| Microorganisms play key roles in ocean biogeochemistry. However, several predominant groups of uncultured bacterioplankton thought to contribute to important biogeochemical processes in the oceans are known primarily from gene cloning studies. Although these studies have greatly expanded our view of microbial diversity in the oceans, they are not quantitative and usually provide no phenotypic information. The vast majority of marine microbes remain uncultivated, and environmental genomic studies have only just begun to identify the genetic diversity within marine bacterioplankton communities. Distribution and abundance, cultivation, and environmental genomic studies are needed to better understand the roles of specific marine bacterioplankton lineages in oceanographic and biogeochemical processes. Ecological inferences derived from distribution and abundance estimates can improve cultivation and environmental genomic strategies, which are essential to further understand the roles of microbes in oceanographic and biogeochemical processes. In this study, direct cell counts supported SAR11 clade dominance in ocean surface waters, and Chloroflexi-related SAR202 cluster prevalence in mesopelagic and deep ocean environments. Members of the SAR11 clade averaged 35%, and up to 50% of the total bacterioplankton community in Atlantic Ocean surface waters (1--140 m), and accounted for an average of 18% of total cell counts in the mesopelagic zone. SAR202 cells were below the limit of detection in surface waters, increased just below the deep chlorophyll maximum (DCM), and persisted in abundance throughout the mesopelagic zone and deep ocean. Members of the SAR202 cluster accounted for an average of 10%, and up to 18% of the total microbial community below 500 m in the Atlantic and Pacific oceans. Nonmetric multidimensional scaling of amplified community 16S rRNA gene fragments supported SAR11 and SAR202 cell distributions and abundances, and identified increases in SAR11, marine Actinobacteria, and OCS116-related lineages following deep convective mixing in the northeaster Atlantic Ocean. Quantitative hybridization of 16S rRNAs supported fragment distribution and abundance estimates observed for SAR11, SAR202, and marine Actinobacteria clusters. Direct cell counts, and relative rRNA and rDNA abundance estimates reported here were in general agreement, and exhibited variability within and between lineages. |
