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.红海是一处独特的海洋生态系统，其南北向存在显著的温度与盐度梯度差异。该海域属于极端贫营养环境，具有全年持续的水体层化、全年高太阳辐射，以及可忽略不计的河流输入与降水补给特征。本研究通过焦磷酸测序法，对从该海域东北部海域采集的表层水样中的细菌16S核糖体RNA（16S rRNA）基因进行测序，以探究当代环境条件是否塑造了浮游细菌群落结构。本研究共获得1855个操作分类单元（OTUs），涵盖自由生活型与颗粒附着型两类浮游细菌。其中隶属于蓝细菌门（Cyanobacteria）与变形菌门（Proteobacteria）的少数优势OTUs占总序列数的约93%，而大量“稀有”OTUs则构成了群落多样性的主体。与表层1a/b SAR11演化支相关的OTUs，以及隶属于高光适应型（HL2）生态型的原绿球藻（Prochlorococcus）相关OTUs，是分布最广泛且占比最高的序列类型。有趣的是，相较于中部海域断面，北部海域断面中典型分布于中层上部带的类群出现频率显著升高，推测这一现象是亚喀巴湾深层对流混合以及北部红海与外海水体交换的直接结果。尽管温度是所有主要细菌类群物种丰富度的最佳预测因子，但空间距离与环境距离均与遗传距离存在显著相关性。本研究结果表明，红海的细菌多样性与其他热带海域相当，并为北部与中部海域浮游群落生物地理学特征存在根本性差异提供了证据。