Bacterial diversity in the South Adriatic Sea during winter of 2016

Bacteria represent the most important microorganisms in the world oceans comprising up to 75% of the total biomass at surface and playing fundamental processes for biogeochemistry. Along the water column, they are useful markers of the trophic state of water masses, therefore representing useful ecological indicators in the surveyed areas. In this study in the southern Adriatic Sea bacteria were quantified by flow cytometry and their diversity assessed by high-throughput sequencing (HTS). In addition, the most represented bacterial groups were quantified by qPCR. The samples were collected from the surface to the seabed, at a total of 16 different depths at four stations in the southern Adriatic Sea, during the late winter BIOTA (BIO-Tracing Adriatic water masses) cruise conducted in 2016. The investigated area showed an unusual circulation characterized by a mixed layer down to 200 m, which differed from the usual winter convection conditions, typical of middle-altitude ecosystems and important for the seasonal picoplankton dynamics in the South Adriatic Sea. Heterotrophic bacteria were separated into HNA (relative High Nucleic Acid content) and LNA (Low Nucleic Acid content) subpopulations with abundances up to 1.8 x 105 and 8.8 x 105 cells/mL, respectively. HNA populations dominated at offshore stations reaching their maximum at depths below the euphotic zone. The bacterial community was dominated by Alphaproteobacteria accounting for > 40% of the total, represented by the SAR11 clade (90.84%), followed by Marinimicrobia (18% of the total), mainly represented by clade SAR406 (8.44%). However, distinctive bacterial groups were found in euphotic layer (Bacteroidetes and Actinobacteria) and aphotic layer samples (Deltaproteobacteria, Marinimicrobia, Chloroflexi, Acidobacteria and Planctomycetes). qPCR assays confirmed HTC results with abundances of targeted bacterial groups found to vary between different sampling points and sampling depths and values ranging from 8.7 x 103 to 9.13 x 105 copy number/mL for Alphaproteobacteria, from 1.44 x 104 to 5.11 x 105 copy number/mL for Gammaproteobacteria and from 1.8 x 103 to 2.4 x 105 copy number/mL for Bacteriodetes. For all targeted taxa higher abundances were found in the euphotic than in the aphotic zone. Introducing of different methodological approaches to the study allowed us to get better insight into the structure of the bacterial assemblages in the South Adriatic.细菌是全球海洋中最重要的微生物类群，在表层水体中可占总生物量的75%，并在生物地球化学循环中发挥核心作用。在整个水柱中，细菌可作为水团营养状态的有效标志物，因此是研究区域内极具价值的生态指示物。本研究针对亚得里亚海南部海域的细菌开展研究：通过流式细胞术（flow cytometry）对细菌进行定量，通过高通量测序（high-throughput sequencing, HTS）分析其多样性；此外，通过定量聚合酶链式反应（quantitative polymerase chain reaction, qPCR）对丰度最高的细菌类群进行定量。本次采样依托2016年冬末开展的BIOTA（亚得里亚海水团生物追踪，BIO-Tracing Adriatic water masses）科考航次，在亚得里亚海南部的4个站位采集了从表层到海底共16个不同深度的海水样品。研究区域呈现出异常的环流特征：混合层深度可达200米，与地中海中等深度海域典型的冬季对流条件存在显著差异，而该对流条件对亚得里亚海南部海域的季节性超微型浮游生物（picoplankton）动态至关重要。异养细菌可根据核酸含量分为高核酸（High Nucleic Acid, HNA）与低核酸（Low Nucleic Acid, LNA）两个亚群，其丰度上限分别可达1.8×10^5 个/mL与8.8×10^5 个/mL。HNA亚群在远海站位占据优势，其丰度峰值出现在真光层以下的深度。细菌群落以α-变形菌门（Alphaproteobacteria）为主，占总丰度的40%以上，其优势类群为SAR11演化支（占该门的90.84%）；其次为Marinimicrobia菌门，占总丰度的18%，其中以SAR406演化支为主要代表（占该门的8.44%）。不过，真光层与无光层样品中分别存在特征性细菌类群：真光层以拟杆菌门（Bacteroidetes）和放线菌门（Actinobacteria）为代表，无光层则以δ-变形菌门（Deltaproteobacteria）、Marinimicrobia菌门、绿弯菌门（Chloroflexi）、酸杆菌门（Acidobacteria）和浮霉菌门（Planctomycetes）为代表。定量聚合酶链式反应（qPCR）结果验证了高通量测序（HTS）的分析结果（原文此处疑似笔误，原写为HTC），目标细菌类群的丰度在不同采样站位与采样深度间存在显著差异：α-变形菌门的丰度范围为8.7×10^3 至9.13×10^5 拷贝数/mL，γ-变形菌门（Gammaproteobacteria）为1.44×10^4 至5.11×10^5 拷贝数/mL，拟杆菌门为1.8×10^3 至2.4×10^5 拷贝数/mL。所有目标类群的丰度均在真光层显著高于无光层。本研究采用多种方法学手段，得以更深入地解析亚得里亚海南部海域细菌群落的结构组成。