ABSTRACT: Single-celled, eukaryotic microorganisms, known as protists, are responsible for 2 important, yet opposing, metabolic activities within aquatic food webs. They are major primary producers and highly active predators in marine and fresh water systems. While genomics has accelerated in recent years for this taxonomically diverse group, our understanding of the metabolic capabilities of most protists remains limited. It is also poorly understood how protist trophic mode is affected by biotic and abiotic factors, and therefore it is difficult to predict how events such as global climate change will affect the balance between autotrophic and heterotrophic activities in protist communities. To address open questions regarding how protist metabolic versatility is influenced by their environment, we characterized the potential for carbon fixation versus organic carbon degradation using enzymatic assays (RubisCO and β-D-glucosaminidase, respectively) within the water columns of ice-covered lakes in McMurdo Dry Valleys (MDV), Antarctica. Steep physical and chemical gradients in the water columns, microorganism domination and minimal allochthonous inputs makes the MDV lakes uniquely suited to investigate environment-microbe interactions. Spatial trends in RubisCO and β-D-glucosaminidase activities were lake-specific and vertically stratified within the water columns. Moreover, bottom-up drivers controlling the activity of C-fixation vs. organic C-degradation among the MDV protist communities were distinct between the upper photic vs. the deep, aphotic zones. We conclude that differential controls over major C-cycling enzymes have important implications on the influence of environmental change on the carbon and nutrient cycles in the MDV lakes.

中文翻译：

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化学分层南极湖中原生动物群落自养和异养的驱动因素

摘要：单细胞的真核微生物，被称为原生生物，是水生食物网中2种重要但相互对立的代谢活动的原因。它们是海洋和淡水系统的主要主要生产者和高度活跃的捕食者。尽管近年来对于这个分类学上多样化的群体而言，基因组学已在加速发展，但我们对大多数生物的代谢能力的理解仍然有限。人们对生物和非生物因素如何影响原生生物的营养模式也知之甚少，因此很难预测诸如全球气候变化之类的事件将如何影响原生生物群落的自养和异养活动之间的平衡。要解决有关其环境如何影响原生代谢多功能性的公开问题，我们利用南极麦克默多干旱谷（MDV）冰雪覆盖的湖泊水柱内的酶法测定法（分别为RubisCO和β-D-氨基葡萄糖苷酶）表征了碳固定与有机碳降解的潜力。水柱中陡峭的物理和化学梯度，微生物的控制以及最小的异源输入使得MDV湖特别适合研究环境与微生物的相互作用。RubisCO和β-D-氨基葡萄糖苷酶活性的空间趋势是湖泊特有的，并且在水柱内垂直分层。此外，在上层光生区与深层光生区之间，控制MDV有机体群落中C固定与有机C降解活性的自下而上的驱动器是截然不同的。