Surviving in a marine desert: the sponge loop retains resources within coral reefs

Ever since Darwin’s early descriptions of coral reefs, scientists have debated how one of the world’s most productive and diverse ecosystems can thrive in the marine equivalent of a desert. It is an enigma how the flux of dissolved organic matter (DOM), the largest resource produced on reefs, is transferred to higher trophic levels. Here we show that sponges make DOM available to fauna by rapidly expelling filter cells as detritus that is subsequently consumed by reef fauna. This “sponge loop” was confirmed in aquarium and in situ food web experiments, using 13C- and 15N-enriched DOM. The DOM-sponge-fauna pathway explains why biological hot spots such as coral reefs persist in oligotrophic seas—the reef’s paradox—and has implications for reef ecosystem functioning and conservation strategies.

自达尔文早年对珊瑚礁的记述以来，科学界始终争论不休：为何全球生产力最高、生物多样性最丰富的生态系统之一，能够在堪比海洋荒漠的环境中存续繁衍。珊瑚礁生成的最大资源——溶解有机物（Dissolved Organic Matter, DOM）——其向更高营养级的传递机制，长期以来都是一道未解的谜题。本研究证实，海绵可通过快速将滤食细胞以碎屑形式排出的方式，使DOM得以被动物群落利用，而这些碎屑随后会被珊瑚礁动物摄食。这一‘海绵循环（sponge loop）’机制，借助13C与15N同位素标记的DOM，在水族箱与原位食物网实验中均得到了验证。这条DOM-海绵-动物通路，解释了珊瑚礁等生物热点为何能够在贫营养海域存续——这便是著名的珊瑚礁悖论——同时也为珊瑚礁生态系统的功能维持与保护策略提供了重要理论参考。