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

有氧无氧光异养生物（AAPs）及其特征色素细菌叶绿素a （BChla）最近在开放海洋的表层水域中被发现（Kolber, 2000; Kolber, 2001）。AAPs占总微生物群落的11%以上，它们在全球范围内分布于光带，并表现出高水平的光合活性。从不同的海洋盆地获得了20多个分离株。这些生物是兼性光异养生物，代表了海洋微生物群落中潜在的重要组成部分，但迄今为止尚未被认识。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aap的光合作用、碳固定和呼吸机制，以确定控制光养与异养表达的因素，并估计不同环境条件下aap特异性能量、碳和氧化还原的程度。AAPs很容易在缺乏有机物的培养基上培养。研究人员正在建立一个开放海洋分离物库，并确定不同地理区域aap之间的系统发育关系。aap利用光合作用产生ATP并固定二氧化碳。研究人员正在表征AAPs的光合特性，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）掺入的影响。在DOM丰富的地方，aap主要进行异养代谢，呼吸可用的有机碳。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。研究人员正在评估控制这些转变的程度和因素。由于不能氧化水，AAPs依靠其他底物作为电子给体。尽管AAPs中的大多数光合电子传递是循环的，但需要外部电子供体来补充光合作用碳固定或外部电子受体失去的电子。研究人员正在研究上层海洋中存在的哪些有机/无机还原剂可以作为aap的潜在电子供体，并探索aap介导的氧化还原反应，特别强调还原硫的氧化，以及铁和氮的还原。光养减轻了AAPs对有机碳的依赖，降低了上层海洋中碳再矿化的速率，从而影响了新生产的程度。AAPs除能有效还原铁和氮外，还会影响富氧光养生物的生理机能。通过解决这些问题，该项目提供了有关AAPs在上层海洋生态中潜在作用的最基本知识。

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.