13C and 15N assimilation and organic matter translocation by the endolithic community in the massive coral Porites lutea.

Laddawan Sangsawang,Beatriz Estela Casareto,Hung Manh Vu,Thamasak Yeemin,Aussanee Meekaew,Hideo Ohba,Yoshimi Suzuki,Toshiyuki Suzuki

doi:10.1098/rsos.171201

Abstract

Corals evolved by establishing symbiotic relationships with various microorganisms (the zooxanthellae, filamentous algae, cyanobacteria, bacteria, archaea, fungi and viruses), forming the ‘coral holobiont'. Among them, the endolithic community is the least studied. Its main function was considered to be translocation of photo-assimilates to the coral host, particularly during bleaching. Here, we hypothesize that (i) endolithic algae may show similar primary production rates in healthy or bleached corals by changing their pigment ratios, and therefore that similar production and translocation of organic matter may occur at both conditions and (ii) diazotrophs are components of the endolithic community; therefore, N2 fixation and translocation of organic nitrogen may occur. We tested these hypotheses in incubation of Porites lutea with 13C and 15N tracers to measure primary production and N2 fixation in coral tissues and endoliths. Assimilation of the 13C atom (%) was observed in healthy and bleached corals when the tracer was injected in the endolithic band, showing translocation in both conditions. N2 fixation was found in coral tissues and endolithic communities with translocation of organic nitrogen. Thus, the endolithic community plays an important role in supporting the C and N metabolism of the holobiont, which may be crucial under changing environmental conditions.

Highlights

Stony corals or reef-building corals are the most important components of the coral reef ecosystems that are distributed in tropical areas of the world’s oceans
We provide a quantification of the primary production and nitrogen fixation of the endolithic community and the coral tissues in terms of organic carbon and nitrogen; we evaluated the proportion of translocated photo-assimilates under both normal and bleached conditions
Associated microorganisms in the coral holobiont show the capacity to rapidly acclimatize to environmental changes by modifying their population growth and physiological responses

Summary

Introduction

Stony corals or reef-building corals are the most important components of the coral reef ecosystems that are distributed in tropical areas of the world’s oceans. Corals evolved to adapt to oligotrophic environments by establishing symbiotic relationships with a variety of microbes forming the so-called ‘coral holobiont’ [1,2,3,4]. In addition to the well-known symbiotic algae zooxanthellae, other associated organisms play crucial roles in environmental adaptation of the coral holobiont by providing organic and inorganic carbon [5] and organic nitrogen [6,7], and protecting the coral host from pathogens [8,9,10]. The coral holobiont has the capacity to acclimatize (to some extent) to these environmental changes by rapidly altering the population of associated microbes and their functioning in a dynamic way. The ‘coral probiotic hypothesis’ was proposed to explain this concept [11]

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Conclusion