Tittel Jörg, Bissinger Vera, Zippel Barbara, Gaedke Ursula, Bell Elanor, Lorke Andreas, Kamjunke Norbert
Department of Ecology and Ecosystem Modeling, University of Potsdam, Maulbeerallee 2, D-14469 Potsdam, Germany.
Proc Natl Acad Sci U S A. 2003 Oct 28;100(22):12776-81. doi: 10.1073/pnas.2130696100. Epub 2003 Oct 20.
The majority of organisms can be grouped into those relying solely on photosynthesis (phototrophy) or those relying solely on the assimilation of organic substances (heterotrophy) to meet their requirements for energy and carbon. However, a special life history trait exists in which organisms combine both phototrophy and heterotrophy. Such "mixotrophy" is a widespread phenomenon in aquatic habitats and is observed in many protozoan and metazoan organisms. The strategy requires investment in both photosynthetic and heterotrophic cellular apparatus, and the benefits must outweigh these costs. In accordance with mechanistic resource competition theory, laboratory experiments revealed that pigmented mixotrophs combined light, mineral nutrients, and prey as substitutable resources. Thereby, they reduced prey abundance below the critical food concentration of competing specialist grazers [Rothhaupt, K. O. (1996) Ecology 77, 716-724]. Here, we demonstrate the important consequences of this strategy for an aquatic community. In the illuminated surface strata of a lake, mixotrophs reduced prey abundance steeply. The data suggest that, as a consequence, grazers from higher trophic levels, consuming both the mixotrophs and their prey, could not persist. Thus, the mixotrophs escaped from competition with and losses to higher grazers. Furthermore, the mixotrophs structured prey abundance along the vertical light gradient, creating low densities near the surface and a pronounced maximum of their algal prey at depth. Such deep algal accumulations are typical features of nutrient-poor aquatic habitats, previously explained by resource availability. We hypothesize instead that the mixotrophic grazing strategy is responsible for deep algal accumulations in many aquatic environments.
大多数生物可分为仅依靠光合作用（光养）的生物或仅依靠有机物质同化作用（异养）来满足其能量和碳需求的生物。然而，存在一种特殊的生活史特征，即生物同时结合了光养和异养。这种“混合营养”在水生生境中是一种普遍现象，在许多原生动物和后生动物中都有观察到。这种策略需要在光合和异养细胞机制上都进行投入，且其带来的益处必须超过这些成本。根据机械资源竞争理论，实验室实验表明，有色素的混合营养生物将光、矿物质营养和猎物作为可替代资源。由此，它们将猎物丰度降低到了竞争性专性食草动物的临界食物浓度以下[罗斯豪普特，K. O.（1996年）《生态学》77卷，第716 - 724页]。在此，我们证明了这种策略对水生群落的重要影响。在湖泊有光照的表层，混合营养生物急剧降低了猎物丰度。数据表明，结果是，来自较高营养级的食草动物，由于既捕食混合营养生物又捕食其猎物，无法持续生存。因此，混合营养生物避免了与较高食草动物的竞争以及被其捕食。此外，混合营养生物沿着垂直光梯度构建猎物丰度，在水面附近形成低密度，而在深处其藻类猎物密度显著最高。这种深层藻类聚集是贫营养水生生境的典型特征，以前是用资源可利用性来解释的。相反，我们假设混合营养的捕食策略是许多水生环境中深层藻类聚集的原因。
