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

Abstract

Mutualistic nutrient cycling in the coral-algae symbiosis depends on limited nitrogen (N) availability for algal symbionts. Denitrifying prokaryotes capable of reducing nitrate or nitrite to dinitrogen could thus support coral holobiont functioning by limiting N availability. Octocorals show some of the highest denitrification rates among reef organisms, however little is known about the community structures of associated denitrifiers and their response to environmental fluctuations. Combining 16S rRNA gene amplicon sequencing with nirS in-silico PCR and quantitative PCR, we found differences in bacterial community dynamics between two octocorals exposed to excess dissolved organic carbon (DOC) and concomitant warming. While bacterial communities of the gorgonian Pinnigorgia flava remained largely unaffected by DOC and warming, the soft coral Xenia umbellata exhibited a pronounced shift towards Alphaproteobacteria dominance under excess DOC. Likewise, the relative abundance of denitrifiers was not altered in P. flava , but decreased by one order of magnitude in X. umbellata under excess DOC likely due to decreased proportions of Ruegeria spp. Given that holobiont C:N ratios remained stable in P. flava but showed a pronounced increase with excess DOC in X. umbellata host, our results suggest that microbial community dynamics may reflect the nutritional status of the holobiont. Hence, denitrifier abundance may be directly linked to N availability. This suggests a passive regulation of N cycling microbes, which could help stabilize nutrient limitation in the coral-algal symbiosis and thereby support holobiont functioning in a changing environment. Importance Octocorals are important members of reef-associated benthic communities that can rapidly replace scleractinian corals as the dominant ecosystem engineers on degraded reefs. Considering the substantial change in the (a)biotic environment that is commonly driving reef degradation, maintaining a dynamic and metabolically diverse microbial community might contribute to octocoral acclimatization and ecological adaptation. Nitrogen (N) cycling microbes, in particular denitrifying prokaryotes, may support holobiont functioning by limiting internal N availability, but little is known about the identity and (a)biotic drivers of octocoral-associated denitrifiers. Here, we show contrasting dynamics of bacterial communities associated with two common octocoral species, the soft coral Xenia umbellata and the gorgonian Pinnigorgia flava after a six-week exposure to excess dissolved organic carbon (DOC) under concomitant warming conditions. The specific responses of denitrifier communities associated with the two octocoral species aligned with the nutritional status of holobiont members. This suggests a passive regulation of this microbial trait based on N availability in the coral holobiont.

Coral holobiont cues prime Endozoicomonas for a symbiotic lifestyle

“…In numerous host-microbe systems, bacteria aid holobiont health and functioning via structuring of the microbiome [8][9][10], provisioning of (essential) metabolites or nutrients [11][12][13] mitigating stress responses [14,15], or changes in their host's life history [16]. Bacteria in corals are thought to support holobiont functioning via nutrient cycling [17][18][19][20][21][22], antimicrobial activity [23,24], and antioxidant capacity [25].…”

Individual and combined effect of organic eutrophication (DOC) and ocean warming on the ecophysiology of the Octocoral Pinnigorgia flava

“…Nevertheless, our results match with recent studies conducted on the pulsating octocoral X. umbellata which showed no negative responses to individual DOC treatments regardless of their concentrations at the ecophysiological level ( Simancas-Giraldo et al, 2021 ). Soft corals may display higher tolerance to enriched DOC concentration in the water than hard corals, by either up taking the available DOC as an additional source of energy via the host ( Fabricius & Klumpp, 1995 ), or by regulating internal nutrient availability of the holobiont via the role that the host-associated bacterial communities ( e.g ., denitrifying bacteria) might play under changing environments ( Xiang et al, 2021 ).…”

“…Seawater salinity was kept at 35 ± 0.6 ppt, pH of 8.2 ± 0.1, and temperature at 26.0 ± 0.3 C (mean ± SE) and exposed to a 12:12 light:dark period at constant light intensity (120.8 ± 10.2 µmol m −2 s −1 ), while additional chemical water parameters, such as the pH, KH, Ammonium (NH 4 + ), Nitrite (NO 2 − ), Nitrate (NO 3 − ) and Phosphate contents (PO 4 3− ) were measured and adjusted manually twice per week. A general summary on the chemical parameters through the experiment can be found in Vollstedt et al (2020) while other relevant parameters such as salinity and pH can be found summarized by treatment in Xiang et al (2021) . We additionally present details on the recorded mean values measured per tank through the experiment in our Material S1 .…”

“…In particular, each experimental tank contained four frames, two holding n = 10 P. flava and two holding n = 10 X. umbellata fragments. The additional frames with X. umbellata fragments were used for different studies besides this one ( i.e ., Vollstedt et al, 2020 ; Simancas-Giraldo et al, 2021 and Xiang et al, 2021 ). Our selection of DOC treatment concentrations for this experiment was based on previous studies that manipulated glucose loading ( Kline et al, 2006 ; Pogoreutz et al, 2017 ) alongside previous findings by e.g ., Baum et al (2016) , where some octocoral species were shown to be able to inhabit highly eutrophicated zones in particular reefs.…”

“…P. flava was molecularly identified by Xiang et al (2021) and selected for this study based on its widespread occurrence in the Indo-Pacific ( Vargas et al, 2020 ) and its relatively simple breeding and maintenance in experimental tanks ( Conci, Wörheide & Vargas, 2019 ; Vargas et al, 2020 ; Vargas et al, 2022 ). The identity of the P. flava associated Symbiodiniaceae of our fragments was not confirmed through molecular means.…”