An efficient diazotroph‐derived nitrogen transfer pathway in coral reef system

Abstract

The underlying mechanism for sustaining the high productivity of coral reefs in nutrient deplete water, known as the Darwin Paradox, has long been debated. The input of new nitrogen, for example, derived from N 2 fixation, and efficient nitrogen recycling are hypothesized as two critical factors in resolving the paradox. However, the source of new nitrogen and its connection to nitrogen recycling in the coral reef system remain poorly understood. We measured N 2 fixation rates and investigated the diazotroph community in different components of a tropical coral reef system. Our results show that the reef-building crustose coralline algae demonstrate a significantly higher N 2 fixation rate than the overlying water or the coral holobiont, representing a large source of N 2 fixation to the system. The associated diazotroph community was dominated by the noncyanobacterial Rhizobiales. 15 N 2 -based pulse-chase incubation experiments revealed $ 50% of the diazotrophderived nitrogen was released from the crustose coralline algae holobiont to the overlying water and coral holobiont within 48 h, which was five fold higher than the fraction of nitrogen released from nitrate-based assimilation by crustose coralline algae. Furthermore, the released diazotroph-derived nitrogen was rapidly transferred to the coral holobiont through grazing and was subsequently released back to the overlying water as dissolved organic nitrogen, revealing an efficient pathway that connects the new nitrogen input with the nitrogen recycling in the coral reef ecosystem. Our results provide new insights into the sources and fates of new nitrogen in the coral reefs, helping to resolve the Darwin Paradox in this unique ecosystem.Coral reefs, the ecological icon of tropical seas and one of the most productive (5-10 g C m À2 d À1 ) and diverse ecosystems on Earth, provide extensive ecosystem goods and services to more than 500 million people (Moberg and Folke 1999). However, most coral reef ecosystems thrive in oligotrophic waters, like an oasis in the desert, a paradox first noted by