Excess labile carbon promotes diazotroph abundance in heat-stressed octocorals

Abstract

Nitrogen limitation is the foundation of stable coral-algal symbioses. Diazotrophs, prokaryotes capable of fixing N₂ into ammonia, support the productivity of corals in oligotrophic waters, but could contribute to the destabilization of holobiont functioning when overstimulated. Recent studies on reef-building corals have shown that labile dissolved organic carbon (DOC) enrichment or heat stress increases diazotroph abundance and activity, thereby increasing nitrogen availability and destabilizing the coral-algal symbiosis. However, the (a)biotic drivers of diazotrophs in octocorals are still poorly understood. We investigated diazotroph abundance (via relative quantification of nifH gene copy numbers) in two symbiotic octocorals, the more mixotrophic soft coral Xenia umbellata and the more autotrophic gorgonian Pinnigorgia flava, under (i) labile DOC enrichment for 21 days, followed by (ii) combined labile DOC enrichment and heat stress for 24 days. Without heat stress, relative diazotroph abundances in X. umbellata and P. flava were unaffected by DOC enrichment. During heat stress, DOC enrichment (20 and 40 mg glucose l^-1) increased the relative abundances of diazotrophs by sixfold in X. umbellata and fourfold in P. flava, compared with their counterparts without excess DOC. Our data suggest that labile DOC enrichment and concomitant heat stress could disrupt the nitrogen limitation in octocorals by stimulating diazotroph proliferation. Ultimately, the disruption of nitrogen cycling may further compromise octocoral fitness by destabilizing symbiotic nutrient cycling. Therefore, improving local wastewater facilities to reduce labile DOC input into vulnerable coastal ecosystems may help octocorals cope with ocean warming.

Keywords: N2 fixation; coral reefs; global warming; octocoral prokaryotes; organic eutrophication; symbiosis.

Figure 1.

Experimental design of the 45-day aquarium experiment (a) and relative diazotroph abundances in the octocorals Xenia umbellata and Pinnigorgia flava before the start of experiment (b). Relative fold change expressed as nifH gene copy number in reference to 16S rRNA gene copy numbers and in relation to samples of X. umbellata (n = 3). Three levels of labile DOC enrichment were achieved by daily glucose administration throughout the experiment. A stepwise increase in temperature was applied from day 21 to day 45.

Figure 2.

Relative diazotroph abundances in the octocorals X. umbellata (a) and P. flava (b) under ambient conditions, labile DOC enrichment and combined labile DOC enrichment and heat stress. Relative fold change in copy numbers of the nifH gene referenced to the 16S rRNA gene and in relation to day 0 control samples of X. umbellata (n = 3). All data are presented as mean ± s.e. (n = 3 fragments each). Different letters above bars indicate significant differences between groups of the same time point (Tukey HSD, p < 0.05).