N2 fixation, and the relative contribution of fixed N, in corals from Curaçao and Hawaii

Abstract

Corals from Hawaii (Montipora capitata) and the Caribbean (brown and orange morphs of Montastraea cavernosa) have previously been shown to harbor symbiotic bacteria capable of fixing nitrogen (N2). Using a nitrogen tracer approach, we find that the rates of net photosynthesis and N2 fixation in M. capitata were significantly lower, while steady-state quantum yields of photosystem II (PSII) fluorescence (∆Fv/Fm′) were significantly higher, when compared to both color morphs of M. cavernosa where there was an inverse relationship between rates of photosynthesis and N2 fixation. However, the amount of fixed N contributing to Symbiodiniaceae N demand was consistent with their observed rates of N2 fixation. The lowest values occurred in M. capitata (0.034% ± 0.002 SE), and the brown morph of M. cavernosa had significantly lower values (1.081% ± 0.12 SE) compared to the orange morph (8.141% ± 0.36 SE). Additionally, for both the Symbiodiniaceae and microbial communities there were significant differences between species/color morphs where M. capitata was significantly different from both color morphs of M. cavernosa which were not significantly different from each other. An analysis of predicted metabolic activity, using PICRUSt2, also showed that the corals in this study were not predicted to be differentially enriched in genes involved in carbon metabolism, but genes involved in denitrification were predicted to be significantly enriched in both M. cavernosa orange and brown morphs. Genes involved in N2 fixation were, surprisingly, predicted to be enriched in M. cavernosa brown morphs. We suggest that the inhibition of nitrogenase by hyperoxia is one factor contributing to the low rates of N2 fixation in both M. capitata and M. cavernosa brown morphs.