Biogeography of pelagic bacterioplankton across an antagonistic temperature-salinity gradient in the Red Sea

The Red Sea is a unique marine ecosystem with contrasting gradients of temperature and salinity along its north to south axis. It is an extremely oligotrophic environment that is characterized by perpetual year-round water column stratification, high annual solar irradiation, and negligible riverine and precipitation inputs. In this study, we investigated whether the contemporary environmental conditions shape community assemblages by pyrosequencing 16S rRNA genes of bacteria in surface water samples collected from the northeastern half of this water body. A combined total of 1,855 operational taxonomic units (OTUs) were recovered as free-living and particle-attached bacterioplankton. Here, a few major OTUs affiliated with Cyanobacteria and Proteobacteria accounted for ~93% of all sequences whereas a tail of “rare” OTUs represented most of the diversity. OTUs allied to Surface 1a/b SAR11 clades and Prochlorococcus related to the high light-adapted (HL2) ecotype were the most widespread and predominant sequence types. Interestingly, the frequency of taxa that are typically found in the upper mesopelagic zone was significantly elevated in the northern transects compared to those in the central, presumably as a direct effect of deep convective mixing in the Gulf of Aqaba and waters exchange with the northern Red. Although temperature was the best predictor of species richness across all major lineages, both spatial and environmental distances correlated strongly with genetic distances. Our results suggest that the bacterial diversity of the Red Sea is as high as in other tropical seas and provide evidence for fundamental differences in the biogeography of pelagic communities between the northern and central regions.

红海乃一独特的海洋生态系统，其自北向南的轴线上温度和盐度梯度迥异。该环境极为寡营养，全年水柱分层恒定，年太阳辐射强度极高，河流径流和降水输入微乎其微。本研究通过焦测序法对收集自该水域东北半部的表层水样中细菌的16S rRNA基因进行了分析，以探究当代环境条件是否塑造了群落组成。共恢复1,855个操作分类单元（OTUs），包括自由生活的和颗粒附着的细菌浮游生物。其中，与蓝藻和变形菌有关的一些主要OTUs占所有序列的约93%，而“稀有”OTUs的尾部代表了大部分多样性。与表面1a/b SAR11群系和与高光适应（HL2）生态型相关的Prochlorococcus相关的OTUs分布最广，序列类型最为普遍。有趣的是，与通常在中上层中层水带发现的类群相比，北部横断面中的这些类群的频率显著提高，这可能是阿卡巴湾深层对流混合和与北部红海水域交换的直接效应。尽管温度是预测所有主要谱系物种丰富度的最佳指标，但空间和环境距离与遗传距离的相关性极强。我们的结果表明，红海的细菌多样性与其他热带海域相当，并为北部和中部区域浮游群落生物地理学的基本差异提供了证据。