Do Anoxygenic Aerobic Photoheterotrophs Contribute to the Ecology of the Open Ocean?

好氧产氧光异养生物(AAPS)及其特有的色素--细菌氯化细菌a(BChla)最近在公海表层水域中被发现(Kolber，2000；Kolber，2001)。AAP约占微生物群落总数的11%以上，它们分布在真光带，具有很高的光合作用活性。已从不同的海洋盆地中分离到20多个菌株。这些生物是兼性光异养生物，代表着海洋微生物群落中一个潜在的重要但迄今未被认识的组成部分。它们在上层海洋生态中的潜在作用取决于它们对能量和碳通量的贡献，以及它们与海洋生态系统其他组成部分的相互作用。为了阐明这一作用，本项目正在研究AAPS的光合作用、碳固定和呼吸作用的机制，以确定控制光营养和异养的表达的因素，并估计不同环境条件下AAP的特定能量、碳和氧化还原的程度。AAP很容易在有机贫乏的培养基上培养。研究人员正在建立一个公海分离物的文库，并表征来自不同地理区域的AAP之间的系统发育关系。AAP利用光合作用产生三磷酸腺苷并固定二氧化碳。研究人员正在对AAPS的光合作用特性进行表征，评估光对其碳代谢的影响，并研究光合作用对溶解有机物质(DOM)结合的影响。在DOM丰富的地方，AAP主要进行异养代谢，呼吸有效的有机碳。当DOM受到限制时，AAPS合成光合色素和反应中心，将其代谢转向光养。调查人员正在评估这些转变的程度和控制因素。AAPS不能氧化水，只能依靠其他底物作为电子供体(S)。虽然AAPS中的大部分光合作用电子传递是循环的，但外部电子供体是补充光合作用碳固定或外部电子受体所失去的电子所必需的。研究人员正在调查上层海洋中存在的哪些有机/无机还原剂是AAPS的潜在电子供体，并探索AAP介导的氧化还原反应，特别是还原硫物种的氧化以及铁和氮物种的还原。光营养减少了AAPS对有机碳的依赖，降低了上层海洋中碳的再矿化速率，从而影响了新生产的程度。如果有效地减少铁和氮物种，AAPS还将影响产氧光养生物的生理。通过解决这些问题，该项目提供了关于AAPS在上层海洋生态中的潜在作用的最基本的知识。

Aerobic anoxygenic photoheterotrophs (AAPs), and their characteristic pigment, bacteriochloropyll a (BChla), were recently discovered throughout the surface waters of the open ocean (Kolber, 2000; Kolber, 2001). AAPs comprise upwards of about 11 % of the total microbial community, they are globally distributed in the euphotic zone, and display high levels of photosynthetic activity. More than twenty isolates have been obtained from different ocean basins. These organisms are facultative photoheterotrophs, and represent a potentially significant, but hitherto unrecognized component of the marine microbial community. Their potential role in the ecology of the upper ocean is determined by their contribution to the energy and carbon fluxes, and by their interactions with other components of marine ecosystems. To elucidate this role, this project is investigating mechanisms of photosynthesis, carbon fixation, and respiration in AAPs, to identify factors controlling expression of phototrophy vs. heterotrophy, and to estimate the extent of AAP-specific energy, carbon, and redox under varying environmental conditions. AAPs are readily cultivated on organic-poor media. The investigators are establishing a library of the open ocean isolates, and characterizing the phylogenetic relationship between AAPs from different geographical regions. AAPs use photosynthesis to produce ATP and to fix CO2. The investigators are characterizing the photosynthetic properties of AAPs, evaluating the effect of light on their carbon metabolism, and investigating the effect of photosynthesis on the incorporation of dissolved organic material (DOM). Where DOM is abundant, AAPs perform mostly heterotrophic metabolism, respiring the available organic carbon. When DOM is limited, AAPs synthesize photosynthetic pigments and reaction centers, shifting their metabolism toward phototrophy. The investigators are assessing the extent and the factors controlling these transitions. Unable to oxidize water, AAPs rely on other substrates to serve as electron donor(s). Although most of the photosynthetic electron transport in AAPs is cyclic, external electron donors are necessary to replenish the electrons lost to photosynthetic carbon fixation, or to external electron acceptors. The researchers are investigating which of the organic/inorganic reductants present in the upper ocean serve as potential electron donors to AAPs, and explore the AAP-mediated redox reactions, with particular emphasis on oxidation of the reduced sulfur species, and reduction of iron and nitrogen species. Phototrophy alleviates AAPs' dependence on the organic carbon, reducing the rates of carbon remineralization in the upper ocean, thus affecting the extent of new production. If effective in reducing iron and nitrogen species, AAPs will also affect the physiology of oxygenic phototrophs. By addressing these questions, this project is providing the most basic knowledge regarding the potential role of AAPs in the upper ocean ecology.