Pore water transport and microbial activity in intertidal Wadden Sea sediments

Pore water transport and its implications for mineralization and nutrient release from the sediment were studied in intertidal permeable sands of the German Wadden Sea. During ebb tide, the hydraulic gradient developing between the pore water level and the faster dropping sea water level caused drainage and seepage of pore water from the sediment near the low water line. As a result, pore water nutrient and DIC concentrations were independent of the season and up to 15-times higher than at the upper flat. Nutrient fluxes associated with the seepage exceeded 5 to 8-fold those fluxes caused by the combined effects of diffusion, advection and bioirrigation during inundation and may enhance primary production in the Wadden Sea. The drainage affects deep sediment layers and is characterized by long flow paths and pore water residence times. This 'body circulation' is only active during low tide and can act as buffered nutrient source to the ecosystem.During inundation of the tidal flat, oxygen penetrated deeper into the sediment than during exposure, driven by hydrodynamic forcing. Oxygen consumption rates were high and sulfate reduction contributed 3-25 % to total mineralization. In another intertidal sand flat, oxygen consumption and sulfate reduction rates decreased from the low water line towards the upper flat and were closely linked to inundation time. The advectively flushed surface layer of the sediment is characterized by short flow paths and low pore water residence time. An immediate feedback of benthic mineralization to the ecosystem can be provided by this 'skin circulation' during inundation. Intertidal sands were also shown to support relatively high benthic primary production while submersed. Gross photosynthesis was on average 4 and 11 times higher in the net autotrophic fine and coarse sand than in the net heterotrophic mud, despite higher chlorophyll content in the mud. Light limitation was less severe at the sandy sites, and two to three times more light was available to the microalgae in the sands than in the mud. The advective flushing of the permeable sands removed chlorophyll decomposition products and may have enhanced benthic photosynthesis by counteracting a possible CO2 limitation of the microalgae

Porenwasser-Transport und mikrobielle Aktivität im Gezeitenbereich des Wattenmeeres

Pore water transport and its implications for mineralization and nutrient release from the sediment were studied in intertidal permeable sands of the German Wadden Sea. During ebb tide, the hydraulic gradient developing between the pore water level and the faster dropping sea water level caused drainage and seepage of pore water from the sediment near the low water line. As a result, pore water nutrient and DIC concentrations were independent of the season and up to 15-times higher than at the upper flat. Nutrient fluxes associated with the seepage exceeded 5 to 8-fold those fluxes caused by the combined effects of diffusion, advection and bioirrigation during inundation and may enhance primary production in the Wadden Sea. The drainage affects deep sediment layers and is characterized by long flow paths and pore water residence times. This 'body circulation' is only active during low tide and can act as buffered nutrient source to the ecosystem.During inundation of the tidal flat, oxygen penetrated deeper into the sediment than during exposure, driven by hydrodynamic forcing. Oxygen consumption rates were high and sulfate reduction contributed 3-25 % to total mineralization. In another intertidal sand flat, oxygen consumption and sulfate reduction rates decreased from the low water line towards the upper flat and were closely linked to inundation time. The advectively flushed surface layer of the sediment is characterized by short flow paths and low pore water residence time. An immediate feedback of benthic mineralization to the ecosystem can be provided by this 'skin circulation' during inundation. Intertidal sands were also shown to support relatively high benthic primary production while submersed. Gross photosynthesis was on average 4 and 11 times higher in the net autotrophic fine and coarse sand than in the net heterotrophic mud, despite higher chlorophyll content in the mud. Light limitation was less severe at the sandy sites, and two to three times more light was available to the microalgae in the sands than in the mud. The advective flushing of the permeable sands removed chlorophyll decomposition products and may have enhanced benthic photosynthesis by counteracting a possible CO2 limitation of the microalgae

Nutrient Release from an Exposed Intertidal Sand Flat

We studied pore water seepage and associated nutrient release in the Janssand intertidal sand flat (North Sea) during exposure at low tide. The hydraulic gradient developing at ebb tide between the pore water level in the elevated sand flat and the water level in the tidal gully generated interstitial water flows toward the seepage zone with velocities ranging from 0.5 (March) to 0.9 cm h–1 (July). Pore water was discharged from an approximately 20 m wide zone near the seaward margin of the flat at rates of 2.4 (March) and 4.2 l m–2 d–1 (July). Nutrient and dissolved inorganic carbon (DIC) concentrations of the seepage water exceeded those measured in the pore water of the upper section of the flat by 10- and 5-fold, respectively. Nutrient effluxes through seepage reached 1100 and 7600 µmol m–2 d–1 for NH4, 280 and 2500 µmol m–2 d–1 for PO4 and 140 and 1700 µmol m–2 d–1 for Si(OH)4 in March and July, respectively. Benthic flux chambers revealed that nutrients and DIC were released from the still submerged sediment as soon as the ebb tide exposed the upper section of the elevated flat. A conservative estimate based on our measurements suggests that 168000 l (March) to 294000 l (July) pore water are discharged each day from the sandy northeastern margin of the Janssand (3.5 km length, 70000 m2). Nutrients contained in this water corresponded to 6–25 kg d–1 (90 to 350 mg m–2 d–1) carbon mineralized during March and 42–223 kg d–1 (600 to 3200 mg C m–2 d–1) during July. Our study indicates that the Janssand intertidal flat does not accumulate organic matter but releases mineralization products that can account for all the organic matter that is potentially filtered through the permeable beds during a tidal cycle. Nutrient fluxes associated with seepage exceeded 5- to 8-fold those fluxes caused by the combined effects of diffusion, advection and bioirrigation during inundation, emphasizing the importance of sand flat drainage to the nutrient cycles in the Wadden Sea

Surficial and deep pore water circulation governs spatial and temporal scales of nutrient recycling in intertidal sand flat sediment

This study addresses organic matter decomposition in permeable sediment of a sloping intertidal sand flat (German Wadden Sea) affected by current-induced pore water exchange and pore fluid drainage. Seasonal and spatial scales of aerobic and anaerobic mineralization were investigated at 2 sites, one near the water line and one on the upper flat. Hydrodynamic forcing during inundation caused deeper oxygen penetration through flushing of the uppermost sediment layer. This flushing resulted in higher areal oxygen consumption rates and lower depth integrated sulfate reduction rates in the submerged flat compared to the rates measured during exposure. Mineralization rates in the top 15 cm of the sediment were similar between both study sites and ranged from 38 (winter) to 280 mmol C m–2 d–1 (summer), with sulfate reduction contributing 3 to 25% to total mineralization, depending on the season. At the upper flat, these seasonal differences were reflected in the pore water concentrations of nutrients, dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC). Near the low water line, however, pore water nutrient and DIC concentrations were independent of the season and up to 15 times higher compared to the values recorded in the upper flat. The differences in concentrations of metabolic products between the 2 sites resulted from a low tide drainage extending deep below the uppermost flushed layer and causing seepage of pore water near the low water line. Mineralization and nutrient release in these permeable intertidal sediments is affected by 2 circulation processes that work on distinctly different temporal and spatial scales: (1) rapid ‘skin circulation’ through the uppermost sediment layer during inundation that is characterized by short flow paths, low pore water residence time and immediate feedback to the ecosystem, and (2) slow ‘body circulation’ through deeper sediment layers during low tide that is characterized by long flow paths and pore water residence times, and acts as a buffered nutrient source to the ecosystem

Murine models of hepatitis C: What can we look forward to?

The study of interactions between hepatitis C virus (HCV) with its mammalian host, along with the development of more effective therapeutics and vaccines has been delayed by the lack of a suitable small animal model. HCV readily infects only humans and chimpanzees, which poses logistic, economic and ethical challenges with analyzing HCV infection in vivo. Progress has been made in understanding the determinants that dictate HCV’s narrow host range providing a blueprint for constructing a mouse model with inheritable susceptibility to HCV infection. Indeed, genetically humanized mice were generated that support viral uptake, replication and production of infectious virions – albeit at low levels. These efforts are complemented with attempts to select for viral variants that are inherently more capable of replicating in non-human species. In parallel, engraftment of relevant human tissues into improved xenorecipients is being continuously refined. Incorporating advances in stem-cell-biology and tissue engineering may allow the generation of patient-specific humanized mice. Construction of such mouse “avatars” may allow analyzing functionally patient-specific differences with respect to susceptibility to infection, disease progression and responses to treatment. In this review, we discuss the three, before mentioned approaches to overcome current species barriers and generate a small animal model for HCV infection, i.e. genetic modification of mice to increase their susceptibility to the virus; genetic modification of HCV, to increase its pathogenicity for mice; and the introduction of human liver and immune cells into immunodeficient mice, to create “humanized” mice. Although in the foreseeable future there will not be a single model that perfectly mimics the natural course of HCV in humans there is reason for optimism. The spectrum of murine animal models for hepatitis C provides a broad arsenal for analyzing the disease. These models may play an important role by prioritizing vaccine candidates and possibly refining combination anti-viral drug therapies. This article forms part of a symposium in Anti-viral Research on “Hepatitis C: next steps toward global eradication.