Publications (37)77.05 Total impact
- [Show abstract] [Hide abstract] ABSTRACT: Regular ecological investigations usually on a fortnightly basis have been carried out at the Saidenbach Reservoir (SE Germany) and its tributaries since 1975. The data show a sudden decrease in the phosphorus import of over 60% in 1990, resulting in the abrupt change of the trophic state from eutrophic to weakly mesotrophic. Contrary to expectations, the average annual phytoplankton abundance did not decrease but almost doubled on average over the years after 1990. This was primarily due to mostly warmer winters after 1990 causing longer spring overturns, which minimized sedimentation losses, enabled better utilization of the phosphorus reserves, and resulted in higher yields of the diatom-dominated phytoplankton spring mass development. In the summers after 1990, the mass growth of the diatom Fragilaria crotonensis, which in the past used to dominate in this season and effectively transported phosphorus to the sediment by settling, was considerably reduced because of lower P supply and stronger stratification. However, the decrease in the biomass of Fragilaria was overcompensated by the increased abundance of cyanobacteria. They benefitted not only from declining phosphorus competition by Fragilaria but also from higher thermal stability and temperature in the epilimnion. Hence, even higher summer biomasses were observed despite considerably lowered phosphorus import. The analysis of the long-term dataset clearly illustrates the deciding impact of hydrophysical factors on the phytoplankton growth, also under nutrient deficient conditions. The altered mixing and stratification pattern caused by climate change did not only prevent the re-oligotrophication of the reservoir but even enhanced the phytoplankton production. It seems that global warming modifies the interplay between physical and nutrient limitation mechanisms and the limits and models used in the past to classify trophic-state levels have to be verified. The study shows the enormous significance and indispensability of uninterrupted ecological long-term datasets, including reliable data of the ecosystem's organismic structure, for research about the consequences of climate change.
- [Show abstract] [Hide abstract] ABSTRACT: In 2011 and 2012 the new DWA Standards DWA-A 222 and DWA-M 221 for wastewater treatment plants were published, where rules for biofilm plants with submerged fixed bed and free movable biofilm carriers are included for the first time. In both technologies, the oxygen supply is usually realized by fine-bubble compressed aeration. The DWA-A 222 Standard does not adequately take into account the extensive operational experience with systems that work with submerged fixed bed. Using the specified parameters, a lack of oxygen supply can occur in the high load plant-level of carbon elimination. In this case the cleaning capability may be severely impaired.
- [Show abstract] [Hide abstract] ABSTRACT: Niemand ist in der Lage, genau vorherzusagen, wie sich z.B. in Zukunft das Klima verändern wird. Wasserwirtschaftliche Planung setzt aber zumindest hinreichend determinierte Annahmen über die künftige Entwicklung maßgeblicher Einflussfaktoren voraus. Solche Annahmen (Szenarios) können nur formuliert und unter Anwendung von Modellen analysiert werden, wenn globale und regionale Trends bekannt sind. Sozioökonomische oder Klimaänderungen verlaufen in der Regel allmählich, aber nie gleichmäßig. Sie sind von saisonalen und jährlichen Schwankungen und von stochastischen Veränderungen überlagert. Die Reaktion von Ökosystemen darauf ist noch wesentlich komplizierter. Hier kann es auch bei kontinuierlicher Veränderung der Einflussgrößen zu einem plötzlichen Umschlagen von Strukturen und Funktionen in einen neuen Zustand kommen. Daher erfordert das Erkennen von Trends und struktureller Veränderungen „einen langen Atem“ bei der Erforschung ökologischer Systeme, insbesondere bei aquatischen mit hohen und praxisrelevanten Änderungsraten. Dies gilt auch für die TS-Forschung. Anhand der an der TSS gewonnenen Datensätze lässt sich zeigen, dass selbst bei 20jähriger Dauer der Untersuchungen wesentliche Veränderungen nicht hätten wahrgenommen, geschweige denn erklärt werden können. Ja nicht einmal nach nunmehr 39 Jahren Forschung an der TSS sind wir in der Lage, einen Großteil wesentlicher Fragen zu beantworten und aktuelle Entwicklungen der Wasserbeschaffenheit zweifelsfrei zu erklären. Allerdings wissen wir heute sehr viel mehr als noch vor wenigen Jahren. Und dieses Wissen hat sich bereits in vielfältiger Weise in der Praxis der TS-Bewirtschaftung nutzbringend niedergeschlagen. Und ja, ökologische Langzeitforschung ist teuer. Aber eine einzige, aufgrund falscher Annahmen realisierte Investition an nur einer TS oder in nur einem Wasserwerk kann kostspieliger sein als mehrere Jahre Forschung.
- [Show abstract] [Hide abstract] ABSTRACT: Durch die Änderung der klimaabhängigen, hydrophysikalischen Bedingungen wurde die „Re-Oligotrophierung“ der Talsperre (im Sinne eines Rückganges der Phytoplanktonproduktion) nach 1990 völlig verhindert. Das bedeutet keineswegs, dass Anstrengungen zur Minderung der Phosphorbelastung nicht sinnvoll wären, denn die Untersuchungen zeigen gleichzeitig, dass ohne diese Reduzierung der Phosphorlast nach 1990 die Auswirkungen der veränderten klimatischen Bedingungen auf das Phytoplankton sehr wahrscheinlich wesentlich erheblicher, d.h. noch weit höhere Biomassen die Folge gewesen wären. Die Ergebnisse belegen, dass die Entflechtung der komplexen Steuermechanismen, die Aufdeckung dieser Zusammenhänge und ihre Quantifizierung sowie die Erkenntnis über den enormen Einfluss klimatisch gesteuerter Umweltgrößen tatsächlich nur mit dieser außergewöhnlich langen, ununterbrochenen und detaillierten Dokumentation möglich gewesen sind. Mit einem Datensatz von "nur" 22 Jahren, also ohne die Jahre vor 1990, wäre die gravierende Bedeutung klimagesteuerter Einflüsse nicht sichtbar und nachvollziehbar geworden. Ein eindrucksvollerer Beweis für die Notwendigkeit ökologischer Langzeitforschung zur Aufklärung der Folgen globaler Veränderungen ist kaum möglich.
- [Show abstract] [Hide abstract] ABSTRACT: Enhanced biological phosphorous removal (EBPR) from wastewater has been successfully used for more than three decades and is considered to be an environmentally friendly wastewater-treatment process. Biologically, this process is realized by incorporation of phosphate as polyphosphate (polyP) granules in activated sludge bacteria. Important groups of bacteria responsible for P removal have been identified, but the full microbial diversity involved in this process is still unknown. This paper reports on the microbial composition of activated sludge communities in eight wastewater-treatment plants (WWTPs) with different sizes and modes of operation. The polyphosphate accumulating organisms (PAOs) within this complex biocenosis were identified by fluorescent dye staining and classified by in situ hybridization techniques. Of the bacteria in the aerobic basin, 5–13% contained polyP granules. In addition, flow cytometry was used to quantify PAOs after tetracycline staining and to separate these cells. The phylogenetic affiliation of the sorted PAOs was identified by cloning and sequencing. Both workflows showed similar outcomes. The majority of PAOs in all plants were Betaproteobacteria (22%), Actinobacteria (21%), and Alphaproteobacteria (12%), with differences in the relative abundance. In addition, Bacteroidetes (12%) were detected in the clone libraries, especially Haliscomenobacter, which should be considered further with regard to its influence on the EBPR process. The denaturing gradient gel electrophoresis analyses of sorted PAOs revealed a diverse community composition of Betaproteobacteria, Actinobacteria, and Rhodocyclales in the WWTPs. PAOs were present in EBPR and non-EBPR WWTPs, and no correlation in their abundance and phylogenetic composition to the mode of operation was revealed. This study shows that specific PAO communities existed in the various WWTPs, probably favored by the respective wastewater composition, including so far unvalued PAOs species, but their active contribution in the EBPR process remains to be investigated. Read More: http://ascelibrary.org/doi/abs/10.1061/(ASCE)EE.1943-7870.0000711
- [Show abstract] [Hide abstract] ABSTRACT: Background Biogas is one of the most promising regenerative energies. The simple fundamental principle underlying behind biomethanation is the conversion of biomass into biogas by microorganisms in four steps, whereby proper process regulation is crucial. Even today, the control of biogas plants is a challenging process which is based merely on physical and chemical parameters. A lack of meaningful microbiological control variables that can be quickly and easily determined locally makes it difficult for operators to react immediately if necessary. Methods In this study, different chemical parameters and several enzyme activities as well as their response to process disturbances were analyzed using two-phase anaerobic digestion systems with maize silage as substrate. The pH value, soluble chemical oxygen demand (COD), volatile fatty acids and total inorganic carbon (VFA/TIC) ratio, and biogas composition were examined using standardized cuvette or titration tests and online monitoring equipment. Hydrolase activities of esterase, amylase, protease, alanine-aminopeptidase, carboxymethyl cellulase, and xylanase were determined photometrically. Results Hydrolases (esterase, amylase, protease, alanine-aminopeptidase, carboxymethyl cellulase, and xylanase) represent key enzymes in the first stage of biogas generation. In normal operation, the optimum values of the specific hydrolase activities varied over retention time. This allows for a visualization of the digestion progress. Furthermore, sudden strong activity attenuation has always indicated a disruption, even before any alterations in chemical parameters were affected. Beyond that, a direct correlation between esterase activity and COD discharge could also be identified. Moreover, in terms of the utilized substrate, specific microbial enzyme activities could be discovered as quality indicators. Thus, correlations to both the age of the silage and the effective cumulative COD yield could be determined. Conclusion Based on our analyses, the significance of various microbial enzyme activities and their association with the biogas process was demonstrated. The photometric determination of these enzyme activities allowed a sophisticated control of biomethanation to be carried out, which requires very little effort and equipment.
- [Show abstract] [Hide abstract] ABSTRACT: The underground uranium mine Königstein (Saxony, Germany), currently in the process of remediation, represents an underground acid mine drainage (AMD) environment, that is, low pH conditions and high concentrations of heavy metals including uranium, in which eye-catching biofilm formations were observed. During active uranium mining from 1984 to 1990, technical leaching with sulphuric acid was applied underground on-site resulting in a change of the underground mine environment and initiated the formation of AMD and also the growth of AMD-related copious biofilms. Biofilms grow underground in the mine galleries in a depth of 250 m (50 m above sea level) either as stalactite-like slime communities or as acid streamers in the drainage channels. The eukaryotic diversity of these biofilms was analyzed by microscopic investigations and by molecular methods, that is, 18S rDNA PCR, cloning, and sequencing. The biofilm communities of the Königstein environment showed a low eukaryotic biodiversity and consisted of a variety of groups belonging to nine major taxa: ciliates, flagellates, amoebae, heterolobosea, fungi, apicomplexa, stramenopiles, rotifers and arthropoda, and a large number of uncultured eukaryotes, denoted as acidotolerant eukaryotic cluster (AEC). In Königstein, the flagellates Bodo saltans, the stramenopiles Diplophrys archeri, and the phylum of rotifers, class Bdelloidea, were detected for the first time in an AMD environment characterized by high concentrations of uranium. This study shows that not only bacteria and archaea may live in radioactive contaminated environments, but also species of eukaryotes, clearly indicating their potential influence on carbon cycling and metal immobilization within AMD-affected environment.
- [Show abstract] [Hide abstract] ABSTRACT: Sediments contain a huge number and diversity of microorganisms that are important for the flux of material and are pivotal to all major biogeochemical cycles. Sediments of reservoirs are affected by a wide spectrum of allochthous and autochthonous influences providing versatile environments along the flow of water within the reservoir. Here we report on the microbial diversity in sediments of the mesotrophic drinking water reservoir Saidenbach, Germany, featuring a pronounced longitudinal gradient in sediment composition in the reservoir system. Three sampling sites were selected along the gradient, and the microbial communities in two sediment depths were characterized using catalysed reporter deposition fluorescence in situ hybridization (CARD-FISH) and a bar-coded pyrosequencing approach. Multivariate statistic was used to reveal relationships between sequence diversity and the environmental conditions. The microbial communities were tremendously diverse with a Shannon index of diversity (H') ranging from 6.7 to 7.1. 18,986 sequences could be classified into 37 phyla including candidate divisions, but the full extent of genetic diversity was not captured. While CARD-FISH gave an overview about the community composition, more detailed information was gained by pyrosequencing. Bacteria were more abundant than Archaea. The dominating phylum in all samples was Proteobacteria, especially Betaproteobacteria and Deltaproteobacteria. Furthermore, sequences of Bacteroidetes, Verrucomicrobia, Acidobacteria, Chlorobi, Nitrospira, Spirochaetes, Gammaproteobacteria, Alphaproteobacteria, Chloroflexi, and Gemmatimonadetes were found. The site ammonium concentration, water content and organic matter content revealed to be strongest environmental predictors explaining the observed significant differences in the community composition between sampling sites.
- [Show abstract] [Hide abstract] ABSTRACT: Wastewater treatment often suffers from instabilities and the failure of specific functions such as biological phosphorus removal by polyphosphate accumulating organisms. Since most of the microorganisms involved in water clarification are unknown it is challenging to operate the process accounting for the permanent varying abiotic parameters and the complex composition and unrevealed metabolic capacity of a wastewater microbial community. Fulfilling the demands for water quality irrespective of substrate inflow conditions may emit severe problems if the limited management resources of municipal wastewater treatment plants are regarded. We used flow cytometric analyses of cellular DNA and polyphosphate to create patterns mirroring dynamics in community structure. These patterns were resolved in up to 15 subclusters, the presence and abundances of which correlated with abiotic data. The study used biostatistics to determine the kind and strength of the correlation. Samples investigated were obtained from a primary clarifier and two activated sludge basins. The stability of microbial community structure was found to be high in the basins and low in the primary clarifier. Despite major abiotic changes certain subcommunities were dominantly present (up to 80% stability), whereas others emerged only sporadically (down to 3% stability, both according to equivalence testing). Additionally, subcommunities of diagnostic value were detected showing positive correlation with substrate influxes. For instance blackwater (r(s) = 0.5) and brewery inflow (both r(s) = 0.6) were mirrored by increases in cell abundances in subclusters 1 and 6 as well as 4 and 8, respectively. Phosphate accumulation was obviously positively correlated with nitrate (r(s) = 0.4) and the presence of denitrifying organisms (Rhodacyclaceae). Various other correlations between community structure and abiotic parameters were apparent. The bacterial composition of certain subcommunities was determined by cell sorting and phylogenetic tools like T-RFLP. In essence, we developed a monitoring tool which is quick, cheap and causal in its interpretation. It will make laborious PCR based technique less obligatory as it allows reliable process monitoring and control in wastewater treatment plants.
- [Show abstract] [Hide abstract] ABSTRACT: Modelling of activated sludge processes is a commonly used technique to design and optimize wastewater treatment processes. Since wastewater and activated sludge is characterized by chemical oxygen demand (COD) measurements, units of state variables describing organic matter are expressed as equivalent amounts of COD. However, current procedures for measuring it have several drawbacks, including the production of hazardous wastes, so the utility of other variables for characterizing the organic load in modelling, such as total organic carbon (TOC), warrant re-evaluation. Other advantages of TOC over COD are that it provides matrix-independent analytical results and it can be readily measured online. Proposals for TOC-based models were made in the 1990s, but they seem to have sunk into obscurity. To re-assess the value of TOC for this purpose, we have recalculated the EAWAG module for Bio-P removal coupled to the Activated Sludge Model No. 3 on a TOC basis, and tested it against data acquired in batch experiments with four single carbon sources (acetate, glucose, citrate and casein). The batch test-based calibrations showed a good match with experimental data, following modifications of the model to account for the anaerobic volumes and retention times applied in the tests.
- [Show abstract] [Hide abstract] ABSTRACT: Phase-variable expression of Legionella pneumophila lipopolysaccharide (LPS) has not been described in detail for strains possessing the virulence-associated epitope recognized by the monoclonal antibody (mAb) 3/1 of the Dresden Panel. About 75 % of cases of community-acquired legionellosis are caused by mAb 3/1-positive strains. In this study, the LPS architecture of the mAb 3/1-positive Corby strain was investigated during its life cycle in broth culture and inside monocytic host cells. During the exponential growth phase in broth, the highly acetylated and therefore strongly hydrophobic mAb 3/1 epitope is expressed continuously, but only 3 % of the bacteria can be detected using mAb 59/1, which recognizes a short-chain variant of the Legionella LPS that is less hydrophobic due to missing acetylations of the O-chain. The percentage of mAb 59/1-positive legionellae increases up to 34 % in the post-exponential growth phase. LPS shed in broth during the exponential phase is mAb 59/1-negative, and mAb 3/1-positive components do not possess short-chain molecules. The LPS pattern expressed and shed inside U937 cells and A/J mouse macrophages points to the same regulatory mechanisms. During the so-called 'pregnant pause', the period for establishment of the replicative phagosomes, the mAb 3/1-positive LPS is shed into the phagosome and seems to pass through the phagosomal membrane, while mAb 59/1-positive LPS is detectable only on the bacterial surface. After egress of the legionellae into the cytoplasm followed by host cell lysis, individual bacteria are mAb 3/1-positive and mAb 59/1-negative. Intracellularly formed Legionella clusters consist of surface-located mAb 3/1-positive bacteria, which are predominantly mAb 59/1-negative. They surround less hydrophobic and therefore closely packed mAb 59/1-positive bacteria. Based on the different degrees of hydrophobicity, bacteria are able to support the expression of two functionally different LPS architectures, namely more hydrophobic LPS for surviving in aerosols and more hydrophilic LPS for close-packing of legionellae inside clusters.
- [Show abstract] [Hide abstract] ABSTRACT: Legionella-like isolates, strains W03-356(T), W03-357 and W03-359, from three independent water samples from the river Elbe, Germany, were analysed by using a polyphasic approach. Morphological and biochemical characterization revealed that they were Gram-negative, aerobic, non-spore-forming bacilli with a cut glass colony appearance that grew only on L-cysteine-supplemented buffered charcoal yeast extract agar. Phylogenetic analysis based on sequence comparisons of the 16S rRNA, macrophage infectivity potentiator (mip), gyrase subunit A (gyrA), ribosomal polymerase B (rpoB) and RNase P (rnpB) genes confirmed that the three isolates were distinct from recognized species of the genus Legionella. Phenotypic characterization of strain W03-356(T) based on fatty acid profiles confirmed that it was closely related to Legionella rubrilucens ATCC 35304(T) and Legionella pneumophila ATCC 33152(T), but distinct from other species of the genus Legionella. Serotyping of the isolates showed that they were distinct from all recognized species of the genus Legionella. Strains W03-356(T), W03-357 and W03-359 are thus considered to represent a novel species of the genus Legionella, for which the name Legionella dresdenensis sp. nov. is proposed. The type strain is W03-356(T) (=DSM 19488(T)=NCTC 13409(T)).
- [Show abstract] [Hide abstract] ABSTRACT: The community-level substrate utilization test based on direct incubation of environmental samples in Biolog EcoPlates™ is a suitable and sensitive tool to characterize microbial communities. The aim of this study was to investigate the influence of plant roots and soil structure on the metabolic diversity of microorganisms in a constructed wetland with vertical flow.Sediment samples were taken from different filter depths representing specific filter layers. The color development representing the substrate utilization was measured with the samples over a period of 10 days. The average well color development (AWCD) for all carbon sources was calculated as an indicator of total activity and in order to compensate the influence of the inoculum's density on the color development in the plates. After transformation by dividing by the AWCD, the optical density data were analysed by principal component analysis (PCA). An analysis of the kinetic profile of the AWCD was carried out to increase the analytical power of the method. The corrected data have been successfully fit to the logistic growth equation. Three kinetic model parameters, the asymptote (K), the exponential rate of color change (p) and the time to the midpoint of the exponential portion of the curve (s), were used for statistical analysis of the physiological profile of the microbial community in the different filter layers of the constructed wetland.We found out that in the upper two horizons, which were rooted most densely, mainly easily degradable materials like specific carbohydrates were utilized, while in the lower layers, where only single roots occur, more biochemically inert compounds, e.g. 2-hydroxy benzoic acid, were utilized. Furthermore it could be shown that microorganisms in the surface layer benefited from the plant litter because they can utilize decay products of these. In the lower filter layers specialists took advantage because they had to cope with the biochemically inert materials and the lower nutrient supply.
- [Show abstract] [Hide abstract] ABSTRACT: Wastewater treatment plants with enhanced biological phosphorus removal represent a state-of-the-art technology. Nevertheless, the process of phosphate removal is prone to occasional failure. One reason is the lack of knowledge about the structure and function of the bacterial communities involved. Most of the bacteria are still not cultivable, and their functions during the wastewater treatment process are therefore unknown or subject of speculation. Here, flow cytometry was used to identify bacteria capable of polyphosphate accumulation within highly diverse communities. A novel fluorescent staining technique for the quantitative detection of polyphosphate granules on the cellular level was developed. It uses the bright green fluorescence of the antibiotic tetracycline when it complexes the divalent cations acting as a countercharge in polyphosphate granules. The dynamics of cellular DNA contents and cell sizes as growth indicators were determined in parallel to detect the most active polyphosphate-accumulating individuals/subcommunities and to determine their phylogenetic affiliation upon cell sorting. Phylotypes known as polyphosphate-accumulating organisms, such as a “Candidatus Accumulibacter”-like phylotype, were found, as well as members of the genera Pseudomonas and Tetrasphaera. The new method allows fast and convenient monitoring of the growth and polyphosphate accumulation dynamics of not-yet-cultivated bacteria in wastewater bacterial communities.
- [Show abstract] [Hide abstract] ABSTRACT: An enzyme which regulates the metabolism of phosphate is the polyphosphate kinase (ppk). Therefore, it is an important criterion for the identification of polyphosphate accumulating bacteria (PAO) in activated sludge. With the published primer systems it is currently impossible to detect the ppk gene simultaneously from different microorganisms. The aim of this study was the development of new primer systems for the acquisition of a wide range of ppk sequences. The specific amplification was tested by cloning and sequencing. As an outcome a large diversity of ppk sequences was detected. In addition differences in the composition of the ppk sequences in samples of the activated sludge of the investigated plants were shown by using the terminal restriction fragment length polymorphism analysis (T-RFLP).
- [Show abstract] [Hide abstract] ABSTRACT: Sediments in standing waters are complex habitats that are able to provide favorable living conditions for manifold microbial species. The aim of this study was to investigate the spatial and seasonal composition of bacteria in freshwater sediments and the chemical conditions. Four sampling points were investigated at monthly intervals along the flow of water from a pre-reservoir down to the main dam of the Saidenbach reservoir, a drinking water reservoir located in Saxony, Germany. Selected chemical criteria of the sediments were analyzed, as well as the physical conditions of the water body at the chosen sites. Redox-sensitive chemical criteria, such as nitrogen, sulfur, iron and manganese compounds displayed very steep vertical gradients within the sediment. Phylogenetic composition of the communities was investigated utilizing clone libraries and catalyzed reporter deposition fluorescence in situ hybridization (CARD-FISH). Up to 90% of the cells that were stained with propidium iodide hybridized with probes for Bacteria and Archaea. This amount decreased with sediment depth. The predominant bacterial groups found in the sediment were α-Proteobacteria, especially at the sampling point with a high content of planktogenic organic material closest to the main dam, as well as β-Proteobacteria and the Cytophaga–Flavobacterium cluster. Clone libraries allowed a more detailed view of the microbial diversity within the detected phylogenetic groups. Despite an observed variation of α-Proteobacteria and bacteria belonging to the Cytophaga–Flavobacterium cluster, a regression analysis showed very weak multiple correlation coefficients between chemical criteria and the analyzed broad phylogenetic bacterial groups.
- [Show abstract] [Hide abstract] ABSTRACT: Analysis of environmental bacteria on the single cell level often requires fixation to store the cells and to keep them in a state as near life-like as possible. Fixation procedures should furthermore counteract the increase of autofluorescence, cell clogging, and distortion of surface characteristics. Additionally, they should meet the specific fixation demands of both aerobically and anaerobically grown bacteria. A fixation method was developed based on metal solutions in combination with sodium azide. The fixation efficiencies of aluminium, barium, bismuth, cobalt, molybdenum, nickel, and tungsten salts were evaluated by flow cytometric measurement of the DNA contents as a bacterial population/community stability marker. Statistical equivalence testing was involved to permit highly reliable flow cytometric pattern evaluation. Investigations were carried out with pure cultures representing environmentally important metabolic and respiratory pathways as controls and with activated sludge as an example for highly diverse bacterial communities. A mixture of 5 mM barium chloride and nickel chloride, each and 10% sodium azide was found to be a suitable fixative for all tested bacteria. The described method provided good sample stability for at least 9 days.
Sächsische Akademie der Wissenschaften zu LeipzigLeipzig, Saxony, Germany
Technische Universität Dresden
Dresden, Saxony, Germany
- Institute for Microbiology