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）利用之间的耦合关系。先前对与近乎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，微生物食物网从类似于开放海洋的微生物食物网（原绿球菌和sar11样细菌）转变为由“超异营养菌”主导的群落，与大肠杆菌、葡萄球菌、链球菌等已知病原体最密切相关。这种转变与珊瑚覆盖面积的减少和珊瑚病患病率的增加有关。研究人员之前的研究还表明，与近海水域的测量值（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，他们观察到的DOC测量值低至30 - 40微米，这一值与在太平洋深处观测到的浓度相似。在退化的珊瑚礁上观察到的低DOC测量值与大量的大型藻类分离，人们可能会期望通过向水柱释放光合作用来提高DOC水平。为了解释这个明显的悖论，研究人员提出了以下假设：珊瑚礁退化，以及相关的从珊瑚到藻类优势的相移，导致水柱中藻类渗出物水平升高，这使得微生物群落能够利用半不稳定的DOC。这导致：1)更高的微生物数量和生物量；2)群落向“超级异养生物”转变，这是潜在的珊瑚病原体；3) DOC常备存量减少。该项目将代表珊瑚礁微生物食物网的第一个全球宏基因组研究。序列数据将得到微生物学（例如直接计数和有限养殖）、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的支持。在本研究完成后，pi将：1)利用宏基因组学对全球100个珊瑚礁的微生物群落进行表征；2)确定不同来源的DOM（近海表层DOM、近岸表层DOM、珊瑚间质DOM和草坪藻衍生DOM）在化学特性、微生物群落结构和影响珊瑚礁DOC再矿化方面的差异；3)确定与珊瑚礁相关的藻类是否产生光合分泌物，使微生物群落能够降解半不稳定的DOC；4)珊瑚状况（即覆盖和疾病流行）与DOM的数量和质量以及微生物生物量、分类和代谢的相关性；5)表征了Moorea和Kiritimati附近礁台上水的停留时间。更广泛的影响：在最广泛的层面上，这一跨学科项目将确定珊瑚礁退化过程中化学、微生物和生态过程之间的联系，并将通过确定珊瑚死亡的可能机制为管理和保护提供信息。在公众宣传方面，将扩大“珊瑚礁多媒体计划”，展示有关珊瑚礁不同方面的迷你纪录片和短片。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。该项目将包括24名学生，为期三年，并将上传50多个新视频到网站上。罗沃和史密斯实验室还赞助了一个每月珊瑚讨论小组，这是一个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提供鼓励和建议，以推进他们的项目，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.