Unsilent partners: Constraining the contribution of methanogen-protist symbioses to total methane production in anoxic marine habitats

概述：人们越来越认识到地球上无处不在的微生物共生，但在大多数栖息地，我们对它们的生理学和生态学只有基本的了解。许多真核生物已经进化出与原核生物的特定关联，利用了原核生物代谢的相对多样性。就数量而言，共生体群体在其所处的环境中经常与自由生活的原核生物相匹敌或超过，因此，通过其代谢，共生体可能对其所处的生物和非生物环境产生重大影响。尽管如此，对大多数共生类群的共生功能和代谢活动的了解有限。在许多缺氧环境中，厌氧原生生物与胞内产甲烷古菌之间存在共生关系，但共生产生的甲烷（CH4）对海洋CH4循环的贡献尚未受到限制。在非海洋环境中，如稻田和湖泊，它们被发现对CH4的产生有显著贡献，这表明这些共生关系可能对这种气候活性气体的生物地球化学循环很重要。然而，尽管在对缺氧和缺氧海水中原生生物的一般调查中发现了已知的产甲烷菌的分类群，但迄今为止，它们从未被专门调查过，也没有研究量化它们在这些栖息地中对CH4产生的贡献。此外，除了它们的产甲烷功能外，对它们的共生体的生理能力一无所知。为了进一步了解这些伙伴关系的细节，并解决微生物共生体对CH4循环的贡献，该研究员将研究Saanich Inlet （SI）的甲烷生菌-原生生物共生体的生态学和生理学。Saanich Inlet是加拿大不列颠哥伦比亚省海岸外的一个季节性缺氧峡湾，其深水中积累的CH4（大于1微米）来自一个不受限制的来源。与伍兹霍尔海洋研究所（WHOI）的赞助者Virginia Edgcomb博士一起，该研究员将结合a)基于荧光激活的细胞分选调查甲烷菌宿主原生生物的丰度和多样性，并通过b)自然种群产生CH4的实验测量和c)分析总共生基因含量（即基因组学）来评估其代谢。这种观察和实验方法的有力结合将为这些共生生物的分布、生理和活动提供有价值的见解，使我们能够首次估计原生生物相关的产甲烷菌对缺氧海水中CH4产生的贡献。知识价值：缺氧和低氧海洋水柱被认为贡献了全球海洋甲烷（一种气候活性气体）排放量的50-60%。通常假定这些生境中的CH4来自于已知发生甲烷生成的下伏沉积物。在这些地区，水柱产量被认为是不重要的，因为在充满硫酸盐的海水中，自由生活的产甲烷菌无法成功地与硫酸盐还原菌竞争共同的底物（例如氢）。然而，共生产甲烷菌受到保护，免受硫酸盐还原菌的竞争，因为宿主提供了底物。因此，水柱中原生生物共生体的产甲烷作用可能占这些地区CH4收支的很大一部分。因此，所提出的努力将大大提高我们对缺氧和缺氧海洋栖息地中CH4潜在来源的理解。由于气温上升预计会导致缺氧栖息地的扩大，因此更好地了解这些地区的CH4循环将对我们理解和模拟全球海洋CH4排放变得越来越重要。更广泛的影响：拟议工作的更广泛影响包括通过公开讲座将研究结果传播给科学界和更广泛的社区，并通过在WHOI暑期学生和少数民族奖学金项目中指导1-2名本科生来支持代表性不足的群体参与海洋科学研究。此外，从年度研究金津贴中拨出的经费已编入预算，以便同当地一个非营利性海洋教育基金会建立一个关于海洋共生的教育单位。这个单元将由研究员呈现给来自新英格兰南部贫困社区的K-12学生。

Overview: There is a growing appreciation for the ubiquity of microbial symbioses on earth, yet in most habitats we have only a basic understanding of their physiology and ecology. Many eukaryotes have evolved specific associations with prokaryotes that capitalize on the relative diversity of prokaryotic metabolisms. In terms of numbers, symbiont populations often rival or exceed free-living prokaryotes in their environment, therefore, through their metabolism, symbionts may have a significant effect on their biotic and abiotic environment. Despite this, there is limited knowledge of the symbiotic function and metabolic activity of most symbiotic taxa.Symbioses between anaerobic protists and intracellular, methanogenic archaea have been found in many anoxic habitats, however the contribution of symbiont-produced methane (CH4) to the marine CH4 cycle is yet unconstrained. In non-marine environments, like rice paddies and lakes, they have been found to significantly contribute to the production of CH4, suggesting that these symbioses may be important to the biogeochemical cycling of this climate-active gas. However, though taxa known to host methanogens have been detected in general surveys of protists in oxygen (O2) -poor and anoxic marine water columns, to date, they have never been specifically surveyed, and no studies have quantified their contribution to CH4 production in these habitats. Additionally, outside of their methanogenic function, nothing is known about the physiological capacity of their symbionts. To develop an understanding of the details of these partnerships and to address the contribution of microbial symbionts to the CH4 cycle, the fellow will examine the ecology and physiology of methanogen-protist symbioses in Saanich Inlet (SI), a seasonally anoxic fjord off the coast of British Columbia, Canada that accumulates CH4 (greater than 1 micrometer) in its deep water from a yet unconstrained source. With sponsor Dr. Virginia Edgcomb at Woods Hole Oceanographic Institution (WHOI), the fellow will combine a) fluorescence-activated cell sorting based surveys of the abundance and diversity of methanogen-hosting protists, with assessment of their metabolism through b) the experimental measurement of CH4 production by natural populations and c) analysis of total symbiont gene content (i.e., genomics). This robust combination of observational and experimental approaches will provide valuable insight into distribution, physiology and activity of these symbioses, allowing the first estimation of the contribution of protist-associated methanogens to CH4 production in an anoxic marine water column. Intellectual merit: Anoxic and low-O2 marine water columns are thought to contribute 50-60% of the global oceanic emission of CH4, a climate-active gas. It is typically assumed that the CH4 in these habitats originates from the underlying sediment, where methanogenesis is known to occur. Water column production is presumed to be unimportant in these areas, as free-living methanogens cannot compete successfully with sulfate-reducing bacteria for common substrates (e.g, Hydrogen) in sulfate-replete seawater. However, symbiotic methanogens are protected from competition with sulfate-reducing bacteria since substrates are host provided. Consequently, methanogenesis by the symbionts of protists in the water column may account for a significant fraction of the CH4 budget in these areas. Thus, the proposed effort will substantially advance our understanding of the underlying source of CH4 in O2-deficient and anoxic marine habitats. Since rising temperatures are predicted to cause the expansion of oxygen-depleted habitats, better insight into CH4 cycling in these areas will become increasingly important to our understanding and modeling of global oceanic CH4 emissions. Broader Impacts: The broader impacts of the proposed effort include disseminating the findings to the scientific and wider community through public lectures, and supporting participation of underrepresented groups in ocean sciences research through mentorship of 1-2 undergraduates in the WHOI Summer Student and Minority Fellowship programs. Additionally, funds from the annual fellowship allowance have been budgeted to develop an educational unit about marine symbioses with a local non-profit marine education foundation. This unit will be presented by the fellow to K-12 students from disadvantaged communities from the southern New England area.