Reintjes Greta, Arnosti Carol, Fuchs Bernhard M, Amann Rudolf
Department of Molecular Ecology, Max Planck Institute for Marine Microbiology, Bremen, Germany.
Department of Marine Sciences, University of North Carolina-Chapel Hill, Chapel Hill, NC, USA.
ISME J. 2017 Jul;11(7):1640-1650. doi: 10.1038/ismej.2017.26. Epub 2017 Mar 21.
Heterotrophic microbial communities process much of the carbon fixed by phytoplankton in the ocean, thus having a critical role in the global carbon cycle. A major fraction of the phytoplankton-derived substrates are high-molecular-weight (HMW) polysaccharides. For bacterial uptake, these substrates must initially be hydrolysed to smaller sizes by extracellular enzymes. We investigated polysaccharide hydrolysis by microbial communities during a transect of the Atlantic Ocean, and serendipitously discovered-using super-resolution structured illumination microscopy-that up to 26% of total cells showed uptake of fluorescently labelled polysaccharides (FLA-PS). Fluorescence in situ hybridisation identified these organisms as members of the bacterial phyla Bacteroidetes and Planctomycetes and the gammaproteobacterial genus Catenovulum. Simultaneous membrane staining with nile red indicated that the FLA-PS labelling occurred in the cell but not in the cytoplasm. The dynamics of FLA-PS staining was further investigated in pure culture experiments using Gramella forsetii, a marine member of Bacteroidetes. The staining patterns observed in environmental samples and pure culture tests are consistent with a 'selfish' uptake mechanisms of larger oligosaccharides (>600 Da), as demonstrated for gut Bacteroidetes. Ecologically, this alternative polysaccharide uptake mechanism secures substantial quantities of substrate in the periplasmic space, where further processing can occur without diffusive loss. Such a mechanism challenges the paradigm that hydrolysis of HMW substrates inevitably yields low-molecular-weight fragments that are available to the surrounding community and demonstrates the importance of an alternative mechanism of polysaccharide uptake in marine bacteria.
异养微生物群落处理海洋中浮游植物固定的大部分碳，因此在全球碳循环中起着关键作用。浮游植物衍生的底物的一大部分是高分子量（HMW）多糖。对于细菌摄取而言，这些底物必须首先被胞外酶水解成更小的尺寸。我们在大西洋的一个断面研究了微生物群落对多糖的水解作用，并意外地发现——使用超分辨率结构光照显微镜——高达26%的总细胞显示摄取了荧光标记的多糖（FLA-PS）。荧光原位杂交将这些生物体鉴定为拟杆菌门和浮霉菌门的细菌以及γ-变形菌纲的链环菌属成员。用尼罗红同时进行膜染色表明，FLA-PS标记发生在细胞内而非细胞质中。使用福氏革兰氏菌（一种拟杆菌门的海洋成员）进行的纯培养实验进一步研究了FLA-PS染色的动态过程。在环境样品和纯培养测试中观察到的染色模式与肠道拟杆菌所展示的较大寡糖（>600 Da）的“自私”摄取机制一致。从生态学角度来看，这种替代的多糖摄取机制在周质空间中确保了大量底物，在那里可以进一步进行处理而不会有扩散损失。这种机制挑战了高分子量底物水解必然产生可供周围群落利用的低分子量片段的范式，并证明了海洋细菌中多糖摄取替代机制的重要性。
