Collaborative Research: ETBC: The coupling between DOM, algae, and microbes on coral reef platforms

本项目将研究初级生产者与珊瑚礁上海洋异养微生物对溶解有机物（DOM）的利用之间的耦合。先前对与接近原始和退化的珊瑚礁相关的微生物群落的宏基因组研究表明，从类似于开放海洋的微生物食物网（原绿球藻属）和SAR 11样细菌）转化为以“超级异养生物”为主的群落，与已知的病原体如E.大肠杆菌，葡萄球菌属，链球菌属，肠杆菌属和弧菌属。这一转变与珊瑚覆盖面积减少和珊瑚病流行率上升有关。研究人员先前的研究还表明，与近海沃茨的测量结果（60-80微米）相比，珊瑚礁平台上的溶解有机碳（DOC）浓度较低。在退化的珊瑚礁上，他们观察到DOC测量值低至30 - 40微米，这一数值与在太平洋深海观察到的浓度相似。低DOC测量退化的珊瑚礁的观察是脱钩的大型藻类的高丰度，人们可能会期望通过释放到水柱中的光合作用提高DOC的水平。为了解释这一明显的悖论，研究人员提出了以下假设：珊瑚礁退化以及从珊瑚到藻类占主导地位的相关相移导致水柱中藻类分泌物水平升高，这使得微生物群落能够利用半不稳定DOC的常备库存。这导致：1）更高的微生物数量和生物量; 2）社区转向“超级异养”，这是潜在的珊瑚病原体;和3）DOC的现存量较低。该项目将代表第一个全球性的珊瑚礁微生物食物网宏基因组研究。序列数据将得到微生物（例如，直接计数和有限的养殖），水化学（DOM表征），底栖生物覆盖（多样性，覆盖和珊瑚疾病流行），鱼类丰度和物理海洋学元数据。在这项研究完成后，PI将：1）表征微生物群落（使用宏基因组学）从世界各地的100个珊瑚礁; 2）确定DOM的不同来源（近岸表层DOM、近岸表层DOM、珊瑚间隙DOM和藻源DOM）的化学特征不同，有助于构建微生物群落，影响珊瑚礁DOC的生物转化; 3）确定与珊瑚礁相关的藻类是否产生允许微生物群落降解半不稳定DOC的光合分泌物; 4）相关的珊瑚状况（即，覆盖率和疾病流行率）的数量和质量的DOM，以及微生物生物量，分类和代谢，和; 5）其特征在于水的停留时间的珊瑚礁平台周围Moorea和Kiritimati。更广泛的影响：在最广泛的层面上，这一跨学科项目将查明珊瑚礁退化中化学、微生物和生态过程之间的联系，并通过查明珊瑚死亡的可能机制，为管理和养护提供信息。在公众宣传方面，将扩大珊瑚礁多媒体项目，该项目播放有关珊瑚礁不同方面的小型纪录片和录像短片。来自SDSU和SIO的本科生和研究生将使用未经编辑的珊瑚礁视频制作原创电影。该计划将包括24名学生在三年的赠款，并将上传50+新的视频到网站。Rohwer和Smith实验室还赞助每月一次的珊瑚讨论小组，这是博士和博士后科学家展示他们研究的论坛。这个珊瑚俱乐部为研究人员推进他们的项目提供鼓励和建议，并在圣地亚哥进行珊瑚礁研究的两个主要机构之间建立了密切的联系。

This project will investigate the coupling between primary producers and the utilization of dissolved organic matter (DOM) by marine heterotrophic microbes on coral reefs. Previous metagenomic studies of the microbial communities associated with near-pristine and degraded coral reefs demonstrated a shift from a microbial food web similar to the open ocean (Prochlorococcus spp. and SAR11-like bacteria) to a community dominated by "super-heterotrophs", most closely related to known pathogens like E. coli, Staphylococcus spp., Streptococcus spp., Enterobacter spp. and Vibrio spp. This shift is associated with a decline in coral cover and an increase in coral disease prevalence. Previous research by the investigators has also shown that dissolved organic carbon (DOC) concentrations are lower on coral reef platforms compared to measurements of offshore waters (60-80 micro M). On degraded reefs, they have observed DOC measurements as low as 30 - 40 micro M, a value similar to concentrations observed in the deep Pacific Ocean. The observation of low DOC measurements on degraded reefs is decoupled from the high abundance of macroalgae, which one might expect would raise levels of DOC through the release of photosynthate into the water column. To explain this apparent paradox, the investigators suggest the following hypothesis: Reef degradation, and the associated phase-shifts from coral to algal dominance, leads to elevated levels of algal exudates in the water column, which allows the microbial community to utilize the standing stock of semi-labile DOC. This results in: 1) higher microbial numbers and biomass; 2) a community shift to 'super-heterotrophs', which are potential coral pathogens; and 3) lower standing stocks of DOC. This project will represent the first global, metagenomic study of coral reef microbial food webs. The sequence data will be supported by microbiological (e.g., direct counts and limited culturing), water chemistry (DOM characterization), benthic cover (diversity, cover, and coral disease prevalence), fish abundance, and physical oceanographic metadata. At the completion of this research, the PIs will have: 1) characterized the microbial communities (using metagenomics) from 100 coral reefs around the world; 2) determined how different sources of DOM (surface offshore DOM, surface inshore DOM, interstitial coral DOM and, turf algal derived DOM) vary in chemical characterization, help to structure microbial communities, and influence DOC remineralization on coral reefs; 3) determined whether reef-associated algae produce photosynthetic exudates that allow the microbial communities to degrade the semi-labile DOC; 4) correlated coral condition (i.e., cover and disease prevalence) to the quantity and quality of DOM, as well as microbial biomass, taxonomy and metabolisms, and; 5) characterized the residence time of water on the reef platform around Moorea and Kiritimati. Broader impacts: At the broadest level, this cross-disciplinary project will identify the linkages between chemical, microbial and ecological processes in coral reef degradation and will inform management and conservation by identifying a likely mechanism of coral mortality. At the level of public outreach, the Coral Reef Multimedia Project, which displays mini-documentaries and short video clips about different aspects of coral reefs, will be expanded. Undergraduate and graduate students from SDSU and SIO will create original movies using the unedited reef video. This program will include 24 students over the three years of the grant and will upload 50+ new videos to the website. The Rohwer and Smith labs also sponsor a monthly coral discussion group, which is a forum for PhD and post-doctoral scientists to present their research. This Coral Club provides encouragement and suggestions for the researchers to move forward with their projects and has established a close link between the two major institutions conducting reef research in San Diego.