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

Yusuf C El-Khaled,Christian R Voolstra,Nils Rädecker,Burton H Jones,Benjamin Kürten,Florian Roth,Christian Wild,Denis B Karcher,Arjen Tilstra

doi:10.1038/s41598-021-90204-8

Abstract

Coral reefs experience phase shifts from coral- to algae-dominated benthic communities, which could affect the interplay between processes introducing and removing bioavailable nitrogen. However, the magnitude of such processes, i.e., dinitrogen (N2) fixation and denitrification levels, and their responses to phase shifts remain unknown in coral reefs. We assessed both processes for the dominant species of six benthic categories (hard corals, soft corals, turf algae, coral rubble, biogenic rock, and reef sands) accounting for > 98% of the benthic cover of a central Red Sea coral reef. Rates were extrapolated to the relative benthic cover of the studied organisms in co-occurring coral- and algae-dominated areas of the same reef. In general, benthic categories with high N2 fixation exhibited low denitrification activity. Extrapolated to the respective reef area, turf algae and coral rubble accounted for > 90% of overall N2 fixation, whereas corals contributed to more than half of reef denitrification. Total N2 fixation was twice as high in algae- compared to coral-dominated areas, whereas denitrification levels were similar. We conclude that algae-dominated reefs promote new nitrogen input through enhanced N2 fixation and comparatively low denitrification. The subsequent increased nitrogen availability could support net productivity, resulting in a positive feedback loop that increases the competitive advantage of algae over corals in reefs that experienced a phase shift.

Highlights

Nitrogen (N) is vital for all living organisms and is required for primary production and the production of biomass
Likewise, fixed N can be transformed into atmospheric N2 via anaerobic ammonium oxidation (ANAMMOX), a pathway functioning as an additional N removing mechanism in coral reef sponges[15], and hypothetically in other coral reef associated organisms[12]
We carried out acetylene-based incubations (i) to identify the key players that import and/or remove nitrogenous compounds into/from the reef system; and (ii) to provide a relative budget for two counteracting N cycling processes (i.e., N2 fixation and denitrification) in a comparative framework that covers the main species of six key benthic categories that together account for > 98% of the benthic cover on a central Red Sea coral reef with two distinct reef areas

Summary

Introduction

Nitrogen (N) is vital for all living organisms and is required for primary production and the production of biomass. Coral reefs belong to the most productive ecosystems on earth and are regarded as oases in an oceanic desert[6,7,8] In this context, microbial N cycling plays a key role by introducing, recycling and removing N from coral reefs[9]. N cycling is critical for the stability of coral reef ecosystems; it has not been investigated yet how gain (via N2 fixation) and loss (via denitrification) terms of bioavailable N differ quantitatively between coral- and algae-dominated reef states. An altered reef community structure associated with phase-shifts could lead to different total N 2 fixation and denitrification budgets in coral- and algae-dominated reef areas. We carried out acetylene-based incubations (i) to identify the key players that import and/or remove nitrogenous compounds into/from the reef system; and (ii) to provide a relative budget for two counteracting N cycling processes (i.e., N2 fixation and denitrification) in a comparative framework that covers the main species of six key benthic categories that together account for > 98% of the benthic cover on a central Red Sea coral reef with two distinct reef areas

Methods

Results

Conclusion