Peter Peduzzi

Diel, Seasonal, and Depth-Related Variability of Viruses and Dissolved DNA in the Northern Adriatic Sea

The depth-dependent, seasonal, and diel variability of virus numbers, dissolved DNA (D-DNA), and other microbial parameters was investigated in the northern Adriatic Sea. During periods of water stratification, we found higher virus abundances and virus/bacterium ratios (VBRs) as well as a larger variability of D-DNA concentrations at the thermocline, probably as a result of higher microbial biomass. At the two investigated stations, virus densities were highest in summer and autumn (up to 9.5 × 1010 1−1) and lowest in winter (< 109 1−1); D-DNA concentrations were highest in summer and lowest in winter. The VBR as well as an estimated proportion of viral DNA on total D-DNA showed a strong seasonal variability. VBR averaged 15.0 (range, 0.9–89.1), and the percentage of viral DNA in total D-DNA averaged 18.3% (range, 0.1–96.1%). An estimation of the percentage of bacteria lysed by viruses, based on 2-h sample intervals in situ, ranged from 39.6 to 212.2% d−1 in 5 m and from 19.9 to 157.2% d−1 in 22 m. The estimated contribution of virus-mediated bacterial DNA release to the D-DNA pool ranged from 32.9 to 161% d−1 in 5 m and from 10.3 to 74.2% d−1 in 22 m. Multiple regression analysis and the diel dynamics of microbial parameters indicate that viral lysis occasionally could be more important in regulating bacterial abundances than grazing by heterotrophic nanoflagellates.

Complexity of bacterial communities in a river-floodplain system (Danube, Austria)

ABSTRACT Natural floodplains play an essential role in the processing and decomposition of organic matter and in the self-purification ability of rivers, largely due to the activity of bacteria. Knowledge about the composition of bacterial communities and its impact on organic-matter cycling is crucial for the understanding of ecological processes in river-floodplain systems. Particle-associated and free-living bacterial assemblages from the Danube River and various floodplain pools with different hydrological characteristics were investigated using terminal restriction fragment length polymorphism analysis. The particle-associated bacterial community exhibited a higher number of operational taxonomic units (OTUs) and was more heterogeneous in time and space than the free-living community. The temporal dynamics of the community structure were generally higher in isolated floodplain pools. The community structures of the river and the various floodplain pools, as well as those of the particle-associated and free-living bacteria, differed significantly. The compositional dynamics of the planktonic bacterial communities were related to changes in the algal biomass, temperature, and concentrations of organic and inorganic nutrients. The OTU richness of the free-living community was correlated with the concentration and origin of organic matter and the concentration of inorganic nutrients, while no correlation with the OTU richness of the particle-associated assemblage was found. Our results demonstrate the importance of the river-floodplain interactions and the influence of damming and regulation on the bacterial-community composition.

Floating mucilage in the Northern Adriatic Sea : the potential of a microbial ecological approach to solve the mystery

Based on recent findings in aquatic microbial ecology we present two hypotheses which should enable us to elucidate the role of anthropogenic eutrophication in the fonnation of the large standing crop of marine snow in the Northern Adriatic Sea. The first hypothesis postulates diverging control instances for phytoplankton production in oligotrophic and eutrophic systems (...)

Hydrological control of system characteristics of floodplain lakes

Abstract Water bodies within floodplains are strongly governed by their hydrological connectivity with the main river (Heiler et al. 1995). Connectivity is decisive for the general ecosystem characteristics and the balance between input and output of nutrients and organic carbon sources. A major determinant is the abundance of macrophytic vegetation, which is controlled by through-flow, dry-falling and scouring effects. The autochthonous primary production is strongly augmented by local terrestrial carbon sources and by the riverine transport in form of dissolved and particulate organic matter. The significance of these different sources depends on hydrology. Flood pulses represent a disturbance to pelagic communities (followed by successions) and a source of nutrients. High loads of inorganic nutrients produce conditions for prolific algal growth. High availability of allochthonous organic carbon provides the basis for a prolific bacterial secondary production. Our data emphasize that the stimulus of local aquatic primary production enhances carbon utilization by bacteria.

Imaging and quantifying virus fluorescence signals on aquatic aggregates: a new method and its implication for aquatic microbial ecology

The development of accurate methods to detect and enumerate viruses is an important issue in aquatic microbial ecology. In particular, viruses attached to floating aggregates are a largely ignored field both in marine and inland water ecology. Data on the total abundance and the colonization of aggregates by viruses are rare, mainly due to methodological difficulties. In the present study, we used confocal laser scanning microscopy (CLSM) to resolve fluorescence signals of single viruses and bacterial cells in a complex three-dimensional matrix of riverine aggregates. CLSM in combination with different fluorochromes is a very promising approach for obtaining information both on the aggregate architecture and on the spatial distribution of viruses attached to fully hydrated aggregates. Aggregates from the Danube River harbored up to 5.39 x 10(9) viruses cm(-3). We discuss the problems associated with different methods such as sonication or directly counting viruses on aggregates, both combined with epifluorescence microscopy and CLSM, to quantify viruses on suspended particles.

Viruses, bacteria and suspended particles in a backwater and main channel site of the Danube (Austria)

Abstract.A short overview of currently available studies on the ecology of viruses in running waters is provided. Additionally, a survey was conducted on the dynamics of both viruses and bacteria in an isolated floodplain segment of the Danube River and in the main channel near Vienna (Austria) during the hydrologically most dynamic phase (spring – summer). The study evaluates the differences between the main channel and the floodplain segment for suspended particle abundance and quality in relation to bacterial and viral parameters; both free-living forms and those attached to particles are examined. The hydrological disconnection of these two contrasting sampling sites influenced particle abundance and quality as well as the distribution of free-living vs. attached bacteria and viruses. The per-cell activity of bacteria attached to particles was significantly higher than that of the free-living fraction, particularly in the isolated water body. The abundance of bacteria and viruses on particles depended on particle quality (size). In the main channel, bacteria were significantly more abundant on surfaces (per mm2) of suspended matter > 5 μm (aggregates with organic constituents) compared to particles < 5 μm (mostly mineral grains). In the isolated water body, both bacteria and viruses were more abundant on the larger particles/aggregates. Data from both locations revealed a positive correlation between abundance of particles > 5μm and attached viruses; free-living viruses were less abundant at high > 5μm particle loads. Only in the isolated floodplain section was viral abundance positively influenced by elevated per-cell productivity of potential host bacteria. The results demonstrate that system variability on a relatively small topographical scale (within a river-floodplain system) has consequences for microbial life, including viruses.

Structure and Composition of Aggregates in Two Large European Rivers, Based on Confocal Laser Scanning Microscopy and Image and Statistical Analyses

ABSTRACT Floating riverine aggregates are composed of a complex mixture of inorganic and organic components from their respective aquatic habitats. Their architecture and integrity are supplemented by the presence of extracellular polymeric substances of microbial origin. They are also a habitat for virus-like particles, bacteria, archaea, fungi, algae, and protozoa. In this study we present different confocal laser scanning microscopy strategies to examine aggregates collected from the Danube and Elbe Rivers. In order to collect multiple types of information, various approaches were necessary. Small aggregates were examined directly. To analyze large and dense aggregates, limitations of the technique were overcome by cryo-sectioning and poststaining of the samples. The staining procedure included positive staining (specific glycoconjugates and cellular nucleic acid signals) as well as negative staining (aggregate volume) and multichannel recording. Data sets of cellular nucleic acid signals (CNAS) and the structure of aggregates were visualized and quantified using digital image analysis. The Danube and Elbe Rivers differed in their aggregate composition and in the relative contribution of specific glycoconjugate and CNAS volume to the aggregate volume; these contributions also changed over time. We report different spatial patterns of CNAS inside riverine aggregates, depending on aggregate size and season. The spatial structure of CNAS inside riverine aggregates was more complex in the Elbe River than in the Danube River. Based on our samples, we discuss the strengths and challenges involved in scanning and quantifying riverine aggregates.

Dissolved organic matter (DOM) and bacterial growth in floodplains of the Danube River under varying hydrological connectivity

Inorganic nutrients, distribution of low and high molecular weight dissolved organic matter (DOM), bulk DOM constituents, carbohydrate contents and a fl uorescence index of DOM, along with bacterial secondary production (BSP), were determined in a large river fl oodplain system. The aim was to elucidate the signifi cance of DOM in relation to hydrological conditions. Two fl oodplain segments with varying degree of connection to the main channel (and for some parameters also the main channel) of the Danube River downstream of Vienna (Austria) were investigated during two hydrologically dynamic periods. In most cases, inorganic nutrients were signifi cantly coupled to hydrology, but bulk dissolved organic nutrient species in two DOM size fractions were poorly correlated with changing connectivity with the main river. A more specifi c investigation of DOM properties (fl uorescence in- dex based on fulvic acids, carbohydrate content of dissolved organic carbon) revealed good correlation with hydrol- ogy, thus mirroring more directly the effect of changing fl oodplain connectivity and emphasising the importance of local (autochthonous) carbon sources at lower connectivity. BSP, measured during one of the investigated periods, was highly variable, with low values at high discharge and a signifi cant positive dependency on increasing autoch- thonous DOM-sources indicated by higher fl uorescence indices. Hydrological connectivity between the main river and its fl oodplains appears to be a crucial driving force infl uencing the quantity but in particular the quality of DOM as well as related microbial processes.

Online Tool for Analysis of Denaturing Gradient Gel Electrophoresis Profiles

ABSTRACT We present an online tool (EquiBands, http://www.univie.ac.at/IECB/limno/equibands/EquiBands.html ) that quantifies the matching of two bands considered to be the same in different samples, even when samples are applied to different denaturing gradient gel electrophoresis gels. With an environmental example we demonstrate the procedure for the classification of two bands of different samples with the help of EquiBands.

Role of bacteria in decomposition of faecal pellets egested by the epiphyte-grazing gastropod Gibbula umbilicaris

The significance of the microheterotrophic utilization of faecal pellets derived from Gibbula umbilicaris — one of the most important gastropod species in the Posidonia oceanica ecosystem around the Isle of Ischia (Italy) — was investigated by means of microcosm-experiments. Initial total organic carbon (TOC) content of faecal pellets was 32 μgC (mg faeces dry wt)-1 and declined continuously over a 2-month incubation period. The low values of TOC coincided with SEM observations of pellets which were found to consist mainly of diatom frustules and other slowly utilizable material. In a long-term experiment freshly egested faecal pellets were rapidly colonized by bacteria, which reached densities of up to 14x105 cells (mg faeces dry wt)-1 within 12 h. Thereafter bacterial numbers declined and fluctuated in a very narrow range between 2 and 3.5x105 cells (mg faeces dry wt)-1 during the two months of the investigation period. In short-term experiments (over 5 d) similar trends were observed. Peak densities of attached bacteria were followed by a decline of this population. Concurrently the number of free-living bacteria increased. This observation was confirmed by O2 consumption measurements of freshly egested faecal pellets over 24 h, where the highest O2 consumption rates were obtained after 12 h, followed by a rapid decline, thus supporting the view that detachment of bacteria may occur. Moreover, the respiration data indicate that about 4% of faecal TOC are remineralized within the first day.