Coral-associated nitrogen fixation rates and diazotrophic diversity on a nutrient-replete equatorial reef

Abstract

The role of diazotrophs in coral physiology and reef biogeochemistry remains poorly understood, in part because N2 fixation rates and diazotrophic community composition have only been jointly analyzed in the tissue of one tropical coral species. We performed field-based 15N2 tracer incubations during nutrient-replete conditions to measure diazotroph-derived nitrogen (DDN) assimilation into three species of scleractinian coral (Pocillopora acuta, Goniopora columna, Platygyra sinensis). Using multi-marker metabarcoding (16S rRNA, nifH, 18S rRNA), we analyzed DNA- and RNA-based communities in coral tissue and skeleton. Despite low N2 fixation rates, DDN assimilation supplied up to 6% of the holobiont’s N demand. Active coral-associated diazotrophs were chiefly Cluster I (aerobes or facultative anaerobes), suggesting that oxygen may control coral-associated diazotrophy. Highest N2 fixation rates were observed in the endolithic community (0.20 µg N cm−2 per day). While the diazotrophic community was similar between the tissue and skeleton, RNA:DNA ratios indicate potential differences in relative diazotrophic activity between these compartments. In Pocillopora, DDN was found in endolithic, host, and symbiont compartments, while diazotrophic nifH sequences were only observed in the endolithic layer, suggesting a possible DDN exchange between the endolithic community and the overlying coral tissue. Our findings demonstrate that coral-associated diazotrophy is significant, even in nutrient-rich waters, and suggest that endolithic microbes are major contributors to coral nitrogen cycling on reefs.

Fine-scale mapping of physicochemical and microbial landscapes clarifies the spatial structure of the coral skeleton microbiome

“…6). The carbon, nitrogen and sulphur metabolic pathways predicted through our data analysis support the hypothesis that endolithic bacteria can be considered major nutrient recyclers (Fine and Loya 2002;Sangsawang et al 2017;Moynihan et al 2021) and, by showing how the abundance of these pathways changes across the skeleton depth gradient, we provide new insight into the spatial distribution of the coral skeleton biogeochemical cycle.…”

“…In darkness, the pH shifts to pre-illumination values mainly because of CO 2 production through respiration (Shashar and Stambler 1992), but very few spatially resolved pH measurements have been done in coral skeletons (Shashar and Stambler 1992; Reyes-Nivia et al 2013). There is now increasing evidence for various microbial processes involved in carbon, nitrogen, sulphur, and phosphorus transformations in the coral skeleton (Ferrer and Szmant 1988;Yang et al 2016;Sangsawang et al 2017;Yang et al 2019;Moynihan et al 2021), but quanti cation of such processes is rare and it remains unknown how the functions of the skeletal microbiome change with depth and skeletal architecture. By using a combination of hyperspectral imaging, planar optode sensors and spatially explicit microbiome characterization (by metabarcoding), the present study explores correlations between the O 2 and pH microenvironment and the composition and inferred functions of the skeletal microbiome.…”

A Review on the Role of Endophytes and Plant Growth Promoting Rhizobacteria in Mitigating Heat Stress in Plants

“…Nitrogen is a macronutrient in plants. Heat stress promotes N accumulation in the meristems of plant cells via apical blade erosion and plays a vital role in energy metabolism, protein synthesis, and photosynthesis [ 80 , 81 ]. Higher temperatures delay the development of nitrogenase activity in plants, resulting in inhibition of N fixation, leading to stunted growth of plants [ 82 , 83 ].…”

Community structure of coral microbiomes is dependent on host morphology

“…For oligotrophic coral reef ecosystems, the major source of new, versus regenerated, nitrogen is from nitrogen fixation [86], which is mediated primarily by free-living cyanobacteria and heterotrophic bacteria in the oceans [86]. In addition to freeliving diazotrophs, several important members of the reef community (i.e., corals and sponges) have the capability to fix nitrogen through symbiotic associations with diazotrophs [32,87,88]. It is well-known that the microbiomes of scleractinian corals harbor diazotrophic symbionts [34,35,37,38,41,42], as well as other prokaryotes that mediate transformations of inorganic nitrogen in processes such as nitrification, denitrification, and the anaerobic oxidation of ammonium (ANAMMOX) [11,43,[89][90][91].…”