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

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.

Diazotrophic community and associated dinitrogen fixation within the temperate coral Oculina patagonica

“…All together, DDN may help O. patagonica to overcome C limitation, thereby possibly contributing to its high recovery potential from annual bleaching events. Sangsawang and colleagues () also observed a fourfold increased DDN assimilation into the tissue of bleached P. lutea corals, whereas assimilation by endoliths was similar in bleached and non‐bleached corals. However, they identified that part of the tissue‐assimilated DDN was fixed and translocated from the endolithic compartment, and similarly, translocation of DDN between the individual compartments may have occurred in the present study.…”

“…Thereof, the majority was detected within the host tissue (62%) and endoliths (33%), with only a minor proportion present in Symbiodinium (7%) and mucus (0%). Previous 15 N 2 tracer studies on tropical corals reported similar DDN assimilation rates into the tissue compartment of S. pistillata (0.97–3.58 ng N cm −2 h −1 ; Bednarz et al ., ) and Porites lutea (2.47 ± 0.78 ng N cm −2 h −1 ; Sangsawang et al ., ). Although two other studies on S. pistillata traced >95% of DDN within Symbiodinium and suggested this compartment as the primary site for DDN (i.e.…”

“…the end‐product of N 2 fixation) have been observed from the endolithic community (Ferrer and Szmant, ; Szmant et al ., ) and may constitute the fixed N 2 compounds released by endolithic diazotrophs. Only one previous study on the tropical coral Porites lutea differentiated between DDN assimilation into the tissue and by endoliths (Sangsawang et al ., ). DDN assimilation into the tissue was similar between P. lutea and O. patagonica , while assimilation by endoliths was 10 times lower in the former.…”

“…This can be achieved via increased heterotrophic feeding on organic C from the surrounding seawater, although the availability of organic C is rather limiting in many reefs as well as in the oligotrophic Mediterranean Sea. It has been shown in some coral species, including O. patagonica , that an endolithic algal community quickly develops in bleached corals providing an alternative photosynthetic energy source to the coral host when the supply of photoassimilates from Symbiodinium is impaired (Fine and Loya, ; Sangsawang et al ., ). Although we did not quantify the abundance of endolithic algae in bleached O. patagonica , their density possibly increased as indicated by the stable photosynthesis rates despite reduced Symbiodinium density.…”

“…Several previous 15 N 2 tracer studies assessed coral‐associated N 2 fixation within the mucus, Symbiodinium and tissue compartment of corals, but mainly focused on tropical species except for one study including Lophelia pertusa , a Symbiodinium ‐free cold‐water coral (Middelburg et al ., ; Benavides et al ., ; Bednarz et al ., ; Sangsawang et al ., ; Lesser et al ., ). Here, we report N 2 fixation activity within a temperate coral species.…”