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

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 1) 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 2) diazotrophs are components of the endolithic community; therefore, N₂ fixation and translocation of organic nitrogen may occur. We tested these hypotheses in incubation of <i>Porites lutea</i> with ¹³C and ¹⁵N tracers to measure primary production and N₂ fixation in coral tissues and endoliths. Assimilation of 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. <br> <br>

珊瑚通过与各类微生物（虫黄藻（zooxanthellae）、丝状藻类、蓝细菌、细菌、古菌、真菌以及病毒）建立共生关系演化而来，形成了珊瑚全共生体（coral holobiont）。其中，石内群落（endolithic community）是目前研究程度最低的类群。此前学界普遍认为其核心功能是将光合同化产物转运至珊瑚宿主体内，这一过程在珊瑚白化期间尤为关键。本研究提出两项假说：其一，石内藻类可通过调整自身色素配比，在健康与白化珊瑚中维持相近的初级生产速率，因此两种环境下的有机质生产与转运过程可能保持一致；其二，固氮微生物（diazotrophs）是石内群落的组成部分，因此该群落可发生氮气固定与有机氮转运过程。为验证上述假说，本研究以澄黄滨珊瑚（*Porites lutea*）为实验材料，通过向其施加碳13（¹³C）与氮15（¹⁵N）示踪剂开展培养实验，以此测定珊瑚组织与石内群落的初级生产速率及氮气固定能力。当示踪剂被注入石内带后，研究人员在健康与白化珊瑚中均检测到了碳13原子百分比（%）的同化现象，证实两种环境下均存在有机质转运过程。同时，研究团队在珊瑚组织与石内群落中均发现了氮气固定现象，并伴随有机氮的转运过程。综上，石内群落对珊瑚全共生体的碳、氮代谢具有重要支撑作用，这一功能在环境动态变化的背景下可能至关重要。