Nitrogen fixation and denitrification activity differ between coral- and algae-dominated Red Sea reefs

Fish-mediated nutrient flows from macroalgae habitats to coral reefs in the Red Sea

Macroalgae canopies are common in tropical coastlines, and can be feeding grounds for coral reef fishes. We investigated whether fish transfer algal material from Sargassum-dominated macroalgae habitats to coral reefs by collecting gut contents of two herbivorous fish species (Naso elegans and N. unicornis) from coral reefs in the central Red Sea. On inshore reefs close to macroalgae canopies, Sargassum accounted for up to 41% of these species' gut contents while almost no Sargassum was found in the stomachs of fish on offshore reefs farther from macroalgae canopies. Using consumption and excretion rates from literature, we estimate that these fish consume up to 6.0 mmol C/m² reef/day and excrete up to 10.8 μmol N/m² reef/day and 1.0 μmol P/m² reef/day across inshore reefs as a result of Sargassum consumption. Examining fish-mediated connections between habitats illuminates the role of fish as a vector of nutrition to nutrient-poor coral reefs.

Excess labile carbon promotes diazotroph abundance in heat-stressed octocorals

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.

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

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.

Nitrogen fixation contribution to nitrogen cycling during cyanobacterial blooms in Utah Lake

Nitrogen (N) cycling is an essential process in lake systems and N-fixation is an important component of it. Recent studies have also found that nitrate reduction through heterotrophic denitrification in lake systems did not prevent harmful cyanobacterial blooms, but instead, may have favored the dominance of N2-fixing cyanobacteria. The overall objective of this study was to estimate nitrogen fixation rates and the expressions of associated nitrogenase (nif gene) functional gene at several sites at different occasions in freshwater Utah Lake. For comparison purposes, one time sampling was also conducted in the brackish Farmington Bay of Great Salt Lake (GSL). The microbial ecology of the top 20-cm of surface water was investigated to assess the dominant cyanobacterial communities and N-related metabolisms. Our study revealed that Dolichospermum and Nodularia were potential N2-fixers for Utah Lake and brackish Farmington Bay, respectively. The in situ N2-fixation rates were 0-0.73 nmol N hr-1L-1 for Utah Lake and 0-0.85 nmol N hr-1L-1 for Farmington Bay, and these rates positively correlated with the abundance and expressions of the nif gene. In addition, nitrate reduction was measured in sediment (0.002-0.094 mg N VSS-1 hr-1). Significantly positive correlations were found among amoA, nirS and nirK abundance (R = 0.56-0.87, p < 0.05, Spearman) in both lakes. An exception was the lower nirK gene abundance detected at one site in Farmington Bay where high ammonium retentions were also detected. Based on a mass balance approach, we concluded that the amount of inorganic N loss through denitrification still exceeded the N input by N2-fixation, much like in most lakes, rivers, and marine ecosystems. This indicates that N cycling processes such as denitrification mediated by heterotrophic bacteria contributes to N-export from the lakes resulting in N limitations.

Interaction and Assembly of Bacterial Communities in High-Latitude Coral Habitat Associated Seawater

Threatened by climate change and ocean warming, coral reef ecosystems have been shifting in geographic ranges toward a higher latitude area. The water-associated microbial communities and their potential role in primary production contribution are well studied in tropical coral reefs, but poorly defined in high-latitude coral habitats to date. In this study, amplicon sequencing of 16S rRNA and cbbL gene, co-occurrence network, and βNTI were used. The community structure of bacterial and carbon-fixation bacterial communities showed a significant difference between the center of coral, transitional, and non-coral area. Nitrite, DOC, pH, and coral coverage ratio significantly impacted the β-diversity of bacterial and carbon-fixation communities. The interaction of heterotrophs and autotrophic carbon-fixers was more complex in the bottom than in surface water. Carbon-fixers correlated with diverse heterotrophs in surface water but fewer lineages of heterotrophic taxa in the bottom. Bacterial community assembly showed an increase by deterministic process with decrease of coral coverage in bottom water, which may correlate with the gradient of nitrite and pH in the habitat. A deterministic process dominated the assembly of carbon-fixation bacterial community in surface water, while stochastic process dominated t the bottom. In conclusion, the structure and assembly of bacterial and carbon-fixer community were affected by multi-environmental variables in high-latitude coral habitat-associated seawater.

Contrasting microbiome dynamics of putative denitrifying bacteria in two octocoral species exposed to dissolved organic carbon (DOC) and warming

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. Although bacterial communities of the gorgonian Pinnigorgia flava remained largely unaffected by DOC and warming, the soft coral Xenia umbellata exhibited a pronounced shift toward Alphaproteobacteria dominance under excess DOC. Likewise, the relative abundance of denitrifiers was not altered in P. flava but decreased by 1 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, 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 based on N availability, 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. 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 6-week exposure to excess dissolved organic carbon under concomitant warming conditions. The specific responses of denitrifier communities of the two octocoral species aligned with the nutritional status of holobiont members. This suggests a passive regulation based on N availability in the coral holobiont.

Macroalgae reveal nitrogen enrichment and elevated N:P ratios on the Belize Barrier Reef

Macroalgal blooms are increasing on the Belize Barrier Reef (BBR) as scleractinian coral cover declines. Although some have attributed this to reduced grazing, the role of land-based nutrient pollution has not been assessed. Nutrient enrichment was quantified through macroalgal tissue analysis from Belize City to the offshore fore reef and at several central BBR lagoon sites. These recent data were compared to baseline data from the 1980s. Significant nearshore-to-offshore gradients of %N, %P and δ¹³C in macroalgae all indicated land-based sources of these nutrients. Macroalgal δ¹⁵N values were generally enriched in nearshore waters where values matched those reported for human sewage. Notably, the N:P ratios of recent macroalgae measurements were elevated at all sites, more than two-fold higher than values from the 1980s (~30: 1 to 70:1). These results support the hypothesis that nitrogen enrichment from land-based sources has increased phosphorus limitation driving macroalgal blooms and coral stress on the BBR.