Institut de Ciències del Mar (ICM-CSIC), Barcelona, Spain.
Institut de Ciència i Tecnologia Ambientals (ICTA-UAB), Universitat Autònoma de Barcelona, Bellaterra, Spain.
Mol Ecol. 2022 Nov;31(22):5745-5764. doi: 10.1111/mec.16695. Epub 2022 Oct 4.
Subterranean estuaries are biogeochemically active coastal sites resulting from the underground mixing of fresh aquifer groundwater and seawater. In these systems, microbial activity can largely transform the chemical elements that may reach the sea through submarine groundwater discharge (SGD), but little is known about the microorganisms thriving in these land-sea transition zones. We present the first spatially-resolved characterization of the bacterial assemblages along a coastal aquifer in the NW Mediterranean, considering the entire subsurface salinity gradient. Combining bulk heterotrophic activity measurements, flow cytometry, microscopy and 16S rRNA gene sequencing we find large variations in prokaryotic abundances, cell size, activity and diversity at both the horizontal and vertical scales that reflect the pronounced physicochemical gradients. The parts of the transect most influenced by freshwater were characterized by smaller cells and lower prokaryotic abundances and heterotrophic production, but some activity hotspots were found at deep low-oxygen saline groundwater sites enriched in nitrite and ammonium. Diverse, heterogeneous and highly endemic communities dominated by Proteobacteria, Patescibacteria, Desulfobacterota and Bacteroidota were observed throughout the aquifer, pointing to clearly differentiated prokaryotic niches across these transition zones and little microbial connectivity between groundwater and Mediterranean seawater habitats. Finally, experimental manipulations unveiled large increases in community heterotrophic activity driven by fast growth of some rare and site-specific groundwater Proteobacteria. Our results indicate that prokaryotic communities within subterranean estuaries are highly heterogeneous in terms of biomass, activity and diversity, suggesting that their role in transforming nutrients will also vary spatially within these terrestrial-marine transition zones.
地下河口是由于地下淡水含水层地下水与海水混合而形成的具有生物地球化学活性的沿海地点。在这些系统中，微生物活动可以极大地改变可能通过海底地下水排放（SGD）到达海洋的化学元素，但对于在这些陆海过渡区中茁壮成长的微生物知之甚少。我们首次对西北地中海沿海含水层中的细菌组合进行了空间分辨率的特征描述，考虑了整个地下盐度梯度。结合异养活性测量、流式细胞术、显微镜和 16S rRNA 基因测序，我们发现无论是在水平还是垂直尺度上，原核生物丰度、细胞大小、活性和多样性都存在很大的变化，这反映了明显的物理化学梯度。受淡水影响最大的部分具有较小的细胞和较低的原核生物丰度和异养生产力，但在富含亚硝酸盐和铵盐的深层低氧咸地下水地点发现了一些活性热点。整个含水层中观察到的是由变形菌门、巴氏杆菌门、脱硫杆菌门和拟杆菌门为主的多样、异质和高度特有群落，这表明在这些过渡区中，原核生物的生态位明显分化，地下水和地中海海水栖息地之间的微生物连通性很小。最后，实验操作揭示了群落异养活性的大幅增加，这是由一些稀有和特定于地点的地下水变形菌的快速生长驱动的。我们的结果表明，地下河口的原核生物群落在生物量、活性和多样性方面具有高度的异质性，这表明它们在转化营养物质方面的作用也将在这些陆地-海洋过渡区中具有空间变异性。
