Marine Microbial Research Articles

Use of Marine Microorganisms in the Production of Bioactive Materials for the Pharmaceutical Industry

Use of Marine Microorganisms in the Production of Bioactive Materials for the Pharmaceutical Industry

Imaging‐Based Lensless Polarization‐Sensitive Fluid Stream Analyzer for Automated, Label‐Free, and Cost‐Effective Microplastic Classification

The presence of microplastics in the environment is of significant concern, yet the exact extent of this pollution remains largely unknown. The ocean is of particular interest as the monitoring of microplastics presents a challenge in that in situ fluid stream solutions are not readily available and traditional sampling methods are labor‐intensive and costly. This study introduces an imaging‐based lensless polarization‐sensitive fluid stream analyzer (FSA) for automated, label‐free, and cost‐effective detection and classification of microplastics. The FSA incorporates digital in‐line holography and birefringence computation, enabling quantitative polarization‐sensitive imaging and machine‐learning‐based activities including sample classification. Birefringent textures of synthetic polymers are investigated owing to their optical anisotropy. A microplastic classifier is developed for the FSA and integrated to form an end‐to‐end workflow capable of sampling fluid streams and determining marine and microplastic particle presence. Cultures of two phytoplankton species form a simplified marine environment for FSA evaluation. The device is tested in a two‐class configuration for marine microorganisms and microplastics, as well as a five‐class configuration for marine microorganisms and four individual microplastic types (polyethylene, polyethylene terephthalate, polypropylene, and polystyrene). The results demonstrate high classification accuracy, supported by experiments in the simulated marine environment that validate the proposed implementation's efficacy.

Metagenomic Analysis of Microbial Diversity in the Moroccan Coastal Water of the Gibraltar Strait

Coastal waters are known for higher productivity and organic matter levels, which support a high diversity and abundance of microorganisms compared to some aquatic environments. The characterization of marine microbiomes can provide valuable information for evaluating the sustainability of coastal waters that are increasingly subjected to environmental and human impacts. Our study is the first metagenomic study realized on Moroccan Mediterranean coastal seawater. We analyzed and described the Gibraltar Detroit marine microbiome taxonomic and functional profiling using MG-RAST software. Shotgun sequencing revealed a predominance of bacterial taxa, particularly the Proteobacteria (57.29%) and Bacteroidetes (27.06%) phyla, alongside notable populations of eukaryotes, viruses, and archaea. Alphaproteobacteria and Gammaproteobacteria emerged as the dominant bacterial classes, while Flavobacteria represented a significant portion of Bacteroidetes. Functional profiling of the microbial community highlighted a wide array of metabolic pathways, emphasizing genes related to carbohydrate metabolism, amino acid synthesis, and protein processing. The marine microbiome exhibited essential biogeochemical activities, particularly in nitrogen, sulfur, and carbon cycles, with notable pathways including denitrification, thiosulfate oxidation, and carbon fixation. This functional diversity underlines the microbiome’s role in sustaining ecosystem health through nutrient cycling and organic matter degradation. Our findings offer a crucial baseline for understanding microbial community structure and resilience in Mediterranean coastal ecosystems, with implications for assessing future environmental and anthropogenic impacts on these microbial dynamics.

Results from a multi-laboratory ocean metaproteomic intercomparison: effects of LC-MS acquisition and data analysis procedures

Abstract. Metaproteomics is an increasingly popular methodology that provides information regarding the metabolic functions of specific microbial taxa and has potential for contributing to ocean ecology and biogeochemical studies. A blinded multi-laboratory intercomparison was conducted to assess comparability and reproducibility of taxonomic and functional results and their sensitivity to methodological variables. Euphotic zone samples from the Bermuda Atlantic Time-series Study (BATS) in the North Atlantic Ocean collected by in situ pumps and the autonomous underwater vehicle (AUV) Clio were distributed with a paired metagenome, and one-dimensional (1D) liquid chromatographic data-dependent acquisition mass spectrometry analysis was stipulated. Analysis of mass spectra from seven laboratories through a common bioinformatic pipeline identified a shared set of 1056 proteins from 1395 shared peptide constituents. Quantitative analyses showed good reproducibility: pairwise regressions of spectral counts between laboratories yielded R2 values averaged 0.62±0.11, and a Sørensen similarity analysis of the top 1000 proteins revealed 70 %–80 % similarity between laboratory groups. Taxonomic and functional assignments showed good coherence between technical replicates and different laboratories. A bioinformatic intercomparison study, involving 10 laboratories using eight software packages, successfully identified thousands of peptides within the complex metaproteomic datasets, demonstrating the utility of these software tools for ocean metaproteomic research. Lessons learned and potential improvements in methods were described. Future efforts could examine reproducibility in deeper metaproteomes, examine accuracy in targeted absolute quantitation analyses, and develop standards for data output formats to improve data interoperability. Together, these results demonstrate the reproducibility of metaproteomic analyses and their suitability for microbial oceanography research, including integration into global-scale ocean surveys and ocean biogeochemical models.

A novel fine-grained TiZrCu alloy tailored for marine environment with high microbial corrosion-resistance

Titanium alloys, usually known as non-corrodible material, are susceptible to microbiologically influenced corrosion (MIC) in marine environment. While titanium-zirconium (TiZr) alloys have been extensively studied in medical applications, the influence of microorganisms, especially marine microorganisms, on their corrosion behavior has not been explored. In this work, a TiZrCu alloy with a combination of excellent mechanical, anti-corrosion, and antibacterial properties was developed by optimizing the Cu content and grain refinement. Its MIC and antibacterial mechanisms against Pseudomonas aeruginosa, a representative marine microorganism, were systematically investigated. 5.5 wt% was determined as the optimal copper content. The fine-grained Ti-15Zr-5.5Cu (TZC-5.5FG) alloy maintained high MIC resistance, exhibiting a corrosion current of 5.7 ± 0.1 nA/cm2 and an antibacterial rate of 91.8 % against P. aeruginosa. The mechanism of improved corrosion resistance was attributed to the denser passive film with high TiO2 content and the lower surface potential difference ΔE. The release of Cu2+ ions, ΔE, and the generation of ROS are three major factors that contribute to the antibacterial performance of TiZrCu alloys. Compared to other available marine metals, TZC-5.5FG alloy exhibited superior comprehensive performance, including excellent mechanical properties and anti-MIC capacity, which make it a promising material for load-bearing applications in marine environment.

Microbiome divergence of marine gastropod species separated by the Isthmus of Panama.

The rise of the Isthmus of Panama separated the populations of many marine organisms, which then diverged into new geminate sister species currently living in the Eastern Pacific Ocean and the Caribbean Sea. However, we know very little about how such evolutionary divergences of host species have shaped the compositions of their microbiomes. Here, we compared the microbiomes of whole-body and shell-surface samples of geminate species of marine gastropods in the genera Cerithium and Cerithideopsis to those of congeneric outgroups. Our results suggest that the effects of ~3 million years of separation and isolation on microbiome composition varied among host genera and between sample types within the same hosts. In the whole-body samples, microbiome compositions of geminate species pairs tended to be similar, likely due to host filtering, although the strength of this relationship varied among the two groups and across similarity metrics. Shell-surface microbiomes show contrasting patterns, with co-divergence between the host taxa and a small number of microbial clades evident in Cerithideopsis but not Cerithium. These results suggest that (i) isolation of host populations after the rise of the Isthmus of Panama affected microbiomes of geminate hosts in a complex and host-specific manner, and (ii) host-associated microbial taxa respond differently to vicariance events than the hosts themselves.IMPORTANCEWhile considerable work has been done on evolutionary divergences of marine species in response to the rise of the Isthmus of Panama, which separated two previously connected oceans, how this event shaped the microbiomes of these marine hosts remains poorly known. Using whole-body and shell-surface microbiomes of closely related gastropod species from opposite sides of the Isthmus, we show that divergences of microbial taxa after the formation of the Isthmus are often not concordant with those of their gastropod hosts. Our results show that evolutionary responses of marine gastropod-associated microbiomes to major environmental perturbations are complex and are shaped more by local environments than host evolutionary history.

Beyond Earth: Harnessing Marine Resources for Sustainable Space Colonization

The quest for sustainable space exploration and colonization is a challenge in its infancy, which faces scarcity of resources and an inhospitable environment. In recent years, advancements in space biotechnology have emerged as potential solutions to the hurdles of prolonged space habitation. Taking cues from the oceans, this review focuses on the sundry types of marine organisms and marine-derived chemicals that have the potential of sustaining life beyond planet Earth. It addresses how marine life, including algae, invertebrates, and microorganisms, may be useful in bioregenerative life support systems, food production, pharmaceuticals, radiation shielding, energy sources, materials, and other applications in space habitats. With the considerable and still unexplored potential of Earth’s oceans that can be employed in developing space colonization, we allow ourselves to dream of the future where people can expand to other planets, not only surviving but prospering. Implementing the blend of marine and space sciences is a giant leap toward fulfilling man’s age-long desire of conquering and colonizing space, making it the final frontier.

Time Series Data Provide Insights into the Evolution and Abundance of One of the Most Abundant Viruses in the Marine Virosphere: The Uncultured Pelagiphages vSAG 37-F6

Viruses play a pivotal role in ecosystems by influencing biochemical cycles and impacting the structure and evolution of their host cells. The widespread pelagiphages infect Pelagibacter spp., the most abundant marine microbe on Earth, and thus play a significant role in carbon transformation through the viral shunt. Among these viruses, the uncultured lytic pelagiphage vSAG 37-F6, uncovered by single-virus genomics, is likely the most numerous virus in the ocean. While previous research has delved into the diversity and spatial distribution of vSAG 37-F6, there is still a gap in understanding its temporal dynamics, hindering our insight into its ecological impact. We explored the temporal dynamics of vSAG 37-F6, assessing periodic fluctuations in abundance and evolutionary patterns using long- and short-term data series. In the long-term series (7 years), metagenomics showed negative selection acting on all viral genes, with a highly conserved overall diversity over time composed of a pool of yearly emergent, highly similar novel strains that exhibited a seasonal abundance pattern with two peaks during winter and fall and a decrease in months with higher UV radiation. Most non-synonymous polymorphisms occurred in structural viral proteins located in regions with low conformational restrictions, suggesting that many of the viral genes of this population are highly purified over its evolution. At the fine-scale resolution (24 h time series), combining digital PCR and metagenomics, we identified two peaks of cellular infection for the targeted vSAG 37-F6 viral strain (up to approximately 103 copies/ng of prokaryotic DNA), one before sunrise and the second shortly after midday. Considering the high number of co-occurring strains of this microdiverse virus, the abundance values at the species or genus level could be orders of magnitudes higher. These findings represent a significant advancement in understanding the dynamics of the potentially most abundant oceanic virus, providing valuable insights into ecologically relevant marine viruses.

Isolation and Characterization of Some Potential Antibacterial Metabolites from Marine Microorganisms Collected in Egypt

Isolation and Characterization of Some Potential Antibacterial Metabolites from Marine Microorganisms Collected in Egypt

Widespread production of plant growth-promoting hormones among marine bacteria and their impacts on the growth of a marine diatom

BackgroundReciprocal exchanges of metabolites between phytoplankton and bacteria influence the fitness of these microorganisms which ultimately shapes the productivity of marine ecosystems. Recent evidence suggests that plant growth-promoting hormones may be key metabolites within mutualistic phytoplankton-bacteria partnerships, but very little is known about the diversity of plant growth-promoting hormones produced by marine bacteria and their specific effects on phytoplankton growth. Here, we aimed to investigate the capacity of marine bacteria to produce 7 plant growth-promoting hormones and the effects of these hormones on Actinocyclus sp. growth.ResultsWe examined the plant growth-promoting hormone synthesis capabilities of 14 bacterial strains that enhance the growth of the common diatom Actinocyclus. Plant growth-promoting hormone biosynthesis was ubiquitous among the bacteria tested. Indeed all 14 strains displayed the genomic potential to synthesise multiple hormones, and mass-spectrometry confirmed that each strain produced at least 6 out of the 7 tested plant growth-promoting hormones. Some of the plant growth-promoting hormones identified here, such as brassinolide and trans-zeatin, have never been reported in marine microorganisms. Importantly, all strains produced the hormone indole-3 acetic acid (IAA) in high concentrations and released it into their surroundings. Furthermore, indole-3 acetic acid extracellular concentrations were positively correlated with the ability of each strain to promote Actinocyclus growth. When inoculated with axenic Actinocyclus cultures, only indole-3 acetic acid and gibberellic acid enhanced the growth of the diatom, with cultures exposed to indole-3 acetic acid exhibiting a two-fold increase in cell numbers.ConclusionOur results reveal that marine bacteria produce a much broader range of plant growth-promoting hormones than previously suspected and that some of these compounds enhance the growth of a marine diatom. These findings suggest plant growth-promoting hormones play a large role in microbial communication and broaden our knowledge of their fuctions in the marine environment.4YJWCKdNErspcm2bUDNM6tVideo

Enrichment of marine microbes to remove nitrogen of urea wastewater under salinity stress

Salinity (NaCl) and urea concentration significantly affect the diversity, structural and physiological function of microbial communities in the biological treatment of wastewater. However, the responses of microbial in high salt and urea wastewater remain elusive. Here, we investigated microbial community function and assembly of four regions using gradient domestication experiment combined with 16S rRNA gene sequencing and statistical methods. The results showed that with the increase of salinity and urea concentration, the consortium Xiamen could still remove most urea, while the other three consortia could not. The alpha diversity of microbial community initially decreased and then increased, showing a recovery trend. After domestication, the consortium Xiamen exhibited high physiological activity and complex network structure, and the community assembly process changed from stochastic to deterministic during the domestication. Furthermore, the keystones with low abundance were associated with urea removal and important for maintain the complexity of the networks, while Arenibacter and Oceanimonas were found to be keystones in maintaining efficient urea removal in harsh environments. To sum up, environmental effects dominated by salinity and urea concentration stress drove the community assembly and species coexistence that underpinned the microbial differentiation pattern at a geographic scale. These results provided new sights for elucidate how microbial response to salinity and urea during wastewater treatment.

Management biofouling of uranium extraction from seawater by ultraviolet technique: Effect of UV wavelength

Marine biofouling is one of the primary limiting factors in uptake of uranium (U(VI)) from seawater. The effects of ultraviolet (UV) wavelength on its elimination capacity for marine biofouling were investigated in detail. V. alginolyticus was used as the representative of marine microorganisms in this work. UV254 irradiation can well reduce the adhesion of V. alginolyticus on poly(amidoxime) (PAO) surface and enhance U(VI) adsorption. The adsorption capacity of PAO for U(VI) increases from 40.7 mg/g to 67.1 mg/g at pH 8.2 and 298 K under UV254 irradiation condition. UV254 irradiation can well damage the DNA structure of V. alginolyticus by photochemistry reaction, and further reduce its extracellular polymeric substances (EPS). The increase of NO3- concentrations promote the yield of ˙OH by UV254 irradiation, and increase the inactivation of V. alginolyticus cells. Biological community results indicate that UV254 irradiation can intercept the reproduction and break the bottleneck of marine microorganisms, and would enhance the extraction of U(VI) from seawater.

Prospecting Marine Natural Products as the Disease-Modifying Treatment of Alzheimer’s Diseases

Alzheimer's disease (AD) is a severe and high costs health problem all over the world. To date, there is no therapy capable of curing AD, making drug discovery a challenging and intriguing field to explore. Targeting marine natural products (MNPs) as a source of drug leads is a suitable choice considering the content of bioactive compounds and promising pharmacological benefits. This article aims to look at MNPs with prospects in several prime targets in AD pathology to cure AD. Marine vertebrates (fishes), marine invertebrates (sponges, tunicates, ascidian, clams, scallops, sand dollars, crabs, cockle, sea cucumber, shrimp, bryozoa, marine worm), algae/seaweed, and marine microorganisms (marine fungi, bacterium, dinoflagellata, microalgae) can be potentially used as disease-modifying treatments (DMTs) for AD. By targeting multiple aspects of AD pathology, these MNPs offer a multifaceted approach to treating and potentially modifying the disease course. This result is an intriguing gap for researchers in the discovery and development of new drugs that can improve AD pathology.

Microbiome Geographic Population Structure (mGPS) Detects Fine-Scale Geography.

Over the past decade, sequencing data generated by large microbiome projects showed that taxa exhibit patchy geographical distribution, raising questions about the geospatial dynamics that shape natural microbiomes and the spread of antimicrobial resistance (AMR) genes. Answering these questions requires distinguishing between local and non-local microorganisms and identifying the source sites for the latter. Predicting the source sites and migration routes of microbiota has been envisioned for decades but was hampered by the lack of data, tools, and understanding of the processes governing biodiversity. State-of-the-art biogeographical tools suffer from low resolution and cannot predict biogeographical patterns at a scale relevant to ecological, medical, or epidemiological applications. Analyzing urban, soil, and marine microorganisms, we found that some taxa exhibit regional-specific composition and abundance, suggesting they can be used as biogeographical biomarkers. We developed the Microbiome Geographic Population Structure (mGPS), a machine-learning-based tool that utilizes microbial relative sequence abundances to yield a fine-scale source site for microorganisms. mGPS predicted the source city for 92% of the samples and the within-city source for 82% of the samples, though they were often only a few hundred meters apart. mGPS also predicted soil and marine sampling sites for 86% and 74% of the samples, respectively. We demonstrated that mGPS differentiated local from non-local microorganisms and used it to trace the global spread of AMR genes. mGPS's ability to localize samples to their waterbody, country, city, and transit stations opens new possibilities in tracing microbiomes and has applications in forensics, medicine, and epidemiology.

Biodiversity of microorganisms in the Baltic Sea: the power of novel methods in the identification of marine microbes.

Until recently, the data on the diversity of the entire microbial community from the Baltic Sea were relatively rare and very scarce. However, modern molecular methods have provided new insights into this field with interesting results. They can be summarized as follows. (i) Although low salinity causes a reduction in the biodiversity of multicellular species relative to the populations of the North-East Atlantic, no such reduction occurs in bacterial diversity. (ii) Among cyanobacteria, the picocyanobacterial group dominates when considering gene abundance, while filamentous cyanobacteria dominate in means of biomass. (iii) The diversity of diatoms and dinoflagellates is significantly larger than described a few decades ago; however, molecular studies on these groups are still scarce. (iv) Knowledge gaps in other protistan communities are evident. (v) Salinity is the main limiting parameter of pelagic fungal community composition, while the benthic fungal diversity is shaped by water depth, salinity, and sediment C and N availability. (vi) Bacteriophages are the predominant group of viruses, while among viruses infecting eukaryotic hosts, Phycodnaviridae are the most abundant; the Baltic Sea virome is contaminated with viruses originating from urban and/or industrial habitats. These features make the Baltic Sea microbiome specific and unique among other marine environments.

Biogeographic shifts in the microbial co-occurrence network features of three domains across complex environmental gradients in subtropical coastal waters

BackgroundBacteria, Archaea, and Microeukaryotes comprise taxonomic domains that interact in mediating biogeochemical cycles in coastal waters. Many studies have revealed contrasting biogeographic patterns of community structure and assembly mechanisms in microbial communities from different domains in coastal ecosystems; however, knowledge of specific biogeographic patterns on microbial co-occurrence relationships across complex coastal environmental gradients remains limited. Using a dense sampling scheme at the regional scale, SSU rRNA gene amplicon sequencing, and network analysis, we investigated intra- and inter-domain co-occurrence relationships and network topology-based biogeographic patterns from three microbial domains in coastal waters that show environmental gradients across the inshore-nearshore-offshore continuum in the East China Sea.ResultsOverall, we found the highest complexity and connectivity in the bacterial network, the highest modularity in the archaeal network, and the lowest complexity, connectivity, and modularity in the microeukaryotic network. Although microbial co-occurrence networks from the three domains showed distinct topological features, they exhibited a consistent biogeographic pattern across the inshore-nearshore-offshore continuum. Specifically, the nearshore zones with intermediate levels of terrestrial impacts reflected by multiple environmental factors (including water temperature, salinity, pH, dissolved oxygen, and nutrient-related parameters) had a higher intensity of microbial co-occurrence for all three domains. In contrast, the intensity of microbial co-occurrence was weaker in both the inshore and the offshore zones at the two ends of the environmental gradients. Archaea occupied a central position in the microbial inter-domain co-occurrence network. In particular, members of the Thaumarchaeota Marine Group I (MGI, now placed within the Family Nitrosopumilaceae of the Phylum Thermoproteota) appeared to be the hubs in the biogeographic shift between inter-domain network modules across environmental gradients.ConclusionsOur work offers new insights into microbial biogeography by integrating network features into biogeographic patterns, towards a better understanding of the potential of microbial interactions in shaping biogeographic patterns of coastal marine microbiota.

Vertical heterogeneity enhances network complexity and stability of co-occurrence microbes in the eastern Indian Ocean

Microbes are core to driving biogeochemical cycles and differ between sun-drenched surface and relatively dark deep oceans. However, their distinct contributions to the organization and association of communities are still remaining elusive. Here, their assembly and co-occurrence stability are systematically researched along the surface and vertical gradients in the eastern Indian Ocean. The distribution of surface microbes was grouped tightly with closer phylogenetic distance and broader niche breadth, and separately from those vertical samples. Clear distance-decay of community similarity was observed in surface microbes with lower richness, while more diverse microeukaryotes and prokaryotes were observed in surface and vertical environments, respectively. Co-occurrence microbes along vertical gradients had a more complex network that was dominated by prokaryotes, while exhibited a lower modularity compared to the surface network. Microbial associations along vertical gradients were more stable and resilient, with lower robustness, higher vulnerability, and a relatively consistent fragmentation. Moreover, prokaryotes contribute greatly to the network topology and stability compared to microeukaryotes in surface environments, emphasizing their distinct functions and survival strategies in maintaining community stability across spatial variations. Environmental selection and community differentiation led to the divergence in organization and potential function of microbes. This study shed light on new perspectives on how marine microbes were associated with and influenced by spatial heterogeneity and their distinct roles in community organization in the face of environmental fluctuations.

Novel oil-associated bacteria in Arctic seawater exposed to different nutrient biostimulation regimes.

The Arctic Ocean is an oligotrophic ecosystem facing escalating threats of oil spills as ship traffic increases owing to climate change-induced sea ice retreat. Biostimulation is an oil spill mitigation strategy that involves introducing bioavailable nutrients to enhance crude oil biodegradation by endemic oil-degrading microbes. For bioremediation to offer a viable response for future oil spill mitigation in extreme Arctic conditions, a better understanding of the effects of nutrient addition on Arctic marine microorganisms is needed. Controlled experiments tracking microbial populations revealed a significant decline in community diversity along with changes in microbial community composition. Notably, differential abundance analysis highlighted the significant enrichment of the unexpected genera Lacinutrix, Halarcobacter and Candidatus Pseudothioglobus. These groups are not normally associated with hydrocarbon biodegradation, despite closer inspection of genomes from closely related isolates confirming the potential for hydrocarbon metabolism. Co-occurrence analysis further revealed significant associations between these genera and well-known hydrocarbon-degrading bacteria, suggesting potential synergistic interactions during oil biodegradation. While these findings broaden our understanding of how biostimulation promotes enrichment of endemic hydrocarbon-degrading genera, further research is needed to fully assess the suitability of nutrient addition as a stand-alone oil spill mitigation strategy in this sensitive and remote polar marine ecosystem.

Identification of Benthic Foraminifera Presence in The Marginal Environments of Biliran Island, Philippines.

Benthic foraminifera are unicellular marine micro-organism with a hard exoskeleton and commonly present in the benthic community of marine ecosystem. This study aimed to identify the benthic foraminifera present along the coastal areas of eight municipalities in Biliran Island, Philippines. Quadrat sampling was conducted and three samples per quadrant transect of 1 m × 1 m divided into nine squares were collected. The samples were then observed under the microscope, and the specimens seen were identified by comparing them with the images of the sample species from the website https://marinespecies.org/. The researchers conducted an in-situ collection of the foraminiferal shells from intertidal areas along shallow water coastlines of the island. Results showed that the coastal environment of Biliran Island has the presence of the genera Spirillina, Quinqueloculina, Marginopora and Sorites. The identified species were classified based on their feeding mechanisms as herbivory and passive suspension feeding. The presence of benthic foraminifera species along the coastal environments of Biliran Island provides a record of the environment where they are found, making them natural bioindicators of water quality. This study provides a baseline for further studies on the distribution and abundance of benthic foraminifera in the area and can contribute to the understanding of the environmental conditions of the coastal areas in Biliran Island.

Whole cell affinity for 4-amino-5-hydroxymethyl-2-methylpyrimidine (HMP) in the marine bacterium Candidatus Pelagibacter st. HTCC7211 explains marine dissolved HMP concentrations.

Vitamin B1 is a universally required coenzyme in carbon metabolism. However, most marine microorganisms lack the complete biosynthetic pathway for this compound and must acquire thiamin, or precursor molecules, from the dissolved pool. The most common version of Vitamin B1 auxotrophy is for thiamin's pyrimidine precursor moiety, 4-amino-5-hydroxymethyl-2-methylpyrimidine (HMP). Frequent HMP auxotrophy in plankton and vanishingly low dissolved concentrations (approximately 0.1-50 pM) suggest that high-affinity HMP uptake systems are responsible for maintaining low ambient HMP concentrations. We used tritium-labelled HMP to investigate HMP uptake mechanisms and kinetics in cell cultures of Candidatus Pelagibacter st. HTCC7211, a representative of the globally distributed and highly abundant SAR11 clade. A single protein, the sodium solute symporter ThiV, which is conserved across SAR11 genomes, is the likely candidate for HMP transport. Experimental evidence indicated transport specificity for HMP and mechanistically complex, high-affinity HMP uptake kinetics. Km values ranged from 9.5 pM to 1.2 nM and were dramatically lower when cells were supplied with a carbon source. These results suggest that HMP uptake in HTCC7211 is subject to complex regulation and point to a strategy for high-affinity uptake of this essential growth factor that can explain natural HMP levels in seawater.