Department Limnology of Stratified Lakes, Leibniz-Institute of Freshwater Ecology and Inland Fisheries, D-16775 Stechlin, Germany.
Proc Natl Acad Sci U S A. 2010 Jun 29;107(26):11959-64. doi: 10.1073/pnas.1000668107. Epub 2010 Jun 14.
Microorganisms and zooplankton are both important components of aquatic food webs. Although both inhabit the same environment, they are often regarded as separate functional units that are indirectly connected through nutrient cycling and trophic cascade. However, research on pathogenic and nonpathogenic bacteria has shown that direct association with zooplankton has significant influences on the bacteria's physiology and ecology. We used stratified migration columns to study vertical dispersal of hitchhiking bacteria through migrating zooplankton across a density gradient that was otherwise impenetrable for bacteria in both upward and downward directions (conveyor-belt hypothesis). The strength of our experiments is to permit quantitative estimation of transport and release of associated bacteria: vertical migration of Daphnia magna yielded an average dispersal rate of 1.3 x 10(5) x cells x Daphnia(-1) x migration cycle(-1) for the lake bacterium Brevundimonas sp. Bidirectional vertical dispersal by migrating D. magna was also shown for two other bacterial species, albeit at lower rates. The prediction that diurnally migrating zooplankton acquire different attached bacterial communities from hypolimnion and epilimnion between day and night was subsequently confirmed in our field study. In mesotrophic Lake Nehmitz, D. hyalina showed pronounced diel vertical migration along with significant diurnal changes in attached bacterial community composition. These results confirm that hitchhiking on migrating animals can be an important mechanism for rapidly relocating microorganisms, including pathogens, allowing them to access otherwise inaccessible resources.
微生物和浮游动物都是水生食物网的重要组成部分。尽管它们都生活在同一环境中，但通常被视为通过营养循环和营养级联间接连接的独立功能单元。然而，对致病性和非致病性细菌的研究表明，与浮游动物的直接关联对细菌的生理和生态有重大影响。我们使用分层迁移柱研究了 hitchhiking 细菌通过迁移的浮游动物在密度梯度中的垂直扩散，这种密度梯度在向上和向下的方向上对细菌都是不可穿透的（输送带假说）。我们实验的优势在于可以定量估计相关细菌的运输和释放：大型水蚤的垂直迁移产生了湖细菌 Brevundimonas sp. 的平均扩散率为 1.3 x 10(5) x 细胞 x Daphnia(-1) x 迁移周期(-1)。对于另外两种细菌，也显示出了由迁移的 D. magna 进行的双向垂直扩散，但扩散率较低。我们的野外研究随后证实了这样的预测，即昼夜迁移的浮游动物在白天和黑夜之间从下湖和上湖获得不同的附着细菌群落。在中营养的 Nehmitz 湖中，D. hyalina 表现出明显的昼夜垂直迁移，同时附着细菌群落组成也发生了显著的昼夜变化。这些结果证实，搭乘迁移动物的顺风车可以成为快速重新定位微生物（包括病原体）的重要机制，使它们能够利用原本无法获得的资源。
