Testing the sponge-loop hypothesis for Caribbean coral reefs

海绵是一种海底动物，在加勒比海珊瑚礁中占据主导地位，因为造礁珊瑚已经减少了几十年。海绵通过过滤大量的海水来进食，这为有机物质的回收提供了一个机制。一种新的理论被称为“海绵圈假说”，这可能是多年来海洋生态学中最重要的新概念，因为它试图解释达尔文的Paradise：高产和多样化的珊瑚礁是如何在沙漠般的热带海洋中生长的？海绵环假说提出，珊瑚礁上的海绵吸收了海藻和珊瑚释放的大量溶解有机碳（如碳水化合物分子），并将其以活细胞和死细胞或其他细胞碎片的形式返回珊瑚礁。该项目将使用一套严格的技术，在加勒比海珊瑚礁上最大和最常见的十种海绵上实地测试海绵环假设。对于每个物种，将测量颗粒和溶解的有机碳对海绵营养的贡献，以及从海绵流出的海水中细胞颗粒的产生。对于选定的海绵物种，将通过实验提高进入海绵的溶解有机碳的浓度，以确定海绵吸收这种潜在食物来源的能力，并衡量其对细胞颗粒产生的影响。该项目将为博士后，研究生和本科生提供STEM教育和培训，并以易于访问的教育视频的形式进行公共宣传。此外，该项目对于了解珊瑚礁的碳循环非常重要，气候变化和海洋酸化的影响可能会使竞争平衡向非造礁生物（如海绵）倾斜。碳从水柱到底栖生物的循环是海洋生态系统功能的核心;对珊瑚礁来说，这一过程始于海藻和珊瑚共生体的光合作用，然后以溶解有机碳的形式释放出相当大一部分固定碳，这些碳可能会随水流和潮汐流失。但是，如果海绵具有巨大的水过滤能力，可以将DOC从水柱返回到珊瑚礁，那么它将成为碳循环的一个主要来源。“海绵环假说”有可能改变我们对珊瑚礁碳循环的理解。基于对巨型桶状海绵研究的初步数据，该项目将调查10种常见桶状、瓶状和管状形成物种的海绵环假设的三个组成部分中的每一个，这些物种跨越了与微生物共生体的一系列关联，从海绵组织中的高微生物丰度（HMA）到低微生物丰度（LMA）。具体而言，实验方法将包括InEx技术（通过海绵之前和之后立即对海水进行比较采样），测速仪和流式细胞仪，以确定每个物种是否消耗DOC并以细胞碎屑的形式产生颗粒有机碳（POC）。然后，对于消耗DOC的物种，将使用相同的技术在操作实验中，增加DOC的量从三个类别（不稳定，半不稳定和耐火），以确定海绵消耗的DOC的类型。除了测试海绵环假说，该项目将使用分子技术来研究HMA和LMA海绵物种之间的差异，目标是可能负责DOC吸收的微生物共生体。

Sponges are bottom-dwelling animals that dominate Caribbean reefs now that reef-building corals have been declining for decades. Sponges feed by filtering huge volumes of seawater, providing a mechanism for recycling organic material back to the reef. A new theory has been proposed called the "sponge-loop hypothesis" that is potentially the most important new concept in marine ecology in many years, because it seeks to explain Darwin's Paradox: how do highly productive and diverse coral reefs grow in desert-like tropical seas? The sponge loop hypothesis proposes that sponges on coral reefs absorb the large quantities of dissolved organic carbon (molecules such as carbohydrates) that are released by seaweeds and corals and return it to the reef as particles in the form of living and dead cells, or other cellular debris. This project will use a rigorous set of techniques to test the sponge-loop hypothesis in the field on ten of the largest and most common sponges on Caribbean reefs. For each species, the contributions of particles and dissolved organic carbon to sponge nutrition will be measured, as well as the production of cellular particles in the seawater flowing out of the sponge. For selected sponge species, the concentration of dissolved organic carbon entering the sponge will be experimentally enhanced to determine the capacity of the sponge to absorb this potential food source, and to gauge its effect on the production of cellular particles. This project will provide STEM education and training for postdoctoral, graduate and undergraduate students and public outreach in the form of easily accessible educational videos. Further, this project is important for understanding the carbon cycle on coral reefs where the effects of climate change and ocean acidification may be tipping the competitive balance toward non-reef-building organisms, such as sponges. The cycling of carbon from the water-column to the benthos is central to marine ecosystem function; for coral reefs, this process begins with photosynthesis by seaweeds and coral symbionts, which then exude a substantial portion of fixed carbon as dissolved organic carbon (DOC) that may be lost to currents and tides. But if sponges, with their enormous water filtering capacity, can return DOC from the water column to the reef, it would represent a major unrecognized source of carbon cycling. The "sponge-loop hypothesis" has the potential to transform our understanding of carbon cycling on coral reefs. Building on preliminary data from studies of the giant barrel sponge, this project will investigate each of the three components of the sponge-loop hypothesis for ten common barrel, vase and tube-forming species that span a range of associations with microbial symbionts, from high microbial abundance (HMA) to low microbial abundance (LMA) in the sponge tissue. Specifically, the experimental approach will include InEx techniques (comparative sampling of seawater immediately before and after passage through the sponge), velocimetry, and flow cytometry to determine whether each species consumes DOC and produces particulate organic carbon (POC) in the form of cellular detritus. Then, for species that consume DOC, the same techniques will be used in manipulative experiments that augment the amount of DOC from three categories (labile, semi-labile and refractory) to determine the types of DOC consumed by sponges. In addition to testing the sponge-loop hypothesis, this project will use molecular techniques to investigate the differences among HMA and LMA sponge species, targeting the microbial symbionts that may be responsible for DOC uptake.