerbated by future warming.

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

“…When comparing the absolute values for P gross and R of studies on X. umbellata, P gross values reach 22 up to 140 mg O 2 m −2 h −1 , while respiration ranges between − 11 and − 60 mg O 2 m −2 h −1 . Despite this range, with a P gross /R ratio of around 2 measured in this study and previously 47,49,52 , X. umbellata has higher photosynthetic productivity compared to other soft corals but is still at the lower edge of productivity compared to hard corals. The measured oxygen fluxes in our experiment remained stable under all added PO 4 concentrations.…”

“…1). The length of the pure eutrophication treatment was chosen (1) as a previous study could already detect an effect of PO 4 and nitrate (NO 3 ) after 2 weeks 55 , and (2) to allow for a better comparison due to similar experimental design of closely related studies conducted on X. umbellata [47][48][49] . To keep the PO 4 enrichment treatments stable, we measured PO 4 concentrations every day after the water exchange with an adjusted protocol of a commercially available PO 4…”

Contrasting Microbiome Dynamics of Putative Denitrifying Bacteria in Two Octocoral Species Exposed to Dissolved Organic Carbon (DOC) and Warming

“…X. umbellata is known for its distinctive pulsation behavior, which may contribute to its ecological resilience by mixing of the surface boundary layer ( 59 , 60 ). Of note, excess DOC remarkably enhanced their pulsation rates ( 61 ) and net primary productions ( 62 ), but whether this increase in pulsation may have an effect on the microbial plasticity of X. umbellata remains to be determined. Some endosymbiotic Symbiodiniaceae are known to feed heterotrophically ( 63 ), suggesting that excess DOC in X. umbellata may also promote the heterotrophic growth of Symbiodiniaceae.…”

Effective asexual reproduction of a widespread soft coral: comparative assessment of four different fragmentation methods

“…We based our assessment of these four methods on (1) success rate, (2) fragmentation time, (3) oxygen fluxes, and (4) pulsating rate. Success rate, defined as the attachment percentage to the desired surface of each method, and fragmentation time were used to determine the time- and labor- efficiency of each method, while oxygen fluxes and pulsating rate measurements were used as a proxy for the health status ( McGillis et al, 2009 ; Jantzen et al, 2013 ; Vollstedt et al, 2020 ; Simancas-Giraldo et al, 2021 ). The method with the best results was then further optimized using three assessment categories: (1) success rate, (2) oxygen fluxes, and (3) pulsating rate.…”

“…Thus, the aim of this study is to (i) assess and compare four fragmentation methods for the pulsating soft coral, the Xenia umbellata , i.e ., the glue, rubber band, tunnel mesh, and plug mesh method, and (ii) provide a protocol for the best method. Among the four methods, the glue and rubber band methods have already been widely used in laboratories ( e.g ., Barton, Willis & Hutson, 2017 ; Ellis & Sharron, 1999 ; Simancas-Giraldo et al, 2021 ; Vollstedt et al, 2020 ). In contrast, the tunnel mesh and plug mesh methods were modified from a recently patented method named the tunnel cube method by Curry (2020) .…”