In recent years the analysis of stable isotope fractionation has increasingly been used for characterizing and quantifying biodegradation of contaminants in aquifers. The correlation of carbon and hydrogen isotope signatures of benzene in a BTEX-contaminated aquifer located in the area of a former hydrogenation plant gave indications that biodegradation mainly occurred under anoxic conditions. This finding was consistent with the investigation of hydrogeochemical conditions within the aquifer. Furthermore, the biodegradation of benzene was calculated by changes in carbon isotope signatures using the Rayleigh-equation-streamline approach. Since contaminant concentrations can be also affected by nonisotope-fractionating abiotic processes such as dilution, volatilization, or irreversible sorption to the aquifer matrix, the Rayleigh-equation-streamline approach was adjusted for scenarios assuming that biodegradation and abiotic processes occur either consecutively or simultaneously along a groundwater flow path between contaminant source and sampling well. The results of the scenarios differed significantly, indicating that an abiotic process (typically dilution) causes a decrease in benzene concentration within the investigated aquifer transect. This comparison of results derived from the different scenarios can help to identify whether biodegradation is the predominant process for decrease in contaminant concentration. However, for a proper quantification of biodegradation, the temporal sequence between biodegradation and dilution needs to be known. The uncertainty associated with the quantification of pollutant biodegradation by the Rayleigh-equation-streamline approach increases when nonisotope-fractionating abiotic processes cause a significant decrease in contaminant concentrations.

Microbial ecologists and environmental engineers share the interest in identifying the key microorganisms responsible for compound turnover in the environment and in estimating the respective transformation rates. For the successful application of Natural Attenuation processes, a reliable assessment of the in situ turnover of a contaminant in an aquifer is essential. Here, we review and present new details of two recently developed approaches concerning the assessment of in situ biodegradation: (i) determination of biodegradation caused by microbial metabolism in a contamination plume by stable isotope fractionation analysis (SIFA) and (ii) determination of the actual degradation under the respective environmental conditions in the aquifer by using in situ microcosms (BACTRAPS®) amended with 13C‐labeled substrates as tracer compounds. Based on stable isotope fractionation analysis, the degradation occurring under anoxic biogeochemical conditions at a respective site can be calculated for the entire plume. This has been shown for benzene and toluene at the Zeitz site and partly for chlorobenzene at the Bitterfeld site. By use of the in situ microcosm approach with 13C‐labeled compounds, the microbial in situ degradation under strictly anaerobic conditions could be proven for benzene and toluene in Zeitz and for chlorobenzene in Bitterfeld. The transformation of 13C‐carbon of the labeled substrate into microbial fatty acids confirmed the assimilation of the pollutant resulting in the formation of biomass. In addition, metabolites such as benzylsuccinic acid were found in the toluene‐amended microcosms indicating anaerobic degradation of toluene. This result corresponds to the geochemical conditions found at the field site and therefore, the microcosm approach with 13C‐labeled compounds can be used to assign the predominant in situ degradation pathways in a contaminated aquifer. Since fatty acids profiles alone are often too unspecific for a community analysis at species level, the composition of the microbial communities was analyzed by genetic profiling and sequencing of partial 16S rRNA genes PCR‐amplified from total DNA extracted directly from the microcosms. Sequences retrieved from the microcosms indicated a dominance of not yet cultivated bacteria. Several sequences were phylogenetically closely related to sequences of bacteria known to be iron and sulfate reducers, typically found at sites polluted with BTEX and/or mineral oil. The results show that the current methods for monitoring microbial in situ activity at present stage are valuable tools for improving environmental control of compound turnover and will speed up engineering approaches.

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Microbial degradation of monochlorobenzene (MCB) under anaerobic conditions was investigated using a stable isotope tracer under in and ex situ conditions. In situ microcosms were incubated directly in an anoxic aquifer and carbon derived from [13C6]-MCB was found to be incorporated into the microbial biomass. In laboratory microcosms, amended with [13C6]-MCB, anaerobic mineralization of MCB was indicated by the production of 13CO2. Further, recovery of the 13C-label in the fatty acids confirmed the assimilation of MCB-derived carbon into microbial biomass. The described approach may be applied to various other organic groundwater contaminants of concern using carbon (13C) as well as other stable isotope tracers, such as nitrogen (15N), allowing direct and sensitive detection of biodegradation.

Both histologic studies and findings with Indocyanin green angiography furnish clear indications of the existence of diabetic choroidopathy. Out of 19,387 fluorescein angiograms made over the last 15 years at the First Ophthalmological Clinic of Vienna University, 902 were selected in which the choroidal vascular system is at least partially visible due to melanin hypopigmentation. The pathologic changes in the choroid observed in these angiograms can be allocated in two basic types of pathologic fluorescence: on the one hand hyperfluorescent lesions such as drusen in Bruch's membrane in suprisingly young diabetics, extremely hyperfluorescent lobules of the choriocapillaris, choroidal aneurysms, and neovascularizations; on the other hand, hypofluorescent lesions or structures were observed, such as delayed filling of the choriocapillaris and, in the late phase of angiography, varicose and tortuous choroidal vessels, drained and appearing dark in contrast to the diffuse background fluorescence.

Within the past 5 years panretinal photocoagulation was performed in both eyes of 36 patients (Type I diabetics) with proliferative retinopathy. The age of these patients was between 21 and 38 years. The average follow-up period was 3.6 years. The effects on visual acuity, visual field, and dark adaptation were examined: in 65.3% of the cases (47 eyes) visual acuity remained unchanged; in 34.7% (25 eyes) central visual acuity was reduced. Depending on the coagulation technique and the number of coagulates applied, the visual field narrowed in a more or less pronounced way. Moreover, in all patients the dark adaptation threshold was elevated after panretinal photocoagulation.