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Abstract Flocculation of riverine dissolved organic matter (DOM) in estuaries is crucial for transforming and removing terrestrial carbon inputs across the land-to-ocean aquatic continuum. We measured variations in chromophoric DOM (CDOM) absorption and fluorescence of riverine DOM through mixing experiments conducted across various seasons and environments, identifying patterns in salt-induced flocculation. Our observations show a systematic reduction in CDOM absorption in the 250–450 nm range at salinity 2, with a sharper decrease at higher wavelengths. Flocculation led to decreased relative fluorescence intensity below emission wavelength of 360 nm and an increased intensity at higher emission wavelengths across the excitation spectrum measured (250–450 nm). We introduce a new metric,red shift ratio, a fluorescence-based metric calculated as the ratio of emission intensity at 300–350 nm to that at 360–500 nm, at excitation wavelengths between 250 and 300 nm, for detecting flocculation-induced changes in CDOM across estuarine systems. The observed sensitivity of CDOM to flocculation in low salinities challenges its use as a conservative tracer in coastal gradients, suggesting that recalibrations are required for remote sensing algorithms and carbon flux estimations across land-sea continuum, particularly in systems with similar characteristics. 

Cyanobacteria are important primary producers, sources of secondary metabolites, and sentinels of environmental change in aquatic ecosystems – including large estuaries. Here, we newly investigated cyanobacterial diversity within the Albemarle Pamlico Sound System (APES) using (16S rRNA) gene amplicon sequencing analyses. Substantial cyanobacterial diversity including lineages lacking current isolates were recovered (46 genera, 17 potentially cyanotoxic), with oligohaline waters of the Albemarle Sound and its tributaries being notable regional hotspot for diversity. Salinity and temperature were influential drivers of cyanobacterial community composition. Picocyanobacteria (cells <3 µm in diameter) were abundant in amplicon sequence libraries (72% of cyanobacterial sequences) – especially populations withinSynechococcusSubClade 5.2. Picocyanobacteria along with picoeukaryotes were large contributors to total phytoplankton biomass comprising ~47% of chlorophyll a. Further, the picocyanobacterial generaSynechococcus,Cyanobium, andSynechocystis(55.4%, 14.8%, and 12.9% of cyanobacterial sequences, respectively) formed a core community spanning from freshwater regions (eastern AST, D949) to polyhaline environments (NRE100 downstream stations to PS5), suggesting resilience to significant salinity fluctuations and associated environmental changes. Amplicon sequence variant (ASV) and environmental data indicate the presence of several putative ecotypes, as well as distinct abundance patterns among closely related populations, highlighting substantial fitness variability among subspecies. Notably, potentially cyanotoxic genera,Synechocystis,Planktothrix,Plectonema, andDolichospermumwere the four more abundant detected in polyhaline APES regions, far beyond conspicuous freshwater sources. These findings reveal previously unrecognized potential sources of cyanotoxics in estuarine food webs and habitats, underscoring the ecological significance of cyanobacterial community dynamics across salinity gradients. 

Abstract Vitamin B1 (thiamin, B1) is an essential micronutrient for cells, yet intriguingly in aquatic systems most bacterioplankton are unable to synthesize it de novo (auxotrophy), requiring an exogenous source. Cycling of this valuable metabolite in aquatic systems has not been fully investigated and vitamers (B1-related compounds) have only begun to be measured and incorporated into the B1 cycle. Here, we identify potential key producers and consumers of B1 and gain new insights into the dynamics of B1 cycling through measurements of B1 and vitamers (HMP: 4-amino-5-hydroxymethyl-2-methylpyrimidine, HET: 4-methyl-5-thiazoleethanol, FAMP: N-formyl-4-amino-5-aminomethyl-2-methylpyrimidine) in the particulate and dissolved pool in a temperate coastal system. Dissolved B1 was not the primary limiting nutrient for bacterial production and was relatively stable across seasons with concentrations ranging from 74–117 pM, indicating a balance of supply and demand. However, vitamer concentration changed markedly with season as did transcripts related to vitamer salvage and transport suggesting use of vitamers by certain bacterioplankton, e.g. Pelagibacterales. Genomic and transcriptomic analyses showed that up to 78% of the bacterioplankton taxa were B1 auxotrophs. Notably, de novo B1 production was restricted to a few abundant bacterioplankton (e.g. Vulcanococcus, BACL14 (Burkholderiales), Verrucomicrobiales) across seasons. In summer, abundant picocyanobacteria were important putative B1 sources, based on transcriptional activity, leading to an increase in the B1 pool. Our results provide a new dynamic view of the players and processes involved in B1 cycling over time in coastal waters, and identify specific priority populations and processes for future study. 

ABSTRACT Vitamin B1 (thiamin) is a vital nutrient for most cells in nature, including marine plankton. Early and recent experiments show that B1 degradation products instead of B1 can support the growth of marine bacterioplankton and phytoplankton. However, the use and occurrence of some degradation products remains uninvestigated, namely N-formyl-4-amino-5-aminomethyl-2-methylpyrimidine (FAMP), which has been a focus of plant oxidative stress research. We investigated the relevance of FAMP in the ocean. Experiments and global ocean meta-omic data indicate that eukaryotic phytoplankton, including picoeukaryotes and harmful algal bloom species, use FAMP while bacterioplankton appear more likely to use deformylated FAMP, 4-amino-5-aminomethyl-2-methylpyrimidine. Measurements of FAMP in seawater and biomass revealed that it occurs at picomolar concentrations in the surface ocean, heterotrophic bacterial cultures produce FAMP in the dark—indicating non-photodegradation of B1 by cells, and B1-requiring (auxotrophic) picoeukaryotic phytoplankton produce intracellular FAMP. Our results require an expansion of thinking about vitamin degradation in the sea, but also the marine B1 cycle where it is now crucial to consider a new B1-related compound pool (FAMP), as well as generation (dark degradation—likely via oxidation), turnover (plankton uptake), and exchange of the compound within the networks of plankton. IMPORTANCEResults of this collaborative study newly show that a vitamin B1 degradation product, N-formyl-4-amino-5-aminomethyl-2-methylpyrimidine (FAMP), can be used by diverse marine microbes (bacteria and phytoplankton) to meet their vitamin B1 demands instead of B1 and that FAMP occurs in the surface ocean. FAMP has not yet been accounted for in the ocean and its use likely enables cells to avoid B1 growth deficiency. Additionally, we show FAMP is formed in and out of cells without solar irradiance—a commonly considered route of vitamin degradation in the sea and nature. Altogether, the results expand thinking about oceanic vitamin degradation, but also the marine B1 cycle where it is now crucial to consider a new B1-related compound pool (FAMP), as well as its generation (dark degradation—likely via oxidation), turnover (plankton uptake), and exchange within networks of plankton. 

B-vitamins are essential micronutrients for marine plankton. Additionally, we now know many marine plankton cannot synthesize B-vitamins de novo (from scratch) and thus are reliant on external supplies. Details of B-vitamin exchange, whether ‘active’ or ‘passive’ (i.e. through cell secretion or mortality), are lacking and as a result we struggle to predict microbial physiology, community composition and biogeochemistry. We argue that significant advances in understanding of the impact of B-vitamin exchange and cycling on marine community structure and biogeochemistry can be made by focusing on unknowns related to the ‘in’s and out’s’ of B-vitamin transport, exchange between plankton, and ecosystem scale processing/transformation of B-vitamins. We point out that it is particularly necessary to reach beyond traditional categorization of populations as B-vitamin auxotrophs (requiring supplied vitamin) or prototrophs ( de novo vitamin synthesizers) and begin addressing which populations are net ‘providers’ and/or ‘consumers’. This is a particularly interesting problem as organisms cannot be confidently categorized as net ‘providers’ and/or ‘consumers’ based on genome-based prediction, and it is possible the two roles may change over time and environmental conditions. We posit that greater knowledge of B-vitamin exchange, e.g. cross-feeding, acquisition and secretion systems, environmental triggers of ‘provision’ and ‘consumption’, will reveal unforeseen networking and novel niches across marine planktonic communities. Last, we advocate for further experiments tracking the responses of isolates or natural communities relative to vitamin availability, tracing flow of B-vitamins between cells using novel approaches (e.g. isotopic, fluorometric), and greater consideration of altered B-vitamin exchange and cycling under future climate scenarios. 

ABSTRACT Microbial communities occupy diverse niches in nature, and community members routinely exchange a variety of nutrients among themselves. While large-scale metagenomic and metabolomic studies shed some light on these exchanges, the contribution of individual species and the molecular details of specific interactions are difficult to track. In this study, we follow the exchange of vitamin B 1 (thiamin) and its intermediates between microbes within synthetic cocultures of Escherichia coli and Vibrio anguillarum . Thiamin contains two moieties, 4-amino-5-hydroxymethyl-2-methylpyrimidine (HMP) and 4-methyl-5-(2-hydroxyethyl)thiazole (THZ), which are synthesized by distinct pathways using enzymes ThiC and ThiG, respectively, and then coupled by ThiE to form thiamin. Even though E. coli Δ thiC , Δ thiE , and Δ thiG mutants are thiamin auxotrophs, we observed that cocultures of Δ thiC -Δ thiE and Δ thiC -Δ thiG mutants are able to grow in a thiamin-deficient medium, whereas the Δ thiE -Δ thiG coculture does not. Further, the exchange of thiamin and its intermediates in V. anguillarum cocultures and in mixed cocultures of V. anguillarum and E. coli revealed that there exist specific patterns for thiamin metabolism and exchange among these microbes. Our findings show that HMP is shared more frequently than THZ, concurrent with previous observations that free HMP and HMP auxotrophy is commonly found in various environments. Furthermore, we observe that the availability of exogenous thiamin in the media affects whether these strains interact with each other or grow independently. These findings collectively underscore the importance of the exchange of essential metabolites as a defining factor in building and modulating synthetic or natural microbial communities. IMPORTANCE Vitamin B 1 (thiamin) is an essential nutrient for cellular metabolism. Microorganisms that are unable to synthesize thiamin either fully or in part exogenously obtain it from their environment or via exchanges with other microbial members in their community. In this study, we created synthetic microbial cocultures that rely on sharing thiamin and its biosynthesis intermediates and observed that some of them are preferentially exchanged. We also observed that the coculture composition is dictated by the production and/or availability of thiamin and its intermediates. Our studies with synthetic cocultures provide the molecular basis for understanding thiamin sharing among microorganisms and lay out broad guidelines for setting up synthetic microbial cocultures by using the exchange of an essential metabolite as their foundation.