Investigation of machine learning algorithms for taxonomic classification of marine metagenomes

Microbial communities play key roles in ocean ecosystems through regulation of biogeochemical processes such as carbon and nutrient cycling, food web dynamics, and gut microbiomes of invertebrates, fish, reptiles, and mammals. Assessments of marine microbial diversity are therefore critical to understanding spatiotemporal variations in microbial community structure and function in ocean ecosystems. With recent advances in DNA shotgun sequencing for metagenome samples and computational analysis, it is now possible to access the taxonomic and genomic content of ocean microbial communities to study their structural patterns, diversity, and functional potential. However, existing taxonomic classification tools depend upon manually curated phylogenetic trees, which can create inaccuracies in metagenomes from less well-characterized communities, such as from ocean water. Herein, we explore the utility of deep learning tools-DeepMicrobes and a novel Residual Network architecture-that leverage natural language processing and convolutional neural network architectures to map input sequence data (k-mers) to output labels (taxonomic groups) without reliance on a curated taxonomic tree. We trained both models using metagenomic reads simulated from marine microbial genomes in the MarRef database. The performance of both models (accuracy, precision, and percent microbe predicted) was compared with the standard taxonomic classification tool Kraken2 using 10 complex metagenomic data sets simulated from MarRef. Our results demonstrate that time, compute power, and microbial genomic diversity still pose challenges for machine learning (ML). Moreover, our results suggest that high genome coverage and rectification of class imbalance are prerequisites for a well-trained model, and therefore should be a major consideration in future ML work. IMPORTANCE Taxonomic profiling of microbial communities is essential to model microbial interactions and inform habitat conservation. This work develops approaches in constructing training/testing data sets from publicly available marine metagenomes and evaluates the performance of machine learning (ML) approaches in read-based taxonomic classification of marine metagenomes. Predictions from two models are used to test accuracy in metagenomic classification and to guide improvements in ML approaches. Our study provides insights on the methods, results, and challenges of deep learning on marine microbial metagenomic data sets. Future machine learning approaches can be improved by rectifying genome coverage and class imbalance in the training data sets, developing alternative models, and increasing the accessibility of computational resources for model training and refinement.

Decoding dissolved information: environmental DNA sequencing at global scale to monitor a changing ocean

The use of environmental DNA (eDNA) technology for environmental monitoring is rapidly expanding, with applications for fisheries, coral reefs, harmful algal blooms, invasive and endangered species, and biodiversity monitoring. By enabling detection of species over space and time, eDNA fulfills a fundamental need of environmental surveys. Traditional surveys are expensive, require significant capital expenditure, and can be destructive; eDNA offers promise for cheaper, less invasive, and higher-resolution (i.e. genetic) assessments of environments and stocks. However, challenges in quantification, detection limits, biobanking capacity, reference databases, and data management and integration remain significant hurdles to efficient eDNA monitoring at global and decadal scale. Here, we consider the current state of eDNA technology and its suitability for the problems for which it is being used. We explore the current best practices, the logistical and social challenges that prevent eDNA from widespread adoption and benefit, and the emerging technologies that may address those challenges.

Tourmaline: A containerized workflow for rapid and iterable amplicon sequence analysis using QIIME 2 and Snakemake

Background

Amplicon sequencing (metabarcoding) is a common method to survey diversity of environmental communities whereby a single genetic locus is amplified and sequenced from the DNA of whole or partial organisms, organismal traces (e.g., skin, mucus, feces), or microbes in an environmental sample. Several software packages exist for analyzing amplicon data, among which QIIME 2 has emerged as a popular option because of its broad functionality, plugin architecture, provenance tracking, and interactive visualizations. However, each new analysis requires the user to keep track of input and output file names, parameters, and commands; this lack of automation and standardization is inefficient and creates barriers to meta-analysis and sharing of results.

Findings

We developed Tourmaline, a Python-based workflow that implements QIIME 2 and is built using the Snakemake workflow management system. Starting from a configuration file that defines parameters and input files—a reference database, a sample metadata file, and a manifest or archive of FASTQ sequences—it uses QIIME 2 to run either the DADA2 or Deblur denoising algorithm; assigns taxonomy to the resulting representative sequences; performs analyses of taxonomic, alpha, and beta diversity; and generates an HTML report summarizing and linking to the output files. Features include support for multiple cores, automatic determination of trimming parameters using quality scores, representative sequence filtering (taxonomy, length, abundance, prevalence, or ID), support for multiple taxonomic classification and sequence alignment methods, outlier detection, and automated initialization of a new analysis using previous settings. The workflow runs natively on Linux and macOS or via a Docker container. We ran Tourmaline on a 16S ribosomal RNA amplicon data set from Lake Erie surface water, showing its utility for parameter optimization and the ability to easily view interactive visualizations through the HTML report, QIIME 2 viewer, and R- and Python-based Jupyter notebooks.

Conclusion

Automated workflows like Tourmaline enable rapid analysis of environmental amplicon data, decreasing the time from data generation to actionable results. Tourmaline is available for download at github.com/aomlomics/tourmaline.

MIxS-SA: a MIxS extension defining the minimum information standard for sequence data from symbiont-associated micro-organisms

The symbiont-associated (SA) environmental package is a new extension to the minimum information about any (x) sequence (MIxS) standards, established by the Parasite Microbiome Project (PMP) consortium, in collaboration with the Genomics Standard Consortium. The SA was built upon the host-associated MIxS standard, but reflects the nestedness of symbiont-associated microbiota within and across host-symbiont-microbe interactions. This package is designed to facilitate the collection and reporting of a broad range of metadata information that apply to symbionts such as life history traits, association with one or multiple host organisms, or the nature of host-symbiont interactions along the mutualism-parasitism continuum. To better reflect the inherent nestedness of all biological systems, we present a novel feature that allows users to co-localize samples, to nest a package within another package, and to identify replicates. Adoption of the MIxS-SA and of the new terms will facilitate reports of complex sampling design from a myriad of environments.

Subsurface automated samplers for eDNA (SASe) for biological monitoring and research

Sampling of environmental DNA (eDNA) in seawater is an increasingly common approach to non-invasively assess marine biodiversity, detect cryptic or invasive species, and monitor specific groups of organisms. Despite this remarkable utility, collection and filtration of eDNA samples in the field still requires considerable time and effort. Recent advancements in automated water samplers have standardized the eDNA collection process, allowing researchers to collect eDNA day or night, sample in locations that are difficult to access, and remove the need for highly trained personnel to perform sampling. However, the high cost of purchasing or building these samplers represents a financial hurdle to widespread application. To overcome this difficulty, we have designed and built a low-cost subsurface automated sampler for eDNA (SASe). Each sampler is submersible to 55 m, can filter a pre-programmable volume of water, and preserves eDNA at the site of collection. SASe samplers have replaceable filters and a low build cost (∼280 USD vs. >100,000 USD for other eDNA samplers), which facilitates repeated field sampling at fine spatial and temporal scales. Lab testing has shown the SASe to be as effective as a standard desktop peristaltic pump for sampling, preserving, and recovering marine eDNA. SASe design files and operating code are open-source, promoting the use of this tool to meet a range of future eDNA research applications, including project-specific customizations to the current design.

Parental perception of a picture-based chronic asthma care management tool in an urban pediatric emergency department

BACKGROUND

National asthma guidelines recommend use of an asthma action plan (AAP) as part of chronic asthma care. Unfortunately, AAPs have not been tailored for use in acute care settings, where many patients at risk for poor chronic asthma care are seen, including those who are non-English-speaking or have low literacy levels. We previously developed a picture-based medication plan (PBMP), a unique type of AAP for use in an ambulatory setting and designed to increase patient use and understanding. However, little is known about how parents seeking emergency department (ED) asthma care would perceive the PBMP.

OBJECTIVE

To assess parental attitudes toward an asthma PBMP in the largest pediatric ED in Los Angeles County.

METHODS

We surveyed a consecutive sample of English- or Spanish-speaking parents of children 2-17 years seeking ED asthma care. Parents used a 5-point Likert scale for various statements regarding their perceptions of the PBMP. Responses were analyzed by sociodemographics, asthma control, and health literacy using Chi-squared and t-tests.

RESULTS

90 parents provided feedback on the PBMP. The majority of parents endorsed the PBMP. Endorsement was 20%-30% higher among Spanish-speaking parents and those who did not complete high-school compared to English-speaking parents and parents with a high school education or higher (p < 0.05 for both comparisons).

CONCLUSION

Spanish-speaking parents and parents with less than a high-school education overwhelmingly endorsed the PBMP. It may be useful to consider incorporating the PBMP as part of patient-centered chronic asthma care strategies for populations seen in ED settings.

MetaMiner: A Scalable Peptidogenomics Approach for Discovery of Ribosomal Peptide Natural Products with Blind Modifications from Microbial Communities

Ribosomally synthesized and post-translationally modified peptides (RiPPs) are an important class of natural products that contain antibiotics and a variety of other bioactive compounds. The existing methods for discovery of RiPPs by combining genome mining and computational mass spectrometry are limited to discovering specific classes of RiPPs from small datasets, and these methods fail to handle unknown post-translational modifications. Here, we present MetaMiner, a software tool for addressing these challenges that is compatible with large-scale screening platforms for natural product discovery. After searching millions of spectra in the Global Natural Products Social (GNPS) molecular networking infrastructure against just eight genomic and metagenomic datasets, MetaMiner discovered 31 known and seven unknown RiPPs from diverse microbial communities, including human microbiome and lichen microbiome, and microorganisms isolated from the International Space Station.

Author Correction: Reproducible, interactive, scalable and extensible microbiome data science using QIIME 2

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Hopanoid-producing bacteria in the Red Sea include the major marine nitrite oxidizers

Hopanoids, including the extended side chain-containing bacteriohopanepolyols, are bacterial lipids found abundantly in the geological record and across Earth's surface environments. However, the physiological roles of this biomarker remain uncertain, limiting interpretation of their presence in current and past environments. Recent work investigating the diversity and distribution of hopanoid producers in the marine environment implicated low-oxygen regions as important loci of hopanoid production, and data from marine oxygen minimum zones suggested that the dominant hopanoid producers in these environments are nitrite-utilizing organisms, revealing a potential connection between hopanoid production and the marine nitrogen cycle. Here, we use metagenomic data from the Red Sea to investigate the ecology of hopanoid producers in an environmental setting that is biogeochemically distinct from those investigated previously. The distributions of hopanoid production and nitrite oxidation genes in the Red Sea are closely correlated, and the majority of hopanoid producers are taxonomically affiliated with the major marine nitrite oxidizers, Nitrospinae and Nitrospirae. These results suggest that the relationship between hopanoid production and nitrite oxidation is conserved across varying biogeochemical conditions in dark ocean microbial ecosystems.

A Novel Sparse Compositional Technique Reveals Microbial Perturbations

The central aims of many host or environmental microbiome studies are to elucidate factors associated with microbial community compositions and to relate microbial features to outcomes. However, these aims are often complicated by difficulties stemming from high-dimensionality, non-normality, sparsity, and the compositional nature of microbiome data sets. A key tool in microbiome analysis is beta diversity, defined by the distances between microbial samples. Many different distance metrics have been proposed, all with varying discriminatory power on data with differing characteristics. Here, we propose a compositional beta diversity metric rooted in a centered log-ratio transformation and matrix completion called robust Aitchison PCA. We demonstrate the benefits of compositional transformations upstream of beta diversity calculations through simulations. Additionally, we demonstrate improved effect size, classification accuracy, and robustness to sequencing depth over the current methods on several decreased sample subsets of real microbiome data sets. Finally, we highlight the ability of this new beta diversity metric to retain the feature loadings linked to sample ordinations revealing salient intercommunity niche feature importance. IMPORTANCE By accounting for the sparse compositional nature of microbiome data sets, robust Aitchison PCA can yield high discriminatory power and salient feature ranking between microbial niches. The software to perform this analysis is available under an open-source license and can be obtained at https://github.com/biocore/DEICODE; additionally, a QIIME 2 plugin is provided to perform this analysis at https://library.qiime2.org/plugins/deicode/.

Species-level functional profiling of metagenomes and metatranscriptomes

Functional profiles of microbial communities are typically generated using comprehensive metagenomic or metatranscriptomic sequence read searches, which are time-consuming, prone to spurious mapping, and often limited to community-level quantification. We developed HUMAnN2, a tiered search strategy that enables fast, accurate, and species-resolved functional profiling of host-associated and environmental communities. HUMAnN2 identifies a community's known species, aligns reads to their pangenomes, performs translated search on unclassified reads, and finally quantifies gene families and pathways. Relative to pure translated search, HUMAnN2 is faster and produces more accurate gene family profiles. We applied HUMAnN2 to study clinal variation in marine metabolism, ecological contribution patterns among human microbiome pathways, variation in species' genomic versus transcriptional contributions, and strain profiling. Further, we introduce 'contributional diversity' to explain patterns of ecological assembly across different microbial community types.

The genome of a novel isolate of Prochlorococcus from the Red Sea contains transcribed genes for compatible solute biosynthesis

Marine microbes possess genomic and physiological adaptations to cope with varying environmental conditions. So far, the effects of high salinity on the most abundant marine photoautotrophic organism, Prochlorococcus, in marine oligotrophic environments, are mostly unknown. Here, we report the isolation of a new Prochlorococcus strain (RSP50) belonging to high-light (HL) clade II from the Red Sea, one of the warmest and most saline bodies of water in the global oceans. A comparative genomic analysis identified a set of 59 genes that were exclusive to RSP50 relative to currently available Prochlorococcus genomes, the majority of which (70%) encode for hypothetical proteins of unknown function. However, three of the unique genes encode for a complete pathway for the biosynthesis of the compatible solute glucosylglycerol, and are homologous to enzymes found in the sister lineage Synechococcus. Metatranscriptomic analyses of this metabolic pathway in the water column of the Red Sea revealed that the corresponding genes were constitutively transcribed, independent of depth and light, suggesting that osmoregulation using glucosylglycerol is a general feature of HL II Prochlorococcus in the Red Sea.

Species-level functional profiling of metagenomes and metatranscriptomes

Functional profiles of microbial communities are typically generated using comprehensive metagenomic or metatranscriptomic sequence read searches, which are time-consuming, prone to spurious mapping, and often limited to community-level quantification. We developed HUMAnN2, a tiered search strategy that enables fast, accurate, and species-resolved functional profiling of host-associated and environmental communities. HUMAnN2 identifies a community's known species, aligns reads to their pangenomes, performs translated search on unclassified reads, and finally quantifies gene families and pathways. Relative to pure translated search, HUMAnN2 is faster and produces more accurate gene family profiles. We applied HUMAnN2 to study clinal variation in marine metabolism, ecological contribution patterns among human microbiome pathways, variation in species' genomic versus transcriptional contributions, and strain profiling. Further, we introduce 'contributional diversity' to explain patterns of ecological assembly across different microbial community types.

Environmental radiation alters the gut microbiome of the bank vole Myodes glareolus

Gut microbiota composition depends on many factors, although the impact of environmental pollution is largely unknown. We used amplicon sequencing of bacterial 16S rRNA genes to quantify whether anthropogenic radionuclides at Chernobyl (Ukraine) impact the gut microbiome of the bank vole Myodes glareolus. Exposure to elevated levels of environmental radionuclides had no detectable effect on the gut community richness but was associated with an almost two-fold increase in the Firmicutes:Bacteroidetes ratio. Animals inhabiting uncontaminated areas had remarkably similar gut communities irrespective of their proximity to the nuclear power plant. Hence, samples could be classified to high-radiation or low-radiation sites based solely on microbial community with >90% accuracy. Radiation-associated bacteria had distinct inferred functional profiles, including pathways involved in degradation, assimilation and transport of carbohydrates, xenobiotics biodegradation, and DNA repair. Our results suggest that exposure to environmental radionuclides significantly alters vertebrate gut microbiota.

Taxon-specific aerosolization of bacteria and viruses in an experimental ocean-atmosphere mesocosm

Ocean-derived, airborne microbes play important roles in Earth’s climate system and human health, yet little is known about factors controlling their transfer from the ocean to the atmosphere. Here, we study microbiomes of isolated sea spray aerosol (SSA) collected in a unique ocean–atmosphere facility and demonstrate taxon-specific aerosolization of bacteria and viruses. These trends are conserved within taxonomic orders and classes, and temporal variation in aerosolization is similarly shared by related taxa. We observe enhanced transfer into SSA of Actinobacteria, certain Gammaproteobacteria, and lipid-enveloped viruses; conversely, Flavobacteriia, some Alphaproteobacteria, and Caudovirales are generally under-represented in SSA. Viruses do not transfer to SSA as efficiently as bacteria. The enrichment of mycolic acid-coated Corynebacteriales and lipid-enveloped viruses (inferred from genomic comparisons) suggests that hydrophobic properties increase transport to the sea surface and SSA. Our results identify taxa relevant to atmospheric processes and a framework to further elucidate aerosolization mechanisms influencing microbial and viral transport pathways.

Factors controlling the transfer of microbes from the ocean to the atmosphere are unclear. Here, Michaud et al. study this process in an enclosed ocean-atmosphere facility, and show that the degree of aerosolization of bacteria and viruses is taxon-specific.

A communal catalogue reveals Earth's multiscale microbial diversity

Our growing awareness of the microbial world's importance and diversity contrasts starkly with our limited understanding of its fundamental structure. Despite recent advances in DNA sequencing, a lack of standardized protocols and common analytical frameworks impedes comparisons among studies, hindering the development of global inferences about microbial life on Earth. Here we present a meta-analysis of microbial community samples collected by hundreds of researchers for the Earth Microbiome Project. Coordinated protocols and new analytical methods, particularly the use of exact sequences instead of clustered operational taxonomic units, enable bacterial and archaeal ribosomal RNA gene sequences to be followed across multiple studies and allow us to explore patterns of diversity at an unprecedented scale. The result is both a reference database giving global context to DNA sequence data and a framework for incorporating data from future studies, fostering increasingly complete characterization of Earth's microbial diversity.

Transcriptional characterization of Vibrio fischeri during colonization of juvenile Euprymna scolopes

The marine bacterium Vibrio fischeri is the monospecific symbiont of the Hawaiian bobtail squid, Euprymna scolopes, and the establishment of this association involves a number of signaling pathways and transcriptional responses between both partners. We report here the first full RNA-Seq dataset representing host-associated V. fischeri cells from colonized juvenile E. scolopes, as well as comparative transcriptomes under both laboratory and simulated marine planktonic conditions. These data elucidate the broad transcriptional changes that these bacteria undergo during the early stages of symbiotic colonization. We report several previously undescribed and unexpected transcriptional responses within the early stages of this symbiosis, including gene expression patterns consistent with biochemical stresses inside the host, and metabolic patterns distinct from those reported in associations with adult animals. Integration of these transcriptional data with a recently developed metabolic model of V. fischeri provides us with a clearer picture of the metabolic state of symbionts within the juvenile host, including their possible carbon sources. Taken together, these results expand our understanding of the early stages of the squid-vibrio symbiosis, and more generally inform the transcriptional responses underlying the activities of marine microbes during host colonization.

Deblur Rapidly Resolves Single-Nucleotide Community Sequence Patterns

High-throughput sequencing of 16S ribosomal RNA gene amplicons has facilitated understanding of complex microbial communities, but the inherent noise in PCR and DNA sequencing limits differentiation of closely related bacteria. Although many scientific questions can be addressed with broad taxonomic profiles, clinical, food safety, and some ecological applications require higher specificity. Here we introduce a novel sub-operational-taxonomic-unit (sOTU) approach, Deblur, that uses error profiles to obtain putative error-free sequences from Illumina MiSeq and HiSeq sequencing platforms. Deblur substantially reduces computational demands relative to similar sOTU methods and does so with similar or better sensitivity and specificity. Using simulations, mock mixtures, and real data sets, we detected closely related bacterial sequences with single nucleotide differences while removing false positives and maintaining stability in detection, suggesting that Deblur is limited only by read length and diversity within the amplicon sequences. Because Deblur operates on a per-sample level, it scales to modern data sets and meta-analyses. To highlight Deblur's ability to integrate data sets, we include an interactive exploration of its application to multiple distinct sequencing rounds of the American Gut Project. Deblur is open source under the Berkeley Software Distribution (BSD) license, easily installable, and downloadable from https://github.com/biocore/deblur. IMPORTANCE Deblur provides a rapid and sensitive means to assess ecological patterns driven by differentiation of closely related taxa. This algorithm provides a solution to the problem of identifying real ecological differences between taxa whose amplicons differ by a single base pair, is applicable in an automated fashion to large-scale sequencing data sets, and can integrate sequencing runs collected over time.

Metagenomic covariation along densely sampled environmental gradients in the Red Sea

Oceanic microbial diversity covaries with physicochemical parameters. Temperature, for example, explains approximately half of global variation in surface taxonomic abundance. It is unknown, however, whether covariation patterns hold over narrower parameter gradients and spatial scales, and extending to mesopelagic depths. We collected and sequenced 45 epipelagic and mesopelagic microbial metagenomes on a meridional transect through the eastern Red Sea. We asked which environmental parameters explain the most variation in relative abundances of taxonomic groups, gene ortholog groups, and pathways-at a spatial scale of <2000 km, along narrow but well-defined latitudinal and depth-dependent gradients. We also asked how microbes are adapted to gradients and extremes in irradiance, temperature, salinity, and nutrients, examining the responses of individual gene ortholog groups to these parameters. Functional and taxonomic metrics were equally well explained (75-79%) by environmental parameters. However, only functional and not taxonomic covariation patterns were conserved when comparing with an intruding water mass with different physicochemical properties. Temperature explained the most variation in each metric, followed by nitrate, chlorophyll, phosphate, and salinity. That nitrate explained more variation than phosphate suggested nitrogen limitation, consistent with low surface N:P ratios. Covariation of gene ortholog groups with environmental parameters revealed patterns of functional adaptation to the challenging Red Sea environment: high irradiance, temperature, salinity, and low nutrients. Nutrient-acquisition gene ortholog groups were anti-correlated with concentrations of their respective nutrient species, recapturing trends previously observed across much larger distances and environmental gradients. This dataset of metagenomic covariation along densely sampled environmental gradients includes online data exploration supplements, serving as a community resource for marine microbial ecology.

Gene Expression Patterns during Light and Dark Infection of Prochlorococcus by Cyanophage

Cyanophage infecting the marine cyanobacteria Prochlorococcus and Synechococcus require light and host photosystem activity for optimal reproduction. Many cyanophages encode multiple photosynthetic electron transport (PET) proteins, which are presumed to maintain electron flow and produce ATP and NADPH for nucleotide biosynthesis and phage genome replication. However, evidence suggests phage augment NADPH production via the pentose phosphate pathway (PPP), thus calling into question the need for NADPH production by PET. Genes implicated in cyclic PET have since been identified in cyanophage genomes. It remains an open question which mode of PET, cyclic or linear, predominates in infected cyanobacteria, and thus whether the balance is towards producing ATP or NADPH. We sequenced transcriptomes of a cyanophage (P-HM2) and its host (Prochlorococcus MED4) throughout infection in the light or in the dark, and analyzed these data in the context of phage replication and metabolite measurements. Infection was robust in the light, but phage were not produced in the dark. Host gene transcripts encoding high-light inducible proteins and two terminal oxidases (plastoquinol terminal oxidase and cytochrome c oxidase)-implicated in protecting the photosynthetic membrane from light stress-were the most enriched in light but not dark infection. Among the most diminished transcripts in both light and dark infection was ferredoxin-NADP+ reductase (FNR), which uses the electron acceptor NADP+ to generate NADPH in linear photosynthesis. The phage gene for CP12, which putatively inhibits the Calvin cycle enzyme that receives NADPH from FNR, was highly expressed in light infection. Therefore, both PET production of NADPH and its consumption by carbon fixation are putatively repressed during phage infection in light. Transcriptomic evidence is thus consistent with cyclic photophosphorylation using oxygen as the terminal electron acceptor as the dominant mode of PET under infection, with ATP from PET and NADPH from the PPP producing the energy and reducing equivalents for phage nucleotide biosynthesis and replication.

Microbial community assembly and metabolic function during mammalian corpse decomposition

Vertebrate corpse decomposition provides an important stage in nutrient cycling in most terrestrial habitats, yet microbially mediated processes are poorly understood. Here we combine deep microbial community characterization, community-level metabolic reconstruction, and soil biogeochemical assessment to understand the principles governing microbial community assembly during decomposition of mouse and human corpses on different soil substrates. We find a suite of bacterial and fungal groups that contribute to nitrogen cycling and a reproducible network of decomposers that emerge on predictable time scales. Our results show that this decomposer community is derived primarily from bulk soil, but key decomposers are ubiquitous in low abundance. Soil type was not a dominant factor driving community development, and the process of decomposition is sufficiently reproducible to offer new opportunities for forensic investigations.

Physiology and evolution of nitrate acquisition in Prochlorococcus

Prochlorococcus is the numerically dominant phototroph in the oligotrophic subtropical ocean and carries out a significant fraction of marine primary productivity. Although field studies have provided evidence for nitrate uptake by Prochlorococcus, little is known about this trait because axenic cultures capable of growth on nitrate have not been available. Additionally, all previously sequenced genomes lacked the genes necessary for nitrate assimilation. Here we introduce three Prochlorococcus strains capable of growth on nitrate and analyze their physiology and genome architecture. We show that the growth of high-light (HL) adapted strains on nitrate is ∼17% slower than their growth on ammonium. By analyzing 41 Prochlorococcus genomes, we find that genes for nitrate assimilation have been gained multiple times during the evolution of this group, and can be found in at least three lineages. In low-light adapted strains, nitrate assimilation genes are located in the same genomic context as in marine Synechococcus. These genes are located elsewhere in HL adapted strains and may often exist as a stable genetic acquisition as suggested by the striking degree of similarity in the order, phylogeny and location of these genes in one HL adapted strain and a consensus assembly of environmental Prochlorococcus metagenome sequences. In another HL adapted strain, nitrate utilization genes may have been independently acquired as indicated by adjacent phage mobility elements; these genes are also duplicated with each copy detected in separate genomic islands. These results provide direct evidence for nitrate utilization by Prochlorococcus and illuminate the complex evolutionary history of this trait.

Distribution and diversity of Prochlorococcus ecotypes in the Red Sea

Photosynthetic prokaryotes of the genus Prochlorococcus play a major role in global primary production in the world's oligotrophic oceans. A recent study on pelagic bacterioplankton communities in the northern and central Red Sea indicated that the predominant cyanobacterial 16S rRNA gene sequence types were from Prochlorococcus cells belonging to a high-light-adapted ecotype (HL II). In this study, we analyzed microdiversity of Prochlorococcus sp. at multiple depths within and below the euphotic zone in the northern, central, and southern regions of the Red Sea, as well as in surface waters in the same locations, but in a different season. Prochlorococcus dominated the communities in clone libraries of the amplified 16S-23S rRNA internal transcribed spacer (ITS) region. Almost no differences were found between samples from coastal or open-water sites, but a high diversity of Prochlorococcus ecotypes was detected at 100-meter depth in the water column. In addition, an unusual dominance of HL II-related sequences was observed in deeper waters. Our results indicate that the Red Sea harbors diverse Prochlorococcus lineages, but no novel ecotypes, despite its unusual physicochemical properties.

Patterns of ecological specialization among microbial populations in the Red Sea and diverse oligotrophic marine environments

Large swaths of the nutrient-poor surface ocean are dominated numerically by cyanobacteria (Prochlorococcus), cyanobacterial viruses (cyanophage), and alphaproteobacteria (SAR11). How these groups thrive in the diverse physicochemical environments of different oceanic regions remains poorly understood. Comparative metagenomics can reveal adaptive responses linked to ecosystem-specific selective pressures. The Red Sea is well-suited for studying adaptation of pelagic-microbes, with salinities, temperatures, and light levels at the extreme end for the surface ocean, and low nutrient concentrations, yet no metagenomic studies have been done there. The Red Sea (high salinity, high light, low N and P) compares favorably with the Mediterranean Sea (high salinity, low P), Sargasso Sea (low P), and North Pacific Subtropical Gyre (high light, low N). We quantified the relative abundance of genetic functions among Prochlorococcus, cyanophage, and SAR11 from these four regions. Gene frequencies indicate selection for phosphorus acquisition (Mediterranean/Sargasso), DNA repair and high-light responses (Red Sea/Pacific Prochlorococcus), and osmolyte C1 oxidation (Red Sea/Mediterranean SAR11). The unexpected connection between salinity-dependent osmolyte production and SAR11 C1 metabolism represents a potentially major coevolutionary adaptation and biogeochemical flux. Among Prochlorococcus and cyanophage, genes enriched in specific environments had ecotype distributions similar to nonenriched genes, suggesting that inter-ecotype gene transfer is not a major source of environment-specific adaptation. Clustering of metagenomes using gene frequencies shows similarities in populations (Red Sea with Pacific, Mediterranean with Sargasso) that belie their geographic distances. Taken together, the genetic functions enriched in specific environments indicate competitive strategies for maintaining carrying capacity in the face of physical stressors and low nutrient availability.

Genomic analysis of oceanic cyanobacterial myoviruses compared with T4-like myoviruses from diverse hosts and environments

T4-like myoviruses are ubiquitous, and their genes are among the most abundant documented in ocean systems. Here we compare 26 T4-like genomes, including 10 from non-cyanobacterial myoviruses, and 16 from marine cyanobacterial myoviruses (cyanophages) isolated on diverse Prochlorococcus or Synechococcus hosts. A core genome of 38 virion construction and DNA replication genes was observed in all 26 genomes, with 32 and 25 additional genes shared among the non-cyanophage and cyanophage subsets, respectively. These hierarchical cores are highly syntenic across the genomes, and sampled to saturation. The 25 cyanophage core genes include six previously described genes with putative functions (psbA, mazG, phoH, hsp20, hli03, cobS), a hypothetical protein with a potential phytanoyl-CoA dioxygenase domain, two virion structural genes, and 16 hypothetical genes. Beyond previously described cyanophage-encoded photosynthesis and phosphate stress genes, we observed core genes that may play a role in nitrogen metabolism during infection through modulation of 2-oxoglutarate. Patterns among non-core genes that may drive niche diversification revealed that phosphorus-related gene content reflects source waters rather than host strain used for isolation, and that carbon metabolism genes appear associated with putative mobile elements. As well, phages isolated on Synechococcus had higher genome-wide %G+C and often contained different gene subsets (e.g. petE, zwf, gnd, prnA, cpeT) than those isolated on Prochlorococcus. However, no clear diagnostic genes emerged to distinguish these phage groups, suggesting blurred boundaries possibly due to cross-infection. Finally, genome-wide comparisons of both diverse and closely related, co-isolated genomes provide a locus-to-locus variability metric that will prove valuable for interpreting metagenomic data sets.

Choreography of the transcriptome, photophysiology, and cell cycle of a minimal photoautotroph, prochlorococcus

The marine cyanobacterium Prochlorococcus MED4 has the smallest genome and cell size of all known photosynthetic organisms. Like all phototrophs at temperate latitudes, it experiences predictable daily variation in available light energy which leads to temporal regulation and partitioning of key cellular processes. To better understand the tempo and choreography of this minimal phototroph, we studied the entire transcriptome of the cell over a simulated daily light-dark cycle, and placed it in the context of diagnostic physiological and cell cycle parameters. All cells in the culture progressed through their cell cycles in synchrony, thus ensuring that our measurements reflected the behavior of individual cells. Ninety percent of the annotated genes were expressed, and 80% had cyclic expression over the diel cycle. For most genes, expression peaked near sunrise or sunset, although more subtle phasing of gene expression was also evident. Periodicities of the transcripts of genes involved in physiological processes such as in cell cycle progression, photosynthesis, and phosphorus metabolism tracked the timing of these activities relative to the light-dark cycle. Furthermore, the transitions between photosynthesis during the day and catabolic consumption of energy reserves at night— metabolic processes that share some of the same enzymes — appear to be tightly choreographed at the level of RNA expression. In-depth investigation of these patterns identified potential regulatory proteins involved in balancing these opposing pathways. Finally, while this analysis has not helped resolve how a cell with so little regulatory capacity, and a ‘deficient’ circadian mechanism, aligns its cell cycle and metabolism so tightly to a light-dark cycle, it does provide us with a valuable framework upon which to build when the Prochlorococcus proteome and metabolome become available.

Prevalence and evolution of core photosystem II genes in marine cyanobacterial viruses and their hosts

Cyanophages (cyanobacterial viruses) are important agents of horizontal gene transfer among marine cyanobacteria, the numerically dominant photosynthetic organisms in the oceans. Some cyanophage genomes carry and express host-like photosynthesis genes, presumably to augment the host photosynthetic machinery during infection. To study the prevalence and evolutionary dynamics of this phenomenon, 33 cultured cyanophages of known family and host range and viral DNA from field samples were screened for the presence of two core photosystem reaction center genes, psbA and psbD. Combining this expanded dataset with published data for nine other cyanophages, we found that 88% of the phage genomes contain psbA, and 50% contain both psbA and psbD. The psbA gene was found in all myoviruses and Prochlorococcus podoviruses, but could not be amplified from Prochlorococcus siphoviruses or Synechococcus podoviruses. Nearly all of the phages that encoded both psbA and psbD had broad host ranges. We speculate that the presence or absence of psbA in a phage genome may be determined by the length of the latent period of infection. Whether it also carries psbD may reflect constraints on coupling of viral- and host-encoded PsbA–PsbD in the photosynthetic reaction center across divergent hosts. Phylogenetic clustering patterns of these genes from cultured phages suggest that whole genes have been transferred from host to phage in a discrete number of events over the course of evolution (four for psbA, and two for psbD), followed by horizontal and vertical transfer between cyanophages. Clustering patterns of psbA and psbD from Synechococcus cells were inconsistent with other molecular phylogenetic markers, suggesting genetic exchanges involving Synechococcus lineages. Signatures of intragenic recombination, detected within the cyanophage gene pool as well as between hosts and phages in both directions, support this hypothesis. The analysis of cyanophage psbA and psbD genes from field populations revealed significant sequence diversity, much of which is represented in our cultured isolates. Collectively, these findings show that photosynthesis genes are common in cyanophages and that significant genetic exchanges occur from host to phage, phage to host, and within the phage gene pool. This generates genetic diversity among the phage, which serves as a reservoir for their hosts, and in turn influences photosystem evolution.

Analysis of 33 cultured cyanophages of known family and host range, as well as viral DNA from field samples, reveals the prevalence of photosynthesis genes in cyanophages and demonstrates significant genetic exchanges between host and phage.

Prenatal drug use in Minneapolis-St Paul, Minn. A 4-year trend

OBJECTIVES

To determine the rates of maternal ingestion of cocaine, marijuana, and opiates in women from Minneapolis-St Paul, Minn, in 1993 and compare them with rates observed in 1989; and to compare outcomes of newborns born to those women with and without evidence of prenatal drug ingestion.

STUDY DESIGN

The meconium from newborns of a representative cluster-based sample of women from Minneapolis-St Paul was analyzed for metabolites of cocaine, marijuana, and opiates. The newborns were consecutive births in four urban and suburban hospitals. Maternal demographic information and newborn outcome data were collected and matched to the meconium samples. The race, age, and socioeconomic status of the mothers whose newborns were screened were the same as the demographic characteristics of all women delivering babies in Minneapolis-St Paul in 1990 and 1991.

SETTING

Metropolitan hospitals of Minneapolis-St Paul.

MAIN RESULTS

Of the 1333 samples, 27 (2.0%) were cocaine positive, 35 (2.6%) were tetrahydrocannabinol positive, and 16 (1.2%) were opiate positive. There were 168 women (22.6%) reporting that they smoked. Detection of tetrahydrocannabinol and cocaine was more common in newborns of women of color, those receiving medical assistance, and those over age 23 years. Newborns with meconium samples positive for cocaine or tetrahydrocannabinol had slightly lower average birth weights but were no more likely to be premature or to require neonatal intensive care unit admission than newborns with no drugs detected in their meconium. Newborns of mothers who smoked throughout pregnancy had lower average birth weights and higher rates of prematurity and neonatal intensive care unit admissions. Standardized rates of cocaine detection in the four hospitals decreased from 4.0% in 1989 to 2.0% in 1993.

CONCLUSIONS

Rates of perinatal cocaine detection have declined in the Twin Cities of Minneapolis-St Paul over the past 4 years. In this population, self-reported smoking was associated with more serious adverse outcomes of the newborns than was the detection of cocaine, marijuana, or opiates.

In vivo studies on the carcinogenic potential of an orthodontic bonding resin

Data from another laboratory have indicated that the individual components of an orthodontic bonding resin might contain a carcinogen. Since that report, the formulation of the product was changed. The purpose of this study was to determine whether the new product is safe. Three groups of rats were used for the experiment. One group served as a control, while the other groups ingested the sealant resin or sealant catalyst. The materials were suspended in an alcohol-aqueous mixture and the solutions were given to the animals as their only source of fluid. The exposure was for 1 year. After this period of time, all the rats were given tap water and observed until day 600. The animals were autopsied at time of death or at the end of the experiment. During the treatment, there were significant differences (p less than 0.01) in water intake among the three groups. The average intake per day for the animals in the control group, the resin group, and the catalyst group was 50.2 cc, 37.8 cc, and 42.2 cc, respectively. Several animals died during the experiment, but there was no significant differences in the life expectancy of the animals in the three groups. The autopsies uncovered one malignant neoplasm, an undifferentiated sarcoma, in a rat from the control group and four benign tumors in rats from the three groups. All of these results indicate that the new formulation of the orthodontic bonding resin is not carcinogenic when ingested at a dose level of 50 ppm.

Validation of a whole blood method for cholinesterase monitoring

Traditional cholinesterase monitoring methods requiring several milliliters of blood are too invasive for prolonged routine monitoring. After several years' experience with a whole blood method requiring only 50 microL of blood for each test, a statistical evaluation of 1 year's monitoring data was undertaken to determine the normal variation of the test procedures. The results showed excellent sensitivity and reproducibility. On the basis of these data and other obvious advantages, this method is recommended as a more acceptable procedure for cholinesterase monitoring in field situations as well as plant settings.

Leaching of unpolymerized materials from orthodontic bonding resin

The unpolymerized material extracted from cured orthodontic bonding resin was analyzed by UV spectrophotometry. Under certain conditions, substantial amounts of the material (approximately 14%) were leached from bracketed teeth. The possible carcinogenicity of these unpolymerized materials is discussed.

A technique for intraradicular dental implants

A technique utilizing nonvital roots for the support of intraradicular dental implants has been developed and tested. Using this technique it may be possible to preserve the roots of teeth that would normally be lost because of periodontal involvement. In this technique, a suitable tooth is treated endodontically. The crown is then removed to the height of the alveolar crest, and the root is drilled and tapped to accept a threaded stainless steel Heli-Coil insert. The root is then covered with a gingival flap. This allows for the regeneration of periodontal tissues around the root while it isolated from the oral environment. Following the period of natural healing, a stainless steel post is threaded into the Heli-Coil root implant. In the present studies (canine roots in a rhesus monkey) the implant posts are fitted with Adaptic crowns. These implants have been in place for more than 6 months with no sign of implant failure.

Effect of pneumatic tourniquet application to the distal extremities of the horse: blood gas, serum electrolyte, osmolality, and hematologic alterations

With 120 minutes of pneumatic tourniquet application to the distal extremity in the horse, the following effects were noted in the tourniqueted limb vein (TLV): (i) local venous acidemia, (ii) increase in serum K+ concentrations, (iii) minimal changes in plasma total solids, Na+, or osmolality, and (iv) apparent reduction in hematocrit values when compared with the same measurements in the control leg. Tourniquet release after 120 minutes produced a prompt return to base line for PCV and PO2 in the TLV; however, pH, PCO2 and K+ values in the TLV required 10 to 15 minutes to reach base line (TLV or control leg vein).

Autopsy study of cerebrovascular disease in Japanese men who lived in Hiroshima, Japan, and Honolulu, Hawaii

Evidence of cerebrovascular disease at autopsy was compared in 2 groups of men: 186 long-time residents of Hiroshima, Japan, and 253 men of Japanese ancestry long resident in Honolulu, Hawaii. They were 45 to 71 years-of-age at death. Atherosclerosis of the circle of Willis and its major branches, sclerosis of the intraparenchymal arteries and the frequency of cerebral hemorrhage and cerebral infarct were compared in the 2 populations. The Honolulu subjects had significantly more atherosclerosis of the circle of Willis, but less intraparenchymal artery sclerosis and less cerebral infarction. Cerebral hemorrhage was equally frequent in the 2 cities. It was concluded that cerebral infarction is more frequent in Japanese men in Hiroshima than Honolulu, and that men of Japanese ancestry in Honolulu are spared an appreciable risk of cerebral infarction through decreased frequency of intraparenchymal arterial sclerosis despite higher levels of atherosclerosis of large intracranial arteries.

Dissociation of the DNA replicase system of bovine lymphocyte nuclei

Exposure of S-phase nuclei or subnuclear preparations from phytohemagglutinin-stimulated bovine lymphocytes to 0.02 M ATP caused an immediate and almost total loss of their ability to replicate DNA in vitro. Other ribonucleoside and deoxyribonucleoside triphosphates caused a similar inhibition of DNA replication. Levels of ATP which inhibit replication cause the release of DNA polymerases alpha and beta and small pieces of DNA from these nuclei. This release occurs both at 4 and 37 degrees C. The data support the conclusion that high levels of ATP or other nucleoside triphosphates inhibit DNA replication in nuclei by dissolution of the DNA replication complex. The limited success in reconstitution of the DNA replicase complexes is discussed.