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Elucidating the chemical structure of dissolved organic matter (DOM) is key to understanding this large yet enigmatic carbon pool. Over the last two decades much progress has been made in assigning exceptionally accurate molecular formulas of DOM owing to the application of ultrahigh resolution mass spectrometry, but little is known about the number of isomers in each molecular formula, a question essentially related to the total number of organic molecules in DOM. Such information is critical for a further understanding of the formation and long-term stability of refractory DOM in the ocean. In this work, we used ion mobility quadrupole time of flight liquid chromatography tandem mass spectrometry (IM Q-TOF LC/MS/MS) to analyze DOM samples collected in different aquatic environments including south Texas rivers, Gulf of Mexico, and South China Sea. Our data showed that generally less than 23% of all detected DOM formulas, which shared a small fraction of common molecules (ca. 12%) detected by direct-infusion Fourier transform ion cyclotron resonance MS, contained structurally distinct isomers (represented by “isomer clusters”). In addition, isomer diversity, in terms of how different the structures are, decreased with degradation in both natural and incubation samples. Specifically, the number of structurally distinct isomers was lower at river mouth and open ocean than coastal waters, where organic matter tends to be fresher due to high primary production; and with depth in water column, isomer diversity of DOM also decreased. Results from a set of incubation experiments also showed that the percentages of riverine DOM molecules that have multiple isomer clusters decreased with time, suggesting that biodegradation decreases the diversity of molecules from an isomer perspective. Overall, these results suggested that isomers, at least in a certain fraction of DOM, are highly constrained, and that degradation decreases its isomeric diversity.

One of the major knowledge gaps in the study of plastic pollution is the understanding of residence times of these anthropogenic particles once they reach our oceans. Observations report a mismatch between estimates of plastic loads from worldwide plastic production and mismanaged plastic waste and actual plastic concentration seen floating at the sea-surface. Surveys of the water column -from the surface to the deep sea- are rare. Most of the recent efforts have thus addressed this question with modeling approaches or laboratory experiments that individuate in biofouling an important factor for the removal efficiency of plastics at sea and a likely explanation for the “missing plastic”. For the first time, we provide in-situ measured fluxes and removal rates of microplastics using deployments of drifting sediment traps in the North Atlantic Gyre from 50 m down to 600 m depth. We identified and quantified plastic contents with two different analytical approaches, FTIR and Py-GC/MS to determine polymer mass and particle distribution over depth. From derived data, interaction with biogenic polymers and thus particles transfer from the surface to the deep ocean are reconstructed. These findings shed a light on important pathways that regulate microplastics fate in marine ecosystems, from possible harmful repercussions on marine biota to impacts on fundamentals elements cycles.

Tropical cyclones are major disturbances for coastal systems. Hurricane Harvey made landfall in Texas, USA, on August 25, 2017 as a category 4 storm. There were two distinct disturbances associated with this storm that were spatially decoupled: (1) high winds causing direct damage and storm surge, and (2) high rains causing scouring floods and significant discharge of fresh water carrying carbon and nutrients to estuaries. Here, we provide a synthesis of the effects of Hurricane Harvey on biogeochemical, hydrographic, and biotic components of freshwater and estuarine systems and their comparative resistance and resilience to wind- and rain-driven disturbances. Wind-driven disturbances were most severe along the coastal barrier islands and lower estuaries, damaging mangroves and seagrass and increasing sediment coarseness. Rain-driven disturbances were most pronounced within freshwater streams and the upper estuaries. Large volumes of freshwater run-off reduced the abundance of riverine fauna and caused hypoxic and hyposaline conditions in the estuaries for over a week. In response to this freshwater input event, benthic fauna diversity and abundance decreased, but mobile fauna such as estuarine fishes did not markedly change. Although hydrographic and biogeochemical components were highly perturbed, they returned to baseline conditions within days. In contrast, biotic components demonstrated lower magnitude changes, but some of these organisms, particularly the sedentary flora and fauna, required weeks to months to return to pre-storm conditions, and some did not recover within the 6 months reported here. Our synthesis illustrates that resistance and resilience of system components may negatively co-vary and that structural components of coastal systems may be the most vulnerable to long-term changes following tropical cyclones.

Methane produced by methanogenic archaea has an important influence on Earth’s changing climate. Methanogenic archaea are phylogenetically diverse and widespread in anoxic environments. These microorganisms can be divided into two subgroups based on whether or not they use b-type cytochromes for energy conservation. Methanogens with b-type cytochromes have a wider substrate range and higher growth yields than those without them. To date, methanogens with b-type cytochromes were found exclusively in the phylum “Ca. Halobacteriota” (formerly part of the phylum Euryarchaeota). Here, we present the discovery of metagenome-assembled genomes harboring methyl-coenzyme M reductase genes reconstructed from mesophilic anoxic sediments, together with the previously reported thermophilic “Ca. Methylarchaeum tengchongensis”, representing a novel archaeal order, designated the “Ca. Methylarchaeales”, of the phylum Thermoproteota (formerly the TACK superphylum). These microorganisms contain genes required for methyl-reducing methanogenesis and the Wood-Ljundahl pathway. Importantly, the genus “Ca. Methanotowutia” of the “Ca. Methylarchaeales” encode a cytochrome b-containing heterodisulfide reductase (HdrDE) and methanophenazine-reducing hydrogenase complex that have similar gene arrangements to those found in methanogenic Methanosarcinales. Our results indicate that members of the “Ca. Methylarchaeales” are methanogens with cytochromes and can conserve energy via membrane-bound electron transport chains. Phylogenetic and amalgamated likelihood estimation analyses indicate that methanogens with cytochrome b-containing electron transfer complexes likely evolved before diversification of Thermoproteota or “Ca. Halobacteriota” in the early Archean Eon. Surveys of public sequence databases suggest that members of the lineage are globally distributed in anoxic sediments and may be important players in the methane cycle.

Characterizing the nature and effects of oil released into the marine environment is very challenging. It is generally recognized that "environmentally relevant" conditions for exposure involve a range of temporal and spatial conditions, a range of exposure pathways (e.g., dissolved, emulsions, sorbed onto particulates matter), and a multitude of organisms, populations, and ecosystems. Various exposure methodologies have been used to study the effects of oil on aquatic organisms, and uniform protocols and exposure methods have been developed for the purposes of regulatory toxicological assessments. Ultimately, all exposure methods have drawbacks, it is impossible to totally mimic field conditions, and the choice of exposure methodology depends on the specific regulatory, toxicological, or other research questions to be addressed. The aim of this paper is to provide a concise review of the state of knowledge to identify gaps in that knowledge and summarize challenges for the future.

Nurdles, also known as plastic resin pellets, are now a major source of plastic pollution on beaches globally, thus it is important to elucidate their weathering patterns and environmental fates as well as the associated pollutants. In this study we collected nurdles from 24 sites in the coastal bend region of south Texas, covering areas from the near shore railway stations to the adjacent bays and barrier islands. The morphologies of nurdles and associated pollutants including polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), and mercury, were investigated. The results showed that the nurdles varied greatly in color, shape, polymer composition, and oxidation degree. More than 80 % of the nurdles were made with polyethylene, and the rest with polypropylene, polyester, polystyrene, polyethylene-vinyl acetate, and polyvinyl chloride based on Fourier Transform Infrared Spectroscopy (FTIR) analysis. PCBs were not detected on nurdles. PAHs and mercury on nurdles were detected at 12 % and 20 % of the sampling sites. The total concentrations of detectable PAHs ranged from 92.59 to 1787.23 ng/g-nurdle, and the detectable mercury concentrations ranged from 1.23 to 22.25 ng/g-nurdle. Although the concentrations of these pollutants were not at the acute toxic effect level, the presence of PAHs and mercury suggested the potential risk of pollutant exposure to marine organisms in ecosystems, given the fact that nurdles are persistent in the environment.

Using 15N stable isotope as a tracer to quantify N transformation rates in isotope-enrichment experiments improves understanding of the N cycle in various ecosystems. However, measuring 15N-nitrate (15NO3−) in small volumes of water for these experiments is a major challenge due to the inconvenience of preparing samples by traditional techniques. We developed a “REOX/MIMS” method by applying membrane inlet mass spectrometry (MIMS) to determining 15NO3− concentrations in a small volumes of water from isotope-enrichment experiments after converting the dissolved inorganic N to N2. The nitrates (NO3− + NO2−) were reduced to NH4+ with zinc powder, and the ammonium (NH4+) was then oxidized to N2 by hypobromite iodine solution. The resulting 29N2 and 30N2 were measured via MIMS. This optimized protocol provides a sensitive (~0.1 μM) and precise (relative standard deviation = 0.1–4.37%) approach to quantify 15NO3− concentrations (0.1–500 µM) in water samples over a wide range of salinities (0–35‰) and in 2 M KCl solution with excellent calibration curves (R2 ≥ 0.9996, p

Nurdles, the pre-production plastic pellets, are a major source of plastic pollution in marine environments due to unregulated spills during production and transportation. We analyzed the types of plastics and associated organic pollutants on nurdles collected along the shoreline of Gulf of Mexico in Texas. Our results showed that the nurdles were made from polyethylene (81.9%) and polypropylene (18.1%). Polycyclic aromatic hydrocarbons (PAHs, 16 US EPA priority) and polychlorinated biphenyls (PCBs, 7 commercial congeners) sorbed to the nurdles were in concentration ranges of 1.6–14,700 ng/ g and 0–642 ng/ g, respectively. Heavily weathered nurdles tended to have higher concentrations of PAHs and PCBs than lightly weathered ones. The bioaccessibility of sorbed contaminants was evaluated using a simulated intestinal fluid. The results showed that the associated PAHs were more bioaccessible in lightly weathered nurdles (13.1 ± 2.3%) than heavily weathered one (5.3 ± 0.1%), and that no PCBs were bioaccessible. These findings are informative for toxicity evaluation and resource management of plastic debris in coastal environments.

The association between phytoplankton blooms and oil spills is still controversial despite numerous studies. Surprisingly, to date, there have been no studies on the effect of bacterial communities (BCs) exposed to crude oil on phytoplankton growth, even though crude oil changes BCs, which can then affect phytoplankton growth and species composition. Co-culture with crude oil-exposed BCs significantly stimulated the growth of Prorocentrum texanum in the laboratory. To gain more direct evidence, oil-degrading bacteria from oil-contaminated sediment collected after the Texas City "Y" oil spill were isolated, and changes in dinoflagellate growth when co-cultured with single bacterial isolates was investigated. The oil-degrading bacterial isolates significantly stimulated the growth of dinoflagellates (axenic and xenic cultures) through releasing growth-promoting substances. This study provides new evidence for the potential role of oil-degrading bacteria in the formation of phytoplankton blooms after an oil spill.

In response to the Deepwater Horizon oil spill, critical research has tracked the changes in petroleum hydrocarbons with environmental weathering. There are limitations, however, whereby single analytical techniques cannot always identify the wide breadth of petroleum and petroleum-derived compounds. We explore the analytical capabilities of ramped pyrolysis-gas chromatography–mass spectrometry (Py-GC–MS) to evaluate environmental samples of petroleum hydrocarbons from the Deepwater Horizon oil spill. We show that bulk flow Py-GC–MS can quantify the overall degree of petroleum hydrocarbon weathering. Furthermore, thermal slicing Py-GC–MS can quantify specific compounds in the “thermal desorption zone” (50–370 °C), as well as characterize pyrolyzed fragments from non-GC-amenable petroleum hydrocarbons (including oxygenated hydrocarbons) in the “cracking zone” (370–650 °C). Our data also suggest an increase in thermochemical stability, concentration of oxygenated products and complexity of high molecular weight and/or polar components with advanced weathering. This analysis not only elucidates weathering trends in Deepwater Horizon oil over several years, but also illustrates the analytical capacity of this method for future petroleum hydrocarbon investigations, filling a void in research connecting Py-GC–MS and environmentally weathered oil samples.

The Deepwater Horizon (DWH) blowout resulted in the deposition to the seafloor of up to 4.9% of 200 million gallons of oil released into the Gulf of Mexico. The petroleum hydrocarbon concentrations near the wellhead were high immediately after the spill, but returned to background levels a few years after the spill. Microbial communities in the seafloor are thought to be responsible for the degradation of hydrocarbons, however, our knowledge is primarily based upon gene diversity surveys and hydrocarbon concentration in field sediment samples. Here, we investigated the oil degradation potential and changes in bacterial community by amending seafloor sediment collected near the DWH site with crude oil and both oil and Corexit dispersant. Polycyclic aromatic hydrocarbons were rapidly degraded during the first 30 days of incubation, while alkanes were degraded more slowly. With the degradation of hydrocarbons, the relative abundances of Colwelliaceae, Alteromonadaceae, Methylococales, Alcanivorax, Bacteriovorax, and Phaeobacter increased remarkably. However, the abundances of oil-degrading bacteria changed with oil chemistry. Colwelliaceae decreased with increasing oil degradation, whereas Alcanivorax and Methylococcales increased considerably. We assembled seven genomes from the metagenome, including ones belonging to Colwellia, Alteromonadaceae, Rhodobacteraceae, the newly reported genus Woeseia, and candidate phylum NC10, all of which possess a repertoire of genes for hydrocarbon degradation. Moreover, genes related to hydrocarbon degradation were highly enriched in the oiled treatment, suggesting that the hydrocarbons were biodegraded, and that the indigenous microflora have a remarkable potential for the natural attenuation of spilled oil in the deep-sea surface sediment.

Extracellular hydrolysis cleaves large compounds into small molecules to be available to bacteria, which is a key step in decomposition of dissolved organic matter (DOM) in the ocean. To decipher factors controlling initial peptide hydrolysis, tetrapeptide ala-val-phe-ala (AVFA) was incubated for 2 h in coastal seawater treated as unfiltered, 0.2 μm-filtered, mercury chloride (HgCl 2)-added, autoclaved, and 0.2 μm-filtered autoclaved. Similar hydrolysis patterns were observed between unfiltered and 0.2 μm-filtered treatments, indicating initial peptide hydrolysis was due to free rather than attached enzymes. Hydrolysis was negligible in autoclaved treatments but significant in the HgCl2 treatment, suggesting that the hydrolytic enzymes were denatured by the high temperature during autoclaving but not affected by HgCl2 . Initial AVFA hydrolysis by free enzymes was dominated by aminopeptidases, derived from the hydrolyzed fragments. This study demonstrates the important role of free enzymes in initial peptide hydrolysis in coastal seawater, acting as the immediate responders to labile substrates in the DOM cycling.

The lability of small peptides was investigated along the salinity gradient of the James River estuary and lower Chesapeake Bay using two fluorescent analogs, Lucifer Yellow Anhydride-alanine-valine-phenylalanine-alanine (LYA-AVFA) and LYA-tetraalanine (LYA-AlA4). Hydrolysis rates of these compounds were compared with each other, and with uptake rates of potential hydrolysis products and other smaller derivatives (e.g., glutamic acid and dialanine). Results suggest that rates of peptide hydrolysis and uptake of hydrolysis products were not always correlated with each other along the salinity gradient, or to other environmental parameters measured in the James River and lower Chesapeake Bay. This may be because diverse input and removal processes can influence both peptide hydrolysis and uptake, but not necessarily simultaneously. Rates of both peptide hydrolysis and free amino acid uptake were strongly associated with particles, particularly those freshly produced. This suggests more rapid turnover of enzymatically available organic nitrogen in regions with elevated phytoplankton biomass. Changes in the abundance and composition of dissolved amino acids in these waters were also examined. Dissolved amino acid compositions, but not concentrations, varied with salinity along a gradient from the tidal fresh James River to the mouth of Chesapeake Bay. These compositional variations demonstrate the mixing of terrestrial organic nitrogen and in-situ production along the salinity transect.

Tin phosphide (Sn4P3) has arisen to be an anode material toward lithium-ion batteries (LIBs) with great potential for its high theoretic Li-storage capacity. Nevertheless, unavoidable capacity fading as well as poor rate capability have severely hindered the extended application of Sn4P3. In this paper, a hierarchical hybrid (Sn4P3-NC) has been fabricated by an easy freeze-drying followed by an annealing and phosphorization treatment. In this hybrid, Sn4P3 nanoparticles are well encapsulated into honeycomb-like nitrogen-doped carbon frameworks. Porous carbon provides multiple electron expressway and robust mechanical support. Furthermore, Sn4P3 nanoparticles are well confined in carbon matrix so as to effectively alleviate the volume expansion. Having profited from the desirable nanostructures and significantly enhanced synergetic effect, Sn4P3-NC as an anode for LIB achieves an extremely improved electrochemical property. A high capacity of ~881 mAh g−1 could be retained after 150 cycles at 100 mA g−1 and an enhanced rate and cycle performance with the discharge capacity of ~507 mAh g−1 at 1000 mA g−1 through 400 cycles is also exhibited.

The hydrolysis pathway of a given peptide or protein in seawater, or how it is hydrolyzed by extracellular enzymes, may depend on its chemical structure or specific amino acid composition. This knowledge is important for a better understanding of the first steps on decomposition of labile organic matter that is often dominated by proteins. The hydrolysis pathway can be estimated from the hydrolyzed fragments of small peptides through incubation experiments. Here we incubated a group of small peptides with different amino acid compositions in coastal seawater, including tetrapeptides alanine-valine-phenylalanine-alanine (AVFA) and its modifications of the N terminal A with valine (V), serine (S), arginine (R) and aspartic acid (D) (i.e., peptides VVFA, SVFA, RVFA, DVFA). Through these incubations, we evaluated the effect of chemical structure or specific amino acid composition on peptide hydrolysis pathways and the roles of different types of peptidases. Our results showed that aminopeptidases preferentially hydrolyzed tetrapeptides with N terminal A and R (29–100% of the total peptidases), followed by V (30–90%) and S (20–51%), but were ineffective with acidic amino acid D (0–14%). This pattern indicates that the N-terminal amino acid affects how a peptide is hydrolyzed in a given environment. For N-terminal amino acids with uncharged side chains, aminopeptidases preferred hydrophobic A, V to polar uncharged S, and for those with charged side chains, they preferred positively-charged R to negatively-charged D. Our results also suggest that if aminopeptidases could not access the N-terminal amino acid of a peptide, carboxypeptidases and/or endopeptidases will hydrolyze it, leading to similar peptide hydrolysis rates among different peptides although their hydrolysis pathways may differ. The effect of amino acid composition on peptide hydrolysis pathways is further demonstrated by comparing trialanine (AAA), AVF, AVFA, VFA, phenylalanine-alanine-serine-tryptophan-glycine-alanine (FASWGA) and SWGA. Overall, results from this study show that the chemical structure of a peptide affects its hydrolysis pathway in coastal seawater, and shed new lights on the early steps of organic nitrogen degradation and further nitrogen cycling in marine environments.

Cyanobacterial harmful algal blooms (CyanoHABs) are enhanced by anthropogenic pressures, including excessive nutrient (nitrogen, N, and phosphorus, P) inputs and a warming climate. Severe eutrophication in aquatic systems is often manifested as non-N2-fixing CyanoHABs (e.g., Microcystis spp.), but the biogeochemical relationship between N inputs/dynamics and CyanoHABs needs definition. Community biological ammonium (NH4+) demand (CBAD) relates N dynamics to total microbial productivity and NH4+ deprivation in aquatic systems. A mechanistic conceptual model was constructed by combining nutrient cycling and CBAD observations from a spectrum of lakes to assess N cycling interactions with CyanoHABs. Model predictions were supported with CBAD data from a Microcystis bloom in Maumee Bay, Lake Erie, during summer 2015. Nitrogen compounds are transformed to reduced, more bioavailable forms (e.g., NH4+ and urea) favored by CyanoHABs. During blooms, algal biomass increases faster than internal NH4+ regeneration rat...

An important aspect of oil spill science is understanding how the compounds within spilled oil, especially toxic components, change with weathering. In this study we follow the evolution of petroleum hydrocarbons, including n-alkanes, polycyclic aromatic hydrocarbons (PAHs) and alkylated PAHs, on a Louisiana beach and salt marsh for three years following the Deepwater Horizon spill. Relative to source oil, we report overall depletion of low molecular weight n-alkanes and PAHs in all locations with time. The magnitude of depletion, however, depends on the sampling location, whereby sites with highest wave energy have highest compound depletion. Oiled sediment from an enclosed bay shows high enrichment of high molecular weight PAHs relative to 17α(H),21β(H)-hopane, suggesting the contribution from sources other than the Deepwater Horizon spill, such as fossil fuel burning. This insight into hydrocarbon persistence as a function of hydrography and hydrocarbon source can inform policy and response for future spills.

Gelatinous zooplankton are known for their capacity to excrete copious amounts of mucus that can be utilized by other organisms. The release of mucus is exacerbated by stressful conditions. Despite the recognized importance of cnidarian mucus to production and material flux in marine ecosystems, the role of gelatinous zooplankton in influencing the fate of oil spills is unknown. In this study we used laboratory experiments to observe the influence of mucus from the moon jellyfish (Aurelia aurita) on the aggregation and degradation of crude oil. The results show that jellyfish swimming in a dispersed solution of oil droplets produced copious amounts of mucus and the mucus aggregates that were shed by the animals contained 26 times more oil than the surrounding water. Incubation experiments showed that hydrocarbon degrading bacteria cell densities more than doubled in the presence of mucus and after 14 days, resulted in a significant increase in oil degradation. These results suggest that jellyfish can aggregate dispersed oil droplets and embed them within a matrix that favors hydrocarbon degrading bacteria. While this study lends support to the hypothesis that the presence of gelatinous zooplankton can impact oil spills large scale mesocosm studies will be needed to fully quantify the influence on a natural system.

Riverine dissolved organic matter (DOM) is a major source of reduced carbon exported from land to marine environments, and the inflow of riverine organic matter greatly affects biogeochemical cycling in estuaries and bays. Thus, any change in DOM composition, such as changes caused by flood waters as a result of storms and hurricanes, can subsequently affect estuarine environments. To investigate the impact of high flow events on riverine DOM, multidimensional molecular level information on DOM from four South Texas Rivers (Aransas, Lavaca, Mission, and Nueces Rivers) was acquired using high-resolution Ion Mobility Quadrupole Time of Flight Liquid Chromatography Mass Spectrometry (IM Q-TOF LCMS). Base-flow samples were collected in May, July, and October of 2016, June of 2017, and March of 2018, while high-flow samples were collected in September of 2017, as well as June and September of 2018. Based on the molecular formulas assigned from IM Q-TOF LCMS, the H/C ratio decreased during high-flow events (1.52 to 1.51 in ESI+ and 1.19 to 1.07 in ESI−), while the O/C ratio increased (0.31 to 0.33 in ESI−). Furthermore, DOM shifted from a protein-like and lipid-like dominated community at base flow conditions, to a lignin, tannin and condensed aromatic structure dominated community during high flow events, based on MS and tandem MS data. These changes in high-flow riverine DOM indicate an increase of terrestrial signal, which is likely a result of mobilization of terrestrial organic matter from the watersheds by flooding. The mobilized DOM, though refractory at high-flow conditions in rivers, could be reactive in coastal regions when conditions change, and thus could potentially fuel microbial activities downstream. In addition, about 3.76–21.8% of DOM molecules contain structural isomers among different flow conditions. This low number of isomer percentages suggests that DOM, as the products of various enzymatic biochemical reactions, is constrained in the number of isomers. Taken together, results from our study provide insights into structural changes of riverine DOM in response to extreme climate events in subtropical regions and have implications in understanding biogeochemical changes in estuaries under a changing climate.

Proteins and peptides account for 20–75% of marine biota biomass, of which a major fraction is metabolized by bacteria, thus deciphering interactions between bacteria and peptides is important in understanding marine carbon and nitrogen cycling. To better understand capabilities of different bacterial strains on peptide decomposition, four Gammaproteobacteria (Pseudoalteromonas atlantica, Alteromonas sp., Marinobacterium jannaschii, Amphritea japonica) were incubated in autoclaved seawater amended with tetrapeptide alanine-valine-phenylalanine-alanine (AVFA), a fragment of RuBisCO. While AVFA was decomposed greatly by Pseudoalteromonas atlantica and Alteromonas sp, it remained nearly intact in the Marinobacterium jannaschii and Amphritea japonica incubations. Pseudoalteromonas and Alteromonas decomposed AVFA mainly through extracellular hydrolysis pathway, releasing 71–85% of the AVFA as hydrolysis products to the surrounding seawater. Overall, this study showed that Gammaproteobacterial strains differ greatly in their capabilities of metabolizing peptides physiologically, providing insights into interactions of bacteria and labile organic matter in marine environments.

During oil spills in the field or for laboratory incubation studies, different oil concentrations are often encountered or applied, yet how initial oil concentration affects biodegradation rates of hydrocarbons and the development of oil degraders remains unclear. We incubated seawater for 50 d with different oil concentrations (0, 50, 100, 200, 400 and 800 ppm). n-Alkanes and polycyclic aromatic hydrocarbons (PAHs), and the bacterial community were analyzed periodically. Results show that the biodegradation rates of alkanes, derived from first order kinetics, decreased with increasing oil concentration, but percent residual was ~50% regardless of the initial concentration. In contrast, the biodegradation rates of PAHs increased with concentration, and the percent residual increased with oil concentration. Increasing oil concentration resulted in increased abundances of Rhodobacterales, Altererythrobacter, and Neptuniibacter. However, Alcanivorax abundance was barely detected in 400 and 800 ppm. Overall, oil concentration critically affected the degradation of hydrocarbons and the bacterial community.

Photooxidation was an important weathering process for spilled oil at the water surface following the Deepwater Horizon oil spill, leading to the rapid formation of polar or oxygenated hydrocarbons, or photo-generated asphaltenes. While the great extent and rapid time scale of this process have been well documented, little is known about the formation pathways and chemical structures of photo-generated asphaltenes. To address this knowledge gap, we conducted a 44-day natural sunlight exposure experiment using Louisiana light sweet crude oil amended in seawater. The saturate, aromatic, resin and asphaltene fractions were separated and quantified by adsorption chromatography. Molecular details were obtained by thermal slicing ramped pyrolysis-gas chromatography–mass spectrometry, while bulk structural changes were quantified by 13C solid-state nuclear magnetic resonance. These analyses illustrate that, in comparison to the “native” petrogenic asphaltenes in crude oil, photo-generated asphaltenes had larger bridged-island structures with smaller aromatic cores and longer alkyl tails in each island. Our results also demonstrated that the large aliphatic fraction of the photo-generated asphaltene was sourced from the alkyl moieties of the aromatic fraction rather than the saturate fraction. A group of intact hydrocarbons, predominantly n-alkanes (C16–C28), was occluded in both native and photo-generated asphaltene matrices. Overall, these results provide insight into the formation mechanisms and structural details of photo-generated asphaltenes in oils after exposure to natural sunlight in Gulf of Mexico waters. This knowledge is valuable for developing remediation strategies for oil spills, in particular those under strong solar irradiance.

Wetland sediments undergo dry-wet cycles that may change their structural properties and affect geochemical behavior of associated organic compounds. In this study, we examined the effect of drying on particle size distributions and the rapid (24h) sorption reactions of polycyclic aromatic hydrocarbons (PAHs) with salt marsh sediments in Nueces Delta, South Texas. Drying reduced the fraction of fine particles in organically richer sediments, indicating structural rearrangement of organic matter and mineral aggregates. Among the 16 EPA priority PAHs examined, dried sediment preferentially released 1.0-7.5% of phenanthrene, fluoranthene and pyrene to added seawater (solid: water mass ratio of 1/100) - significantly greater than release from sediments maintained in the wet state. On the other hand, drying also increased the affinity of sedimentary organic matter (SOM) for experimentally amended (deuterated) phenanthrene relative to continually wet sediments. Further, deuterated phenanthrene was even more effectively retained when it was added to wet sediment that was subsequently dried and rewetted. These apparently contradictory results can be reconciled and explained by SOM having a heterogeneous distribution of hydrophobic and hydrophilic zones - e.g., a zonal model. We propose that drying changed the orientation of amphiphilic SOM, exposing hydrophobic zones and promoting the release of some of their native PAHs to water. Freshly amended PAHs were only able to penetrate into the surface hydrophobic zone and/or deeper but rapidly accessible ("kinetic") zone in wet sediments due to the brief adsorption contact time. Subsequent drying presumably then induced structural changes in SOM that isolated these amended PAHs in sites inaccessible to water exchange in the next rewetting. These results provide insights into structural changes of SOM upon drying, and help predict the fate of compounds such as organic contaminants during drought/flood oscillations.

This study evaluated impacts of the BP Deepwater Horizon ( DWH ) oil spill on petroleum hydrocarbons in surface waters of the Louisiana continental shelf in northern Gulf of Mexico. Surface water (~top 5 cm) without visible oil was collected from three cruises in May 2010 during the oil spill, August 2010 after the well was capped, and May 2011 one year after the spill. Concentrations of total dissolved n -alkanes (C 9 –C 35 ) in surface seawater were more than an order of magnitude higher in May 2010 than August 2010 and May 2011, indicating contamination by the DWH oil spill. This conclusion was further supported by more abundant smaller n -alkanes (C 9 –C 13 ), together with pristane and phytane, in May than August 2010 samples. In contrast, even carbon-numbered dissolved n -alkanes (C 14 –C 20 ) dominated the May 2011 samples, and this distribution pattern of dissolved n -alkanes is the first documentation for water samples in the northern Gulf of Mexico. However, this pattern was not observed in May 2011 suspended particles except for Sta. OSS. This decoupling between dissolved and particle compositions suggests that either these even carbon-numbered n -alkanes originated from bacteria rather than algae, or that the alkanes in the shelf were transported from elsewhere. Concentrations of polycyclic aromatic hydrocarbons (PAHs) in suspended particles were 5 times higher on average in May 2010 (83–252 ng L −1 ) than May 2011 (7.2–83 ng L −1 ), also indicating contamination by the DWH oil spill. Application of a biomarker ratio of 17α(H),21β(H)-30-norhopane over 17α(H),21β(H)-hopane confirmed that suspended particles from at least two stations were contaminated by the DWH oil spill in May 2010. Taken together, these results showed that surface waters of the sampling area in May 2010 were contaminated by the oil spill, but also that rapid weathering and/or physical dilution quickly reduced hydrocarbon levels by August 2010.

Peptide analogs, such as leucine-methylcoumarinylamide (Leu-MCA) and Lucifer yellow anhydride-tetraalanine (LYA-Ala4), are often used as proxies to quantify extracellular peptidase activities, but how accurately these analogs represent natural peptides remains unclear. Here we compared hydrolysis rates of tetrapeptides alanine-valine-phenylalanine-alanine (AVFA) and tetraalanine (Ala4) with their corresponding LYA analogs along a salinity gradient in the northern Gulf of Mexico Mississippi River plume. Hydrolysis rates were generally similar, but not always, between natural peptides and their analogs in the plume water. For example, hydrolysis rates were similar between LYA-AVFA and AVFA at all stations except one. In contrast, hydrolysis rates of LYA-Ala4 were significantly higher than those of Ala4 at low-salinity (18–19) stations, but significantly lower at one high-salinity (36) station. These results suggest that relative activities of different types of peptidases, measured by the analogs or peptides, depended on biological and chemical properties at each station, such as abundance of certain “opportuni-trophic” bacteria, including Flavobacterium , Ruegeria and Roseobacter . The produced fragments and amino acids from peptide hydrolysis implied further that the hydrolysis pathway depended on specific peptide substrate, and that the LYA tag affected the hydrolysis pathway. Taken together, this study validates the technique of using LYA analogs to study extracellular enzymatic activities, particularly the overall hydrolysis rate, and provides insights into the patterns of extracellular enzymatic activities in estuarine environments.

Dissolved organic matter (DOM) is one of the key carbon substrates transported by rivers from land to sea, yet only a small fraction of it has been characterized at the molecular level. Here we characterize high molecular weight DOM (HMW-DOM, >1 kDa) isolated by ultrafiltration from nine North American rivers using thermal slicing ramped pyrolysis gas chromatography mass spectrometry (TSRP-GC–MS), carbon and nitrogen stable isotopes, elemental composition and total hydrolyzable amino acids (THAA). Samples from the more anthropogenically altered watersheds carried more enriched δ15N-HMW-DOM signatures than observed in samples from the more pristine watersheds. The TSRP-GC–MS results, coupled with THAA data, constrained DOM degradation process. Specifically, the diversity of pyrolyzates of HMW-DOM samples correlated well with the degradation index derived from the THAA data, with more degraded samples being more thermally stable and less molecularly diverse. The relationship between degradation index and pyrolyzate diversity observed in the HMW-DOM samples from across such a broad spatial scale suggests that degradative processes reduce riverine DOM complexity through the loss of more reactive moieties. Overall, TSRP-GC–MS, in combination with specific compound or isotope analyses, illuminates processes controlling DOM composition in different North American river systems with varying degrees of bio-lability and anthropogenic influence.

To examine the bioavailability of dissolved organic carbon (DOC) and nitrogen (DON) in riverine dissolved organic matter (DOM) discharged to the coastal ocean, we conducted a series of month-long (24 days) incubation experiments with filtered samples collected from five southern Texas rivers (Lavaca, San Antonio, Mission, Aransas, and Nueces) inoculated using the same natural coastal microbial assemblages during summer (June) and winter (January) in 2016. The bioavailable fractions of DOC and DON (BDOC% and BDON%) varied substantially in different rivers and seasons, ranging respectively from 6–11% and 15–38% during winter, and 0–6% and 9–15% during summer. Relatively higher BDOC% and BDON% occurred in the San Antonio and Aransas Rivers, which are impacted more by human activities through discharge from wastewater treatment plants. Seasonally, the riverine DOM was more bioavailable in winter than in summer when DOM may have been extensively degraded in situ due to the low base flow (or long residence time) and the elevated temperature in river water in summer. The principal component analysis on amino acid composition further confirmed that DOM was less degraded in winter than in summer. Functional gene abundance data revealed that winter riverine DOM was relatively labile as evidenced by an increase in N-metabolism pathways and functional genes during the winter incubation, whereas the opposite pattern was observed in summer. The findings of the varying bioavailability of DOM among rivers and seasons have important implications about the fate of riverine DOM and their potential contributions to nutrient supplies as southern Texas bays and estuaries are often nitrogen limited.

Deciphering molecular structures of dissolved organic matter (DOM) components is key to understanding the formation and transformation of this globally important carbon pool in aquatic environments. Such a task depends on the integrated use of complementary analytical techniques. We characterize the molecular structure of natural DOM using an ion mobility quadrupole time of flight liquid chromatography mass spectrometer (IM Q-TOF LC/MS), which provides multidimensional structural information on DOM molecules. Geometric conformation of DOM molecules is introduced into molecular-level analysis via the ion mobility (IM) in the system, and an actual measurement of isomers is achieved for the first time. Our data show that natural DOM molecules from several south Texas rivers and adjacent coastal waters have smaller geometric conformation compared with standard biomolecules. Furthermore, about 10% of all DOM molecules resolved within the detection limit of IM-MS had at least one but no more than four isomers. With acquired geometric and isomeric information, we established a multidimensional database containing 89 natural DOM compounds. This database provides a foundation to expand further, or compare, with DOM data from different seasons and locations.

Lake Taihu, the third largest freshwater lake in China, suffers from harmful cyanobacteria blooms caused by Microcystis spp., which do not fix nitrogen (N). Reduced N (i.e., NH4+, urea and other labile organic N compounds) is an important factor affecting the growth of Microcystis. As the world use of urea as fertilizer has escalated during the past decades, an understanding of how urea cycling relates to blooms of Microcystis is critical to predicting, controlling and alleviating the problem. In this study, the cycling rates of urea-N in Lake Taihu ranged from non-detectable to 1.37 μmol N L−1 h−1 for regeneration, and from 0.042 μmol N L−1 h−1 to 2.27 μmol N L−1 h−1 for potential urea-N removal. The fate of urea-N differed between light and dark incubations. Increased 15NH4+ accumulated and higher quantities of the removed urea-15N remained in the 15NH4+ form were detected in the dark than in the light. A follow-up incubation experiment with 15N-urea confirmed that Microcystis can grow on urea but its effects on urea dynamics were minor, indicating that Microcystis was not the major factor causing the observed fates of urea under different light conditions in Lake Taihu. Bacterial community composition and predicted functional gene data suggested that heterotrophic bacteria metabolized urea, even though Microcystis spp. was the dominant bloom organism.

The Deepwater Horizon Oil Spill (DWH) is recognized as the worst oil spill in the U.S. history and resulted to the release of more than 4.9 million barrels of Macondo crude oil into the Gulf of Mexico. From the depth of 1500 m, oil was deposited to the seafloor, formed a plume in the deep water, ascended to the surface, and contaminated the coastal waters and coastline. As the oil partitioned within and from the deep and open ocean to the shore, the oil chemistry changed as a result of its interactions with different environmental factors. The evolution and fate of the oil following the DWH spill were mainly influenced by bacterial community, temperature, sunlight, nutrients, shoreline energy, and pressure. Here, we discuss how these factors affected the biodegradation of the spilled oil as determined by laboratory incubation studies, environmentally relevant experiments, and actual field observations.