Quantitative mapping of chlorhexidine in natural river biofilms

Soft X-ray scanning transmission X-ray microscopy has been applied to map chlorhexidine, a ubiquitous antimicrobial agent, relative to major biochemical components (proteins, lipids, polysaccharides, Ca2+, K+, CO3(2-)) in natural river biofilms. For the first time, bio-accumulation of chlorhexidine in diatoms has been observed unambiguously. The quantitative results show that chlorhexidine bioaccumulated extensively in lipid-rich regions of diatoms and bacteria. Confocal laser scanning microscopy was used to document changes in the biofilm community. The bioaccumulation provides a significant entry point for chlorhexidine into the aquatic food chain. It results in modification of the biofilm community and it impacts the photosynthetic and protozoan species in particular. X-ray microscopy mapping at high spatial resolution is shown to be a powerful tool for studies of antimicrobial agents in the environment.     

Daphnia magna ecotoxicogenomics provides mechanistic insights into metal toxicity

Toxicogenomics has provided innovative approaches to chemical screening, risk assessment, and predictive toxicology. If applied to ecotoxicology, genomics tools could greatly enhance the ability to understand the modes of toxicity in environmentally relevant organisms. Daphnia magna, a small aquatic crustacean, is considered a "keystone" species in ecological food webs and is an indicator species for toxicant exposure. Our objective was to demonstrate the potential utility of gene expression profiling in ecotoxicology by identifying novel biomarkers and uncovering potential modes of action in D. magna. Using a custom D. magna cDNA microarray, we identified distinct expression profiles in response to sublethal copper, cadmium, and zinc exposures and discovered specific biomarkers of exposure including two probable metallothioneins, and a ferritin mRNA with a functional IRE. The gene expression patterns support known mechanisms of metal toxicity and reveal novel modes of action including zinc inhibition of chitinase activity. By integrating gene expression profiling into an environmentally important organism, this study provides experimental support for the utility of ecotoxicogenomics.     

Larval exposure to 4-nonylphenol and 17beta-estradiol affects physiological and behavioral development of seawater adaptation in Atlantic salmon smolts

Population declines of anadromous salmonids are attributed to anthropogenic disturbances including dams, commercial and recreational fisheries, and pollutants, such as estrogenic compounds. Nonylphenol (NP), a xenoestrogen, is widespread in the aquatic environment due to its use in agricultural, industrial, and household products. We exposed Atlantic salmon yolk-sac larvae to waterborne 10 or 100 microg L(-1) NP (NP-L or NP-H, respectively), 2 microg L(-1) 17beta-estradiol (E2), or vehicle, for 21 days to investigate their effects on smolt physiology and behavior 1 year later. NP-H caused approximately 50% mortality during exposure, 30 days after exposure, and 60 days after exposure. Mortality rates of NP-L and E2 fish were not affected until 60 days after treatment, when they were 4-fold greater than those of controls. Treatment with NP-L or E2 as yolk-sac larvae decreased gill sodium-potassium-activated adenosine triphosphatase (Na+,K(+)-ATPase) activity and seawater (SW) tolerance during smolt development, 1 year after exposure. Exposure to NP-L and E2 resulted in a latency to enter SW and reduced preference for SW approximately 2- and 5-fold, respectively. NP-L-exposed fish had 20% lower plasma insulin-like growth factor I (IGF-I) levels and 35% lower plasma triiodothyronine (T3). Plasma growth hormone and thyroxine (T4) were unaffected. Exposure to E2 did not affect plasma levels of IGF-I, GH, T3, or T4. Both treatment groups exhibited increased plasma cortisol and decreased osmoregulatory capacity in response to a handling stressor. These results suggest that early exposure to environmentally relevant concentrations of NP, and other estrogenic compounds, can cause direct and delayed mortalities and that this exposure can have long-term, "organizational" effects on life-history events in salmonids.     

Quantitative Comparison Between Dislocation Densities in Offcut 4H-SiC Wafers Measured Using Synchrotron X-ray Topography and Molten KOH Etching

Molten KOH etching and x-ray topography have been well established as two of the major characterization techniques used for observing as well as analyzing the various crystallographic defects in both substrates and homoepitaxial layers of silicon carbide. Regarding assessment of dislocation density in commercial wafers, though the two techniques show good consistency in threading dislocation density analysis, significant discrepancy is found in the case of basal plane dislocations (BPDs). In this paper we compare measurements of BPD densities in 4-inch 4H-SiC commercial wafers assessed using both etching and topography methods. The ratio of the BPD density calculated from topographic images to that from etch pits is estimated to be larger than 1/sinθ, where θ is the offcut angle of the wafer. Based on the orientations of the defects in the wafers, a theoretical model is put forward to explain this disparity and two main sources of errors in assessing the BPD density using chemical etching are discussed.