Peter H. Santschi

Isotopic evidence of groundwater and stream water is frequently used to investigate water exchanges with groundwater. Monthly sampling of rain, stream water, and groundwater was conducted at Tims Branch watershed in South Carolina for the oxygen and hydrogen stable isotope (delta 2H and delta 18O) measurement, as well as pH and oxidation-reduction potential (ORP). Together with a mass balance perspective, it was determined that it takes a few weeks to one month for groundwater in the hyporheic zone to fully exchange with stream water. From hydrodynamic modelling, we show that substantial (up to 70 %) groundwater exchange occurs at gaining and losing sites. Groundwater exfiltration, i.e. inflow into stream water, contributes up to 4 % to stream water, with the remainder from upstream exfiltration. A 2-4 % per day renewal rate of adjacent groundwater would indirectly indicate a groundwater residence time in the order of half a month to a full month (assuming either a well-mixed case or large dispersion rate in pulse flow case), in agreement with a greatly reduced variability of delta 2H and delta 18O of groundwater compared to stream water and rain. This reduced variability of stable isotope signal from groundwater confirms our hypothesis that riparian groundwater mixing at Tims Branch is more of a mixed type rather than a pulse flow type. A monthly time scale is sufficient for groundwater to become anoxic at exit points into stream water resulting in the episodic production of natural organic matter- and iron-rich flocs upon oxidation.

Plastics are produced, consumed, and disposed of worldwide, with more than eight million tons of plastic litter entering the ocean each year. Plastic litter accumulates in marine and terrestrial environments through a variety of pathways. Large plastic debris can be broken down into micro- and nano-plastic particles through physical/mechanical mechanisms and biologically or chemically mediated degradation. Their toxicity to aquatic organisms includes the scavenging of pollutant compounds and the production of reactive oxygen species (ROS). Higher levels of ROS cause oxidative damages to microalgae and bacteria; this triggers the release of large amounts of exopolymeric substances (EPSs) with distinct molecular characteristics. This review will address what is known about the molecular mechanisms phytoplankton and bacteria use to regulate the fate and transport of plastic particles and identify the knowledge gaps, which should be considered in future research. In particular, the microbial communities react to plastic pollution through the production of EPSs that can reduce the plastic impacts via marine plastic snow (MPS) formation, allowing plastics to settle into sediments and facilitating their removal from the water column to lessen the plastic burden to ecosystems.

The objective of this study was to determine if U sediment concentrations in a U-contaminated wetland located within the Savannah River Site, South Carolina, were greater in the rhizosphere than in the nonrhizosphere. U concentrations were as much as 1100% greater in the rhizosphere than in the nonrhizosphere fractions; however and importantly, not all paired samples followed this trend. Iron (but not C, N, or S) concentrations were significantly enriched in the rhizosphere. XAS analyses showed that in both sediment fractions, U existed as UO₂²⁺ coordinated with iron(III)-oxides and organic matter. A key difference between the two sediment fractions was that a larger proportion of U was adsorbed to Fe(III)-oxides, not organic matter, in the rhizosphere, where significantly greater total Fe concentrations and greater proportions of ferrihydrite and goethite existed. Based on 16S rRNA analyses, most bacterial sequences in both paired samples were heterotrophs, and population differences were consistent with the generally more oxidizing conditions in the rhizosphere. Finally, U was very strongly bound to the whole (unfractionated) sediments, with an average desorption K_d value (U_sediment/U_aqueous) of 3972 ± 1370 (mg-U/kg)/(mg-U/L). Together, these results indicate that the rhizosphere can greatly enrich U especially in wetland areas, where roots promote the formation of reactive Fe(III)-oxides.

The focus of our project is on understanding natural organic matter (NOM) properties and their influence on U remobilization/immobilization, considering the important roles of watersheds hydrology and microbial dynamics. Laboratory and analytical techniques assessed these dynamic processes at ambient U and OM concentrations, in order to find favorable environmental settings that make the wetland, stream sediment, and flocs an organic C and/or U sink or source. We tested the following hypotheses: Hypothesis 1: Source, abundance and composition of OM in porewater, groundwater and stream water are strongly regulated by the seasonal variation in hydrological conditions of watersheds (Eh, pH, rainfall, stream water discharge, temperature, nutrients loading like nitrate-nitrite, sulfate, etc.), which determines the OM-uranium interaction and thus the immobilization/remobilization of uranium-carrying OM in watersheds. Hypothesis 2: Solid phases in the interfacial zones (wetland surface sediment, subsurface sediment, stream sediment, and flocs) contain distinct OM, the molecular characteristics of which shows seasonal variations, yet selected ubiquitous functionalities of OM are responsible for binding uranium. In addition, the mobility of U-carrying OM molecules of different sources will be determined by their macro-molecular environment (molecular weight, hydrophobicity, lability and association with mineral phases). Hypothesis 3: Microbial communities inhabiting the interfacial zones possess distinctmore » functional traits that regulate the formation and degradation of sediment OM (largely as a function of water-driven changes in redox conditions); thus, shaping OM functionality as it relates to U mobility. NOM/U related hypotheses were tested in laboratory-scale experiments using samples from Tims Branch stream and soil sediments at gaining and loosing sites along the riparian zone, while hydrology related hypotheses were tested using stable isotope measurements of stream and riparian zone groundwater, combined with simulations of a numerical hydrological model. Results revealed 1) numerical simulations of a watershed model could reproduce excursions in stable isotope results from stream and groundwater. This is remarkable given the fact that rainwater data was collected 30 km to the north, given that rains storms often are very local rather than regional. In addition, stable isotope data can only provide a snapshot of the conditions on that part of the day when the rain and stream water sample was taken. 2) Using ESI-FT-ICR-MS results of NOM extracts of various samples, U-containing formulas are more enriched and predominant in CHON groups, followed by CHOS and CHO groups, as compared to the bulk total sedimentary OM. Thus this observation suggests that, U-binding ligands are likely N-containing moieties, especially embedded onto the some big biogeomolecular backbones derived from, like lignin - (40%) and proteins may be especially effective at binding U-like (19%) compounds. U-containing formulas in the ‘mobile’ fraction are more freshly produced with a lower number-averaged-NOSC, lower N,S, and P content, more enriched in CHO group and less aromatic, compared to those in the ‘immobile’ fraction. Iron has likely played an important role in stabilizing sedimentary OM, especially U-containing compounds. U-containing formulas from both the redox interfacial zones (bank and groundwater discharge or ‘gaining’ site), as compared to their counterparts (upland and groundwater recharge or ‘losing’ site), displayed some common trends including 1) smaller MW; 2) lower NOSC; 3) higher contents of COO and/or CONO functional groups; and 4) higher abundance of Fe, U-containing formulas with Fe. « less

Quorum sensing is a communication system by which bacteria use signal molecules to induce a physiological response. In natural marine environments, quorum sensing is suspected to occur in regions with high cell densities. Free-living bacteria, however, are largely believed to exist at concentrations too low to make use of a density-dependent quorum-sensing system. Due to so-called ‘free-living’ bacteria inhabiting marine gels composed of exopolymeric substances occurring in an operationally defined colloidal fraction, it is possible that quorum-sensing molecules would also occur in this fraction. In this study, possible signaling molecules were collected from marine water, separated, and concentrated as some organic matter. Their identification with an Agrobacterium tumefaciens assay indicated the presence of AHLs. In fall 2002 and spring 2003, free-living (single cells) fraction of marine bacterioplankton was sampled and separated. Various AHLs were added to these disperse populations (10⁹ cells/L). The studied bacterial communities responded with change in the thymidine incorporation. The results are discussed as existence of active reaction of marine free-living bacteria to signaling molecules even in sparse inhabiting marine gels. That there is a bacterial response to signaling molecules in so sparse populations is of great significance for marine and environmental sciences.

The aquaculture industry in Taiwan grosses more than USD 1.1 billion annually; however, it also generates considerable waste discharge (causing eutrophication in estuarine and coastal waters) and heavy groundwater withdrawals (causing land subsidence in coastal areas). Many aquaculture facilities using earth ponds are affected by benthic algae, resulting in an earthy odor, and fixed-cage farms are difficult to relocate during cold weather events. In this study, we tested small-scale (~15 ton) mobile cage tanks for the nearshore rearing of white shrimp and grouper in the Yung-An district of Kaohsiung, Taiwan. At the conclusion of the mariculture experiment, the content of free amino acids in shrimp and groupers reared in our mobile tanks surpassed that in animals reared locally in traditional earthy ponds. In a blind taste test involving 42 volunteers, groupers reared in mobile cage tanks were deemed more palatable than those raised in ponds. Our results demonstrate that small-scale mobile cage tanks are a feasible approach to the sustainable rearing of high-quality shrimp or fish. Note that wastewater from the mobile tanks is easily diluted by seawater, thereby reducing the likelihood of eutrophication in coastal regions. The proposed system could also be used for recreational fishing activities to increase income for smallholders of fishermen and/or aquaculture farmers.

Marine gels (nano-, micro-, macro-) and marine snow play important roles in regulating global and basin-scale ocean biogeochemical cycling. Exopolymeric substances (EPS) including transparent exopolymer particles (TEP) that form from nano-gel precursors are abundant materials in the ocean, accounting for an estimated 700 Gt of carbon in seawater. This supports local microbial communities that play a critical role in the cycling of carbon and other macro- and micro-elements in the ocean. Recent studies have furthered our understanding of the formation and properties of these materials, but the relationship between the microbial polymers released into the ocean and marine snow remains unclear. Recent studies suggest developing a (relatively) simple model that is tractable and related to the available data will enable us to step forward into new research by following marine snow formation under different conditions. In this review, we synthesize the chemical and physical processes. We emphasize where these connections may lead to a predictive, mechanistic understanding of the role of gels in marine snow formation and the biogeochemical functioning of the ocean.

Microgels play critical roles in a variety of processes in the ocean, including element cycling, particle interactions, microbial ecology, food web dynamics, air–sea exchange, and pollutant distribution and transport. Exopolymeric substances (EPS) from various marine microbes are one of the major sources for marine microgels. Due to their amphiphilic nature, many types of pollutants, especially hydrophobic ones, have been found to preferentially associate with marine microgels. The interactions between pollutants and microgels can significantly impact the transport, sedimentation, distribution, and the ultimate fate of these pollutants in the ocean. This review on marine gels focuses on the discussion of the interactions between gel-forming EPS and pollutants, such as oil and other hydrophobic pollutants, nanoparticles, and metal ions.

The cycling and fate of polycyclic aromatic hydrocarbons (PAHs) is not well understood in estuarine systems. It is critical now more than ever given the increased ecosystem pressures on these critical coastal habitats. A budget of PAHs and cycling has been created for Galveston Bay (Texas) in the northwestern Gulf of Mexico, an estuary surrounded by 30–50% of the US capacity of oil refineries and chemical industry. We estimate that approximately 3 to 4 mt per year of pyrogenic PAHs are introduced to Galveston Bay via gaseous exchange from the atmosphere (ca. 2 mt/year) in addition to numerous spills of petrogenic PAHs from oil and gas operations (ca. 1.0 to 1.9 mt/year). PAHs are cycled through and stored in the biota, and ca. 20 to 30% of the total (0.8 to 1.5 mt per year) are estimated to be buried in the sediments. Oysters concentrate PAHs to levels above their surroundings (water and sediments) and contain substantially greater concentrations than other fish catch (shrimp, blue crabs and fin fish). Smaller organisms (infaunal invertebrates, phytoplankton and zooplankton) might also retain a significant fraction of the total, but direct evidence for this is lacking. The amount of PAHs delivered to humans in seafood, based on reported landings, is trivially small compared to the total inputs, sediment accumulation and other possible fates (metabolic remineralization, export in tides, etc.), which remain poorly known. The generally higher concentrations in biota from Galveston Bay compared to other coastal habitats can be attributed to both intermittent spills of gas and oil and the bay's close proximity to high production of pyrogenic PAHs within the urban industrial complex of the city of Houston as well as periodic flood events that transport PAHs from land surfaces to the Bay.