Aluminum Biorecovery from Wastewaters.

Aluminum biorecovery is still at an early stage. However, a significant number of studies showing promising results already exist, although they have revealed problems that need to be solved so aluminum biorecovery can have a wider application and industrial upscaling. In this chapter, we revise the existing knowledge on the biorecovery of aluminum from different sources. We discuss the design, overall performance, advantages, technical problems, limitations, and possible future directions of the different biotechnological methods that have been reported so far. Aluminum biorecovery from different sources has been studied (i.e., solid wastes and primary sources of variable origin, wastewater with low concentrations of dissolved aluminum at pH-neutral or weakly acidic conditions, and acidic mine waters with high concentrations of dissolved aluminum and other metal(loid)s) and has shown that the process efficiency strongly depends on factors such as (1) the physicochemical properties of the source materials, (2) the physiological features of the used (micro)organisms, or (3) the biochemical process used. Bioleaching of aluminum from low-grade bauxite or red mud can much be achieved by a diverse range of organisms (e.g., fungi, bacteria) with different metabolic rates. Biorecovery of aluminum from wastewaters, e.g., domestic wastewater, acidic mine water, has also been accomplished by the use of microalgae, cyanobacteria (for domestic wastewater) or by sulfate-reducing bacteria (acidic mine water). In most of the cases, the drawback of the process is the requirement of controlled conditions which involves a continuous supply of oxygen or maintenance of anoxic conditions which make aluminum biorecovery challenging in terms of process design and economical value. Further studies should focus on studying these processes in comparison or in combination to existing economical processes to assess their feasibility.

Microbially influenced corrosion and rust tubercle formation on sheet piles in freshwater systems.

The extent of how complex natural microbial communities contribute to metal corrosion is still not fully resolved, especially not for freshwater environments. In order to elucidate the key processes, we investigated rust tubercles forming massively on sheet piles along the river Havel (Germany) applying a complementary set of techniques. In-situ microsensor profiling revealed steep gradients of O2 , redox potential and pH within the tubercle. Micro-computed tomography and scanning electron microscopy showed a multi-layered inner structure with chambers and channels and various organisms embedded in the mineral matrix. Using Mössbauer spectroscopy we identified typical corrosion products including electrically conductive iron (Fe) minerals. Determination of bacterial gene copy numbers and sequencing of 16S rRNA and 18S rRNA amplicons supported a densely populated tubercle matrix with a phylogenetically and metabolically diverse microbial community. Based on our results and previous models of physic(electro)chemical reactions, we propose here a comprehensive concept of tubercle formation highlighting the crucial reactions and microorganisms involved (such as phototrophs, fermenting bacteria, dissimilatory sulphate and Fe(III) reducers) in metal corrosion in freshwaters.

Microbial functional changes mark irreversible course of Tibetan grassland degradation.

The Tibetan Plateau's Kobresia pastures store 2.5% of the world's soil organic carbon (SOC). Climate change and overgrazing render their topsoils vulnerable to degradation, with SOC stocks declining by 42% and nitrogen (N) by 33% at severely degraded sites. We resolved these losses into erosion accounting for two-thirds, and decreased carbon (C) input and increased SOC mineralization accounting for the other third, and confirmed these results by comparison with a meta-analysis of 594 observations. The microbial community responded to the degradation through altered taxonomic composition and enzymatic activities. Hydrolytic enzyme activities were reduced, while degradation of the remaining recalcitrant soil organic matter by oxidative enzymes was accelerated, demonstrating a severe shift in microbial functioning. This may irreversibly alter the world´s largest alpine pastoral ecosystem by diminishing its C sink function and nutrient cycling dynamics, negatively impacting local food security, regional water quality and climate.

Digital Responsibility Goals - A Framework for a Human-Centered Sustainable Digital Economy with a Focus on Trusted Digital Solutions.

This paper describes the Digital Responsibility Goals, their purpose, and the associated guiding criteria and their relevance particularly for health. In addition, the document makes a first proposal for measuring digital responsibility.

Thermodesulfobium sp. strain 3baa, an acidophilic sulfate reducing bacterium forming biofilms triggered by mineral precipitation.

Sulfate reducing prokaryotes are promising candidates for the remediation of acidic metal-rich waste waters. However, only few acidophilic species have been described to date. Chemolithoautotrophic strain 3baa was isolated from sediments of an acidic mine pit lake. Based on its 16S-rRNA gene sequence it belongs to the genus Thermodesulfobium. It was identified as an acidophile growing in artificial pore water medium in the range of pH 2.6-6.6. Though the highest sulfate reduction rates were obtained at the lower end of this range, elongated cells and extended lag phases demonstrated acid stress. Sulfate reduction at low pH was accompanied by the formation of mineral precipitates strongly adhering to solid surfaces. A structural investigation by laser scanning microscopy, electron microscopy and X-ray microanalysis revealed the formation of Al hydroxides and Fe sulfides which were densely populated by cells. Al hydroxides precipitated first, enabling initial cell attachment. Colonization of solid surfaces coincided with increased sulfate reducing activity indicating more favourable growth conditions within biofilms compared with free-living cells. These findings point out the importance of cell-mineral interaction for biofilm formation and contribute to our understanding how sulfate reducing prokaryotes thrive in both natural and engineered systems at low pH.

Effects of low dose silver nanoparticle treatment on the structure and community composition of bacterial freshwater biofilms.

The application of engineered silver nanoparticles (AgNPs) in a considerable amount of registered commercial products inevitably will result in the continuous release of AgNPs into the natural aquatic environment. Therefore, native biofilms, as the prominent life form of microorganisms in almost all known ecosystems, will be subjected to AgNP exposure. Despite the exponentially growing research activities worldwide, it is still difficult to assess nanoparticle-mediated toxicity in natural environments. In order to obtain an ecotoxicologically relevant exposure scenario, we performed experiments with artificial stream mesocosm systems approaching low dose AgNP concentrations close to predicted environmental concentrations. Pregrown freshwater biofilms were exposed for 14 days to citrate-stabilized AgNPs at a concentration of 600 µg l-1 in two commonly used sizes (30 and 70 nm). Sublethal effects of AgNP treatment were assessed with regard to biofilm structure by gravimetric measurements (biofilm thickness and density) and by two biomass parameters, chlorophyll a and protein content. The composition of bacterial biofilm communities was characterized by t-RFLP fingerprinting combined with phylogenetic studies based on the 16S gene. After 14 days of treatment, the structural parameters of the biofilm such as thickness, density, and chlorophyll a and protein content were not statistically significantly changed by AgNP exposure. Furthermore, t-RFLP fingerprint analysis showed that the bacterial diversity was not diminished by AgNPs, as calculated by Shannon Wiener and evenness indices. Nevertheless, t-RFLP analysis also indicated that AgNPs led to an altered biofilm community composition as was shown by cluster analysis and multidimensional scaling (MDS) based on the Bray Curtis index. Sequence analysis of cloned 16S rRNA genes further revealed that changes in community composition were related with the displacement of putatively AgNP-sensitive bacterial taxa Actinobacteria, Chloroflexi, and Cyanobacteria by taxa known for their enhanced adaptability towards metal stress, such as Acidobacteria, Sphingomonadales, and Comamonadaceae. This measurable community shift, even after low dose AgNP treatment, causes serious concerns with respect to the broad application of AgNPs and their potentially adverse impact on the ecological function of lotic biofilms, such as biodegradation or biostabilization.

Sublethal concentrations of silver nanoparticles affect the mechanical stability of biofilms.

Bacterial biofilms are most likely confronted with silver nanoparticles (Ag NPs) as a pollutant stressor in aquatic systems. In this study, biofilms of Aquabacterium citratiphilum were exposed for 20 h to 30 and 70 nm citrate stabilized Ag NPs in low-dose concentrations ranging from 600 to 2400 µg l-1, and the Ag NP-mediated effects on descriptive, structural, and functional biofilm characteristics, including viability, protein content, architecture, and mechanical stability, were investigated. Viability, based on the bacterial cell membrane integrity of A. citratiphilum, as determined by epifluorescence microscopy, remained unaffected after Ag NP exposure. Moreover, in contrast to information in the current literature, protein contents of cells and extracellular polymeric substances (EPS) and biofilm architecture, including dry mass, thickness, and density, were not significantly impacted by exposure to Ag NPs. However, the biofilms themselves served as effective sinks for Ag NPs, exhibiting enrichment factors from 5 to 8. Biofilms showed a greater capacity to accumulate 30 nm sized Ag NPs than 70 nm Ag NPs. Furthermore, Ag NPs significantly threatened the mechanical stability of biofilms, as determined by a newly developed assay. For 30 nm Ag NPs, the mechanical stability of biofilms decreased as the Ag NP concentrations applied to them increased. In contrast, 70 nm Ag NPs produced a similar decrease in mechanical stability for each applied concentration. Overall, this finding demonstrates that exposure to Ag NPs triggers remarkable changes in biofilm adhesion and/or cohesiveness. Because of biofilm-mediated ecological services, this response raises environmental concerns regarding Ag NP release into freshwater systems, even in sublethal concentrations.

Enrichment of sulfate-reducing bacteria and resulting mineral formation in media mimicking pore water metal ion concentrations and pH conditions of acidic pit lakes.

Acid mine drainage sites are extreme environments with high acidity and metal ion concentrations. Under anoxic conditions, microbial sulfate reduction may trigger the formation of secondary minerals as a result of H2S production and pH increase. This process was studied in batch experiments with enrichment cultures from acidic sediments of a pit lake using growth media set at different pH values and containing elevated concentrations of Fe²âº and Al³âº. At initial pH values of 5 and 6, sulfate reduction occurred shortly after inoculation. Sulfate- reducing bacteria affiliated to the genus Desulfosporosinus predominated the microbial communities as shown by 16S rRNA gene analysis performed at the end of the incubation. At initial pH values of 3 and 4, sulfate reduction and cell growth occurred only after an extended lag phase, however, at a higher rate than in the less acidic assays. At the end of the growth phase, enrichments were dominated by Thermodesulfobium spp. suggesting that these sulfate reducers were better adapted to acidic conditions. Iron sulfides in the bulk phase were common in all assays, but specific aluminum precipitates formed in close association with cell surfaces and may function as a detoxification mechanism of dissolved Al species at low pH.

Importance of different physiological groups of iron reducing microorganisms in an acidic mining lake remediation experiment.

Iron- and sulfate-reducing microorganisms play an important role for alkalinity-generating processes in mining lakes with low pH. In the acidic mining lake 111 in Lusatia, Germany, a passive in situ remediation method was tested in a large scale experiment, in which microbial iron and sulfate reduction are stimulated by addition of Carbokalk (a mixture of the nonsugar compounds of sugar beets and lime) and straw. The treated surface sediment consisted of three layers of different pH and geochemical composition. The top layer was acidic and rich in Fe(III), the second and third layer both showed moderately acidic to circum-neutral pH values, but only the second was rich in organics, strongly reduced and sulfidic. Aim of the study was to elucidate the relative importance of neutrophilic heterotrophic, acidophilic heterotrophic, and acidophilic autotrophic iron-reducing microorganisms in each of the three layers. In order to distinguish between them, the effect of their respective characteristic electron donors acetate, glucose, and elemental sulfur on potential iron reduction rates was investigated. Limitation of iron reduction by the availability of Fe(III) was revealed by the addition of Fe(OH)(3). The three groups of iron-reducing microorganisms were quantified by most probable number (MPN) technique and their community composition was analyzed by cloning and sequencing of 16S rRNA genes. In the acidic surface layer, none of the three electron donors stimulated iron reduction; acetate even had an inhibiting effect. In agreement with this, no decrease of the added electron donors was observed. Iron reduction rates were low in comparison to the other layers. Iron reduction in layers 2 and 3 was enhanced by glucose and acetate, accompanied by a decrease of these electron donors. Addition of elemental sulfur did not enhance iron reduction in either layer. Layer 2 exhibited the highest iron reduction rate (4.08 mmol dm(-3) d(-1)) and the highest cell numbers in MPN media. In MPN enrichments from all layers, Acidithiobacillus-like sequences were frequent. In addition to these, sequences related to Fulvimonas and Clostridium dominated in layer 1. MPN enrichments of layer 2 were diverse, containing Rhodocyclaceae-related sequences and surprisingly low numbers of Geobacteraceae. In layer 3, Sulfobacillus and Trichococcus spp. were also important. It was concluded that in the surface layer mainly acidophilic, probably autotrophic and heterotrophic, iron reducers were active, whereas in layers 2 and 3 mainly neutrophilic heterotrophs were important for iron reduction. These differ from well-studied Fe(III) reducers in other environments, so they deserve further study. The potential for acid-producing sulfur-driven Fe(III) reduction seemed not to be critical for in situ remediation.