Listeria monocytogenes is a Gram-positive, intracellular pathogen that is highly adapted to invade and replicate in the cytosol of eukaryotic cells. Intermediate metabolites in the menaquinone biosynthesis pathway are essential for the cytosolic survival and virulence of L monocytogenes, independent of the production of menaquinone (MK) and aerobic respiration. Determining which specific intermediate metabolite(s) are essential for cytosolic survival and virulence has been hindered by the lack of an identified 1,4-dihydroxy-2-naphthoylcoenzyme A (DHNA-CoA) thioesterase essential for converting DHNA-CoA to DHNA in the MK synthesis pathway. Using the recently identified Escherichia coli DHNA-CoA thioesterase as a query, homology sequence analysis revealed a single homolog in L. monocytogenes, LMRG_02730. Genetic deletion of LMRG_02730 resulted in an ablated membrane potential, indicative of a nonfunctional electron transport chain (ETC) and an inability to aerobically respire. Biochemical kinetic analysis of LMRG_02730 revealed strong activity toward DHNA-CoA, similar to its E. coli homolog, further demonstrating that LMRG_02730 is a DHNA-CoA thioesterase. Functional analyses in vitro, ex vivo, and in vivo using mutants directly downstream and upstream of LMRG_02730 revealed that DHNA-CoA is sufficient to facilitate in vitro growth in minimal medium, intracellular replication, and plaque formation in fibroblasts. In contrast, protection against bacteriolysis in the cytosol of macrophages and tissue-specific virulence in vivo requires the production of 1,4-dihydroxy-2-naphthoate (DHNA). Taken together, these data implicate LMRG_02730 (renamed MenI) as a DHNA-CoA thioesterase and suggest that while DHNA, or an unknown downstream product of DHNA, protects the bacteria from killing in the macrophage cytosol, DHNA-CoA is necessary for intracellular bacterial replication.

The accumulation of Fe and Mn in seasonally stratified drinking water reservoirs adversely impacts water quality. To control issues with Fe and Mn at the source, some drinking water utilities have deployed hypolimnetic oxygenation systems to create well-oxygenated conditions in the water column that are favorable for the oxidation, and thus removal, of Fe and Mn. However, in addition to being controlled by dissolved oxygen (DO), Fe and Mn concentrations are also influenced by pH and metal-oxidizing microorganisms. We studied the response of Fe and Mn concentrations to hypolimnetic oxygenation in a shallow drinking water reservoir in Vinton, Virginia, USA by sequentially activating and deactivating an oxygenation system over two summers. We found that maintaining well-oxygenated conditions effectively prevented the accumulation of soluble Fe in the hypolimnion. However, while the rate of Mn oxidation increased under well-oxygenated conditions, soluble Mn still accumulated in the slightly acidic to neutral (pH 5.6 to 7.5) hypolimnion. In parallel, we conducted laboratory incubation experiments, which showed that the presence of Mn-oxidizing microorganisms increased the rate of Mn oxidation in comparison with rates under oxic, abiotic conditions. Combined, our field and laboratory results demonstrate that increasing DO concentrations in the water column is important for stimulating the oxidation of Fe and Mn, but that the successful management of Mn is also tied to the activity of Mn-oxidizing organisms in the water column and favorable (neutral to alkaline) pH. (C) 2016 Elsevier Ltd. All rights reserved.

Physical and chemical gradients across ecosystems, such as stream-to-lake continua within human-made reservoirs, provide valuable opportunities to examine how organisms respond to changing environments. We quantified the rate of dinoflagellate recruitment across a small reservoir to test the hypothesis that organisms are controlled by different factors along a reservoir continuum. We predicted that recruitment would be tightly coupled with reservoir physics in the riverine zone and closely related to water chemistry in the lacustrine zone. For the dominant dinoflagellate genus in the reservoir, Peridinium, recruitment from the sediments accounted for a median of 16% of increases in pelagic cell abundance throughout the summer. As predicted, Peridinium recruitment rates at the riverine site were correlated with physical variables, while at the lacustrine site, recruitment rates were highly correlated with water chemistry (e.g. nutrient ratios and dissolved oxygen). Recruitment patterns of the second most common genus, Gymnodinium, were not correlated with environmental variables, though Gymnodinium's much lower densities suggest that its dynamics were controlled by other factors. Our results reveal that the physical-biological coupling controlling algal recruitment, which can play a large role in pelagic population growth and bloom formation, can vary substantially on a spatial gradient within even a small reservoir.

The magnitude of lateral dissolved inorganic carbon (DIC) export from terrestrial ecosystems to inland waters strongly influences the estimate of the global terrestrial carbon dioxide (CO2) sink. At present, no reliable number of this export is available, and the few studies estimating the lateral DIC export assume that all lakes on Earth function similarly. However, lakes can function along a continuum from passive carbon transporters (passive open channels) to highly active carbon transformers with efficient in-lake CO2 production and loss. We developed and applied a conceptual model to demonstrate how the assumed function of lakes in carbon cycling can affect calculations of the global lateral DIC export from terrestrial ecosystems to inland waters. Using global data on in-lake CO2 production by mineralization as well as CO2 loss by emission, primary production, and carbonate precipitation in lakes, we estimated that the global lateral DIC export can lie within the range of 0.70(-0.31)(+0.27) 1.52(-0.90)(+1.09) Pg C yr(-1) depending on the assumed function of lakes. Thus, the considered lake function has a large effect on the calculated lateral DIC export from terrestrial ecosystems to inland waters. We conclude that more robust estimates of CO2 sinks and sources will require the classification of lakes into their predominant function. This functional lake classification concept becomes particularly important for the estimation of future CO2 sinks and sources, since in-lake carbon transformation is predicted to be altered with climate change.

Bubble-plume mixing systems are often deployed in eutrophic lakes and reservoirs to manage phytoplankton taxa. Unfortunately, inconsistent outcomes from bubble-plume (induced) mixing are often reported in the literature. The present study investigates the response of phytoplankton to induced mixing using a whole-reservoir field experiment and a three-dimensional hydrodynamic model (Si3D) coupled with the Aquatic EcoDynamics (AED) model through the framework for aquatic biogeochemical modelling (FABM). The coupled Si3D-AED model is validated against a 24-h field mixing experiment and subsequently used for a numerical parametric study to investigate phytoplankton responses to various induced mixing scenarios in which the phytoplankton settling rate, phytoplankton growth rate, reservoir depth, and mixing system diffuser depth were sequentially varied. Field observations during the mixing experiment suggest that the total phytoplankton concentration (measured in mu g/L) across the reservoir was reduced by nearly 10% during the 24-h mixing period. The numerical modeling results show that phytoplankton concentration may be substantially affected by the functional traits of the phytoplankton and the deployment depth of the mixing diffuser. Interestingly, the numerical results indicate that the phytoplankton concentration is controlled by reduced growth rates due to light limitation in deep reservoirs (> 20 m), whereas settling loss is a more important factor in shallow reservoirs during the mixing period. In addition, the coupled Si3D-AED model results suggest that deploying the mixing diffuser deeper in the water column to increase mixing depth may generally improve the successful management of cyanobacteria using bubble-plume mixing systems. Thus, the coupled Si3D-AED model introduced in the present study can assist with the design and operation of bubble-plume mixing systems.

Studies that examine the effects of artificial mixing for water-quality mitigation in lakes and reservoirs often view a water column with a one-dimensional (1-D) perspective (e.g., homogenized epilimnetic and hypolimnetic layers). Artificial mixing in natural water bodies, however, is inherently three dimensional (3-D). Using a 3-D approach experimentally and numerically, the present study visualizes thermal structure and analyzes constituent transport under the influence of artificial mixing in a shallow drinking-water reservoir. The purpose is to improve the understanding of artificial mixing, which may help to better design and operate mixing systems. In this reservoir, a side-stream supersaturation (SSS) hypolimnetic oxygenation system and an epilimnetic bubble-plume mixing (EM) system were concurrently deployed in the deep region. The present study found that, while the mixing induced by the SSS system does not have a distinct 3-D effect on the thermal structure, epilimnetic mixing by the EM system causes 3-D heterogeneity. In the experiments, epilimnetic mixing deepened the lower metalimnetic boundary near the diffuser by about 1 m, with 55% reduction of the deepening rate at 120 m upstream of the diffuser. In a tracer study using a 3-D hydrodynamic model, the operational flow rate of the EM system is found to be an important short-term driver of constituent transport in the reservoir, whereas the duration of the EM system operation is the dominant long-term driver. The results suggest that artificial mixing substantially alters both 3-D thermal structure and constituent transport, and thus needs to be taken into account for reservoir management.

Metalimnetic oxygen minimum zones (MOMs) commonly develop during the summer stratified period in freshwater reservoirs because of both natural processes and water quality management. While several previous studies have examined the causes of MOMs, much less is known about their effects, especially on reservoir biogeochemistry. MOMs create distinct redox gradients in the water column which may alter the magnitude and vertical distribution of dissolved methane (CH4) and carbon dioxide (CO2). The vertical distribution and diffusive efflux of CH4 and CO2 was monitored for two consecutive open-water seasons in a eutrophic reservoir that develops MOMS as a result of the operation of water quality engineering systems. During both summers, elevated concentrations of CH4 accumulated within the anoxic MOM, reaching a maximum of 120 mu M, and elevated concentrations of CO2 accumulated in the oxic hypolinin ion, reaching a maximum of 780 mu M. Interestingly, the largest observed diffusive CH4 effluxes occurred before fall turnover in both years, while peak diffusive CO2 effluxes occurred both before and during turnover. Our data indicate that MOMs can substantially change the vertical distribution of CH4 and CO2 in the water column in reservoirs, resulting in the accumulation of CH4 in the metalimnion (vs. at the sediments) and CO2 in the hypolimnion. (C) 2013 Elsevier B.V. All rights reserved.

Chaoborus spp. (midge larvae) live in the anoxic sediments and h-ypolimnia of freshwater lakes and reservoirs during the day and migrate to the surface waters at night to feed on plankton. It has recently been proposed that Chaoborus take up methane (CH4) from the sediments in their tracheal gas sacs, use this acquired buoyancy to ascend into the surface waters, and then release the CH4, thereby serving as a CH4 "pump" to the atmosphere. We tested this hypothesis using diel surveys and seasonal monitoring, as well as incubations of Chaoborus to measure CH, transport in their gas sacs at different depths and times in a eutrophic reservoir. We found that Chaoborus transported CH, from the hypolimnion to the lower epilimnion at dusk, but the overall rate of CH4 transport was minor, and incubations revealed substantial variability in CH4 transport over space and time. We calculated that Chaoborus transport similar to 0.1 mmol CH4 m(-2) yr(-1) to the epilimnion in our study reservoir, a very low proportion (<1%) of total CH4 diffusive flux during the summer stratified period. Our data further indicate that CH4 transport by Chaoborus is sensitive to water column mixing, Chaoborus density, and Chaoborus species identity.

Organic carbon (OC) mineralization in freshwaters is dependent on oxygen availability near the sediments, which controls whether OC inputs will be buried or respired. However, oxygen dynamics in waterbodies are changing globally due to land use and climate, and the consequences of variable oxygen conditions for OC burial are unknown. We manipulated hypolimnetic oxygen availability in a whole-reservoir experiment and used a mass balance OC model to quantify rates of OC burial. Throughout summer stratification, we observed that OC burial rates were tightly coupled to sediment oxygen concentrations: oxic conditions promoted the mineralization of "legacy" OC that had accumulated over years of sedimentation, resulting in negative OC burial. Moreover, our study demonstrates that fluctuating oxygen conditions can switch ecosystem-scale OC burial in a reservoir between positive and negative rates. Consequently, changing oxygen availability in freshwaters globally will likely have large implications for the role of these ecosystems as OC sinks.

Water column mixing can influence community composition of pelagic phytoplankton in lakes and reservoirs. Previous studies suggest that low mixing favors cyanobacteria, while increased mixing favors green algae and diatoms. However, this shift in community dominance is not consistently achieved when epilimnetic mixers are activated at the whole-ecosystem scale, possibly because phytoplankton community responses are mediated by mixing effects on other ecosystem processes. We conducted two epilimnetic mixing experiments in a small drinking water reservoir using a bubble-plume diffuser system. We measured physical, chemical, and biological variables before, during, and after mixing and compared the results to an unmixed reference reservoir. We observed significant increases in the biomass of cyanobacteria (from 0.8 +/- 0.2 to 2.4 +/- 1.1 g L-1, p = 0.008), cryptophytes (from 0.7 +/- 0.1 to 1.9 +/- 0.6 g L-1, p = 0.003), and green algae (from 3.8 to 4.4 g L-1, p = 0.15) after our first mixing event, likely due to increased total phosphorus from entrainment of upstream sediments. After the second mixing event, phytoplankton biomass did not change but phytoplankton community composition shifted from taxa with filamentous morphology to smaller, rounder taxa. Our results suggest that whole-ecosystem dynamics and phytoplankton morphological traits should be considered when predicting phytoplankton community responses to epilimnetic mixing.