In-depth Spatiotemporal Characterization of Planktonic Archaeal and Bacterial Communities in North and South San Francisco Bay

Distinct diversity patterns and assembly mechanisms of prokaryotic microbial sub-community in the water column of deep-sea cold seeps

Methane leakage from deep-sea cold seeps has a major impact on marine ecosystems. Microbes sequester methane in the water column of cold seeps and can be divided into abundant and rare groups. Both abundant and rare groups play an important role in cold seep ecosystems, and the environmental heterogeneity in cold seeps may enhance conversion between taxa with different abundances. Yet, the environmental stratification and assembly mechanisms of these microbial sub-communities remain unclear. We investigated the diversities and assembly mechanisms in microbial sub-communities with distinct abundance in the deep-sea cold seep water column, from 400 m to 1400 m. We found that bacterial β-diversity, as measured by Sørensen dissimilarities, exhibited a significant species turnover pattern that was influenced by several environmental factors including depth, temperature, SiO32-, and salinity. In contrast, archaeal β-diversity showed a relatively high percentage of nestedness pattern, which was driven by the levels of soluble reactive phosphate and SiO32-. During the abundance dependency test, abundant taxa of both bacteria and archaea showed a significant species turnover, while the rare taxa possessed a higher percentage of nestedness. Stochastic processes were prominent in shaping the prokaryotic community, but deterministic processes were more pronounced for the abundant taxa than rare ones. Furthermore, the metagenomics results revealed that the abundances of methane oxidation, sulfur oxidation, and nitrogen fixation-related genes and related microbial groups were significantly higher in the bottom water. Our results implied that the carbon, sulfur, and nitrogen cycles were potentially strongly coupled in the bottom water. Overall, the results obtained in this study highlight taxonomic and abundance-dependent microbial community diversity patterns and assembly mechanisms in the water column of cold seeps, which will help understand the impacts of fluid seepage from the sea floor on the microbial community in the water column and further provide guidance for the management of cold seep ecosystem under future environmental pressures.

Pelagic metagenome-assembled genomes from an estuarine salinity gradient in San Francisco Bay

San Francisco Bay (SFB) is a large and highly human-impacted estuarine system. We produced 449 metagenome-assembled genomes from SFB waters, collected along the salinity gradient, providing a rich data set to compare the metabolic potential of microorganisms from different salinity zones within SFB and to other estuarine systems.

Ecophysiology and genomics of the brackish water adapted SAR11 subclade IIIa

The Order Pelagibacterales (SAR11) is the most abundant group of heterotrophic bacterioplankton in global oceans and comprises multiple subclades with unique spatiotemporal distributions. Subclade IIIa is the primary SAR11 group in brackish waters and shares a common ancestor with the dominant freshwater IIIb (LD12) subclade. Despite its dominance in brackish environments, subclade IIIa lacks systematic genomic or ecological studies. Here, we combine closed genomes from new IIIa isolates, new IIIa MAGS from San Francisco Bay (SFB), and 460 highly complete publicly available SAR11 genomes for the most comprehensive pangenomic study of subclade IIIa to date. Subclade IIIa represents a taxonomic family containing three genera (denoted as subgroups IIIa.1, IIIa.2, and IIIa.3) that had distinct ecological distributions related to salinity. The expansion of taxon selection within subclade IIIa also established previously noted metabolic differentiation in subclade IIIa compared to other SAR11 subclades such as glycine/serine prototrophy, mosaic glyoxylate shunt presence, and polyhydroxyalkanoate synthesis potential. Our analysis further shows metabolic flexibility among subgroups within IIIa. Additionally, we find that subclade IIIa.3 bridges the marine and freshwater clades based on its potential for compatible solute transport, iron utilization, and bicarbonate management potential. Pure culture experimentation validated differential salinity ranges in IIIa.1 and IIIa.3 and provided detailed IIIa cell size and volume data. This study is an important step forward for understanding the genomic, ecological, and physiological differentiation of subclade IIIa and the overall evolutionary history of SAR11.

Annual dynamics of eukaryotic and bacterial communities revealed by 18S and 16S rRNA metabarcoding in the coastal ecosystem of Sagami Bay, Japan

Sagami Bay, Japan is influenced by both the warm Kuroshio Current and the cold Oyashio Current and rich nutrients are supplied from multiple river sources and the deep-sea, forming a dynamic ecosystem. The aim of the present study was to investigate eukaryotic and bacterial communities in the coastal waters of Sagami Bay, using 16S rRNA and 18S rRNA sequencing and to assess the seasonal and vertical dynamics in relation to physicochemical and biological conditions. Eukaryotic and bacterial communities showed synchronous seasonal and vertical changes along with environmental variability. Diversity of plankton community suspended in the surface was lower than those at the subsurface layers in both the eukaryotes and bacteria communities; however, community diversity showed different characteristics in the subsurface where the eukaryotic community at the deeper layer (100 m) was as low as the surface and highest in intermediate depth layers (10–50 m), while that of bacterial community was highest in the deeper layer (100 m). The annual variability of the coastal microbial communities was driven, not only by the seasonal changes of abiotic and biotic factors and short-term rapid changes by river water inflow and phytoplankton blooms, but also largely influenced by deep-seawater upwellings due to the unique seafloor topography.

Genome-Resolved Metagenomic Insights into Massive Seasonal Ammonia-Oxidizing Archaea Blooms in San Francisco Bay

Ammonia-oxidizing archaea (AOA) are key for the transformation of ammonia to oxidized forms of nitrogen in aquatic environments around the globe, including nutrient-rich coastal and estuarine waters such as San Francisco Bay (SFB). Using metagenomics and 16S rRNA gene amplicon libraries, we found that AOA are more abundant than ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB), except in the freshwater stations in SFB. In South SFB, we observed recurrent AOA blooms of “Candidatus Nitrosomarinus catalina” SPOT01-like organisms, which account for over 20% of 16S rRNA gene amplicons in both surface and bottom waters and co-occur with weeks of high nitrite concentrations (>10 μM) in the oxic water column. We observed pronounced nitrite peaks occurring in the autumn for 7 of the last 9 years (2012 to 2020), suggesting that seasonal AOA blooms are common in South SFB. We recovered two high-quality AOA metagenome-assembled genomes (MAGs), including a Nitrosomarinus-like genome from the South SFB bloom and another Nitrosopumilus genome originating from Suisun Bay in North SFB. Both MAGs cluster with genomes from other estuarine/coastal sites. Analysis of Nitrosomarinus-like genomes show that they are streamlined, with low GC content and high coding density, and harbor urease genes. Our findings support the unique niche of Nitrosomarinus-like organisms which dominate coastal/estuarine waters and provide insights into recurring AOA blooms in SFB.

IMPORTANCE Ammonia-oxidizing archaea (AOA) carry out key transformations of ammonia in estuarine systems such as San Francisco Bay (SFB)—the largest estuary on the west coast of North America—and play a significant role in both local and global nitrogen cycling. Using metagenomics and 16S rRNA gene amplicon libraries, we document a massive, recurrent AOA bloom in South SFB that co-occurs with months of high nitrite concentrations in the oxic water column. Our study is the first to generate metagenome-assembled genomes (MAGs) from SFB, and through this process we recovered two high-quality AOA MAGs, one of which originated from bloom samples. These AOA MAGs yield new insight into the Nitrosopumilus and Nitrosomarinus-like lineages and their potential niches in coastal and estuarine systems. Nitrosomarinus-like AOA are abundant in coastal regions around the globe, and we highlight the common occurrence of urease genes, low GC content, and range of salinity tolerances within this lineage.