Bacterial lipolytic activity in a hypertrophic dead arm of the river Danube in Vienna

The kinetic parameters of lipase, bacterial secondaryproduction (BSP) and bacterial numbers (BN) were determined fortnightlyduringthe development of the summer phytoplankton bloom at twostationsof Alte Donau, a hypertrophic stagnant dead arm of the riverDanubein Vienna. Until the middle of August we observed a gradualincrease in lipase activity as well as BN and BSP rates tothe maximum of 19.9 nmol l^–1 h^–1,4.5×10⁹cells l^–1 and 8.1 g C l^–1 h^–1,respectively. Atthe end of August and during September we found a markeddecreasein all bacterial parameters, coinciding with a progressingincreaseof chlorophyll a concentrations at both sampling sites. Themaximalvalues of lipase V^max were determined in the bottom waterlayer (avg. 13.7±6.5 nmol l^–1 h^–1) probablyowingto the predominating importance of polymeric matter in thesubstrate pool for microheterotrophs in this water zone.Differential filtration experiments showed that 20.1% to56.3% ofthe total lipase activity and 4.2% to 9.0% of the totalbacterialnumbers in Alte Donau water samples occurred in 0.2-mfiltrate. Further experiments indicated that the highcontributionto lipase activity in the 0.2-m filtrate was rather dueto thepresence of 0.2 m filterable bacteria than to solubleenzymemolecules. Moreover, we observed higher bacterial lipaseactivityin 0.2 m filtrate than in unfiltered samples. Thepossibleinfluence of limiting factors on the metabolism of insitubacteria is discussed.     

How many damaged corals in Red Sea reef systems? A quantitative survey

Quantitative coral damage assessment by means of line transects was performed in several northern Red Sea coral reef sites in Israel (Eilat) and Egypt (Hurghada area). Reefs with high and low visitor frequency were compared. For both reef systems, breakage was found to be the most common damage category, being significantly higher on highly frequented reefs. Also, all observed damage (breakage, tissue loss, algal overgrowth) was most frequent within the first ten meters depth. A significant difference in the amount of corals overgrown by algae was found on the reefs near Hurghada as compared to all other reefs. Algal overgrowth was correlated with the occurrence of tissue loss and breakage, being considered as a consequence of pollution or the former damage types. In all cases of damage, Acropora was the most frequently affected genus, while Millepora dichotoma was the most affected species.     

<Emphasis Type="Italic">Drosophila</Emphasis><Emphasis Type="Italic">P</Emphasis> transposons of the urochordata <Emphasis Type="Italic">Ciona intestinalis</Emphasis>

P transposons belong to the eukaryotic DNA transposons, which are transposed by a cut and paste mechanism using a P-element-coded transposase. They have been detected in Drosophila, and reside as single copies and stable homologous sequences in many vertebrate species. We present the P elements Pcin1, Pcin2 and Pcin3 from Ciona intestinalis, a species of the most primitive chordates, and compare them with those from Ciona savignyi. They showed typical DNA transposon structures, namely terminal inverted repeats and target site duplications. The coding region of Pcin1 consisted of 13 small exons that could be translated into a P-transposon-homologous protein. C. intestinalis and C. savignyi displayed nearly the same phenotype. However, their P elements were highly divergent and the assumed P transposase from C. intestinalis was more closely related to the transposase from Drosophila melanogaster than to the transposase of C. savignyi. The present study showed that P elements with typical features of transposable DNA elements may be found already at the base of the chordate lineage. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Contribution of virus-induced lysis and protozoan grazing to benthic bacterial mortality estimated simultaneously in microcosms

In contrast to the water column, the fate of bacterial production in freshwater sediments is still a matter of debate. Thus, the importance of virus-induced lysis and protozoan grazing of bacteria was investigated for the first time simultaneously in a silty sediment layer of a mesotrophic oxbow lake. Microcosms were installed in the laboratory in order to study the dynamics of these processes over 15 days. All microbial and physicochemical parameters showed acceptable resemblance to field data observed during a concomitant in situ study, and similar conclusions can be drawn with respect to the quantitative impact of viruses and protozoa on the bacterial compartment. Viral decay rates ranged from undetectable to 0.078 h⁻¹ (average, 0.033 h⁻¹), and the control of bacterial production from below the detection limit to 36% (average, 12%). The contribution of virus-induced lysis of bacteria to the dissolved organic matter pool as well as to benthic bacterial nutrition was low. Ingestion rates of protozoan grazers ranged from undetectable to 24.7 bacteria per heterotrophic nanoflagellate (HNF) per hour (average, 4.8 bacteria HNF⁻¹ h⁻¹) and from undetectable to 73.3 bacteria per ciliate per hour (average, 11.2 bacteria ciliate⁻¹ h⁻¹). Heterotrophic nanoflagellate and ciliates together cropped up to 5% (average, 1%) of bacterial production. The viral impact on bacteria prevailed over protozoan grazing by a factor of 2.5–19.9 (average, 9.5). In sum, these factors together removed up to 36% (average, 12%) of bacterial production. The high number of correlations between viral and protozoan parameters is discussed in view of a possible relationship between virus removal and the presence of protozoan grazers.     

Weak Coupling between Heterotrophic Nanoflagellates and Bacteria in a Eutrophic Freshwater Environment

In a study on the dynamics and trophic role of the heterotrophic nanoflagellate (HNAN) assemblage in the microbial food web of a eutrophic oxbow lake abundances, biomass, and production rates of HNAN and their potential prey organisms, namely heterotrophic bacteria and autotrophic picoplankton, were monitored for a period of 2 years. No coupling between HNAN abundance and biomass and the abundance and biomass of their picoplanktonic prey was observed for the investigation period. The ratio of heterotrophic bacterial to HNAN abundance ranged from 2.2 x 103 to 8.6 x 103 (mean: 4.2 x 103 +/- 1.8 x 103). HNAN carbon consumption could account for only 10% to 40% of bacterial secondary production. The lack of coupling between HNAN and their potential prey and the low HNAN abundance relative to bacterial abundance suggested (a) that HNAN grazing was an insignificant factor in the regulation of bacterial abundance and (b) that HNAN abundance was regulated by predation rather than by prey abundance. This hypothesis was supported by the fact that HNAN growth rates were high (in the range of 0.45 d-1 to 1.00 d-1 during spring and summer, yearly mean: 0.52 d-1), and only weakly correlated with prey abundance and biomass. The results indicated strong top-down control of HNAN and consequently a weak coupling of HNAN and picoplankton in the investigated eutrophic freshwater environment.     

Carbon fluxes in the microbial loop: Comments

The heterotrophic bacterial community of oceanic aggregates which mediates particle solubilization, displays features (low carbon demand and low turnover) that are difficult to reconcile with the observed high enzyme activities and cell numbers. Hypotheses are proposed to explain this discrepancy. Furthermore, production of both free and attached bacteria may have been underestimated by neglecting the ultramicrobacteria (UMB). Production of UMB may represent up to 28% of the total bacterial production.     

Variability and predictability of the empirical conversion factor for converting 3H-thymidine uptake into bacterial carbon production for a eutrophic lake

To assess the variability and predictability of the empiricalconversion factor (CF) for converting the uptake of ³H-thymidine(TDR) to bacterial carbon production rates in eutrophic environments,we performed 11 dilution culture experiments and results wererelated to a set of important concomitantly recorded environmentalvariables. TDR incorporation data from a field study were thenconverted to carbon production with the established empiricalCF and a theoretical CF and compared to production estimatesderived from leucine incorporation. Mean empirical CF variedbetween 0.5 and 7.0 x 10⁶ cells (pmol TDR)^–1 over theyear and showed highly significant negative correlation withTDR incorporation rate and a highly significant positive correlationto temperature. Values of carbon production derived with thevariable empirical CF were lower than the values obtained bythe use of the theoretical CF of 1 x 10⁶ cells (pmol TDR)^–1.They showed less seasonal variation than values obtained byleucine incorporation, and periods of uncoupling were observed.However, when the empirical CF was calculated from the multipleregression equation including TDR incorporation and temperature,the resulting carbon production rates showed a high correspondencewith the leucine-derived production rates. Results of the analyseswere interpreted as an indication that under favourable conditions(resulting in high TDR incorporation) bacteria may be able tooptimize DNA duplication over protein synthesis as a possiblestrategy to persist and to maintain their potential to divideunder limiting conditions (e.g. decrease in temperature andsubstrate availability). In unfavourable conditions (resultingin low incorporation rates), bacteria may then use already producedDNA copies for rapid growth, when the environmental conditionsturn favourable again. Thus, an experimental set-up causingnutrient enrichment of the sampled water by autoclaving andfiltering, as generally used for dilution culture experiments,does not always reflect in situ situations, especially duringperiods of low nutrient concentrations.     

Development of the bacterial compartment along the Danube River: a continuum despite local influences

Microbial food webs dominate heterotrophic food webs in large rivers with bacterial metabolism being a key component of carbon processing. Thus, analysis of bacterial population dynamics is critical to understanding patterns and mechanisms of material cycling and energy fluxes in large rivers. Within the frame of the Joint Danube Survey (JDS) 2007, the longitudinal development of the natural bacterial community in the Danube in terms of bacterial numbers, morphotype composition, and heterotrophic production of the suspended and particle-attached fractions was followed at a fine spatial resolution of approximately 30 km for the first time in such a large river along a 2,600-km stretch. Twenty-one major tributaries and branches were also included. This allowed us to investigate whether bacterial standing stock and production undergo continuous, linear changes or whether discontinuities and local processes like the merging of tributaries or the potential impact of sewage input drive the bacterial population in the Danube. The presented investigation revealed surprising continuous patterns of changes of bacterial parameters along the Danube River. Despite the presence of impoundments or hydropower plants, large municipalities, and the discharge of large tributaries, most bacterial parameters (standing stock, morphotype succession, and attached bacterial production) developed gradually, indicating that mainly broad-scale drivers and not local conditions shape and control the bacterial community in the midstream of this large river. As most important broad-scale drivers, nutrients (inorganic and organic) and changes in particle concentrations were identified. These data are also in remarkable accordance with the patterns of changes of the genetic bacterial community composition, observed during the first JDS (2001) 6 years before. In contrast, bacterial activity did not follow a continuous trend and was mainly controlled by the input of sewage from large cities in the middle section, leading to a bloom of phytoplankton. The observed patterns and the comparison between the Danube, its tributaries and other large rivers worldwide indicate that the bacterial community in rivers has a powerful indicator function for estimating the ecological status of large river ecosystems once enough information has been collected at various temporal and spatial scales.     

Growth Response of Soda Lake Bacterial Communities to Simulated Rainfall

Moderately saline soda lakes harbor extremely abundant and fast growing bacterial communities. An interesting phenomenon of an explosive bacterial growth in shallow soda lakes in Eastern Austria after dilution with rainwater, concomitantly with a significant decrease in temperature was observed in a former study. In the present study, we tried to identify the factors being responsible for this enhanced bacterial growth in laboratory batch cultures. Three experiments were performed with water taken from two different lakes at different seasons. Natural soda lake water was diluted with distilled water, artificial lake water, sterile filtered soda lake water, and grazer-free water to test (1) for the influence of compatible solutes released to the environment and reduced salt stress after osmotic down-shock, (2) for the influence of nutrients, which may be washed in from the dry areas of the lake bottom after rainfall and (3) for the decrease of grazing pressure due to dilution. The potential influence of (4) viruses was indirectly deduced. The response of the bacterial community to the manipulations was measured by changes in bacterial numbers, the incorporation of ³H-leucine and the concomitant determination of the amount of ³H-leucine uptaking bacteria by microautoradiography. The influence of the environmental factors enhancing bacterial growth after a simulated rainfall event showed variations between the lakes and over the seasons. The addition of nutrients was, in all experiments, the main factor triggering bacterial growth. The decrease in grazing pressure and viral lysis after dilution was of significant importance in two of three experiments. In the experiment with the highest salinity, we could show that either compatible solutes released after osmotic down-shock and used as a source of nutrients for the soda lake bacterial populations or reduced salt stress were most probably responsible for the observed marked enhancement of bacterial growth.     

Effects of Deposit-Feeding Macrofauna on Benthic Bacteria,Viruses, and Protozoa in a Silty Freshwater Sediment

In microcosm experiments, we simultaneously tested the effects of increased numbers of deposit-feeding macrofauna (chironomids, oligochaetes and cladocerans) on the standing stock, activities and interactions of heterotrophic bacteria, viruses, and bacterivorous protozoa (heterotrophic nanoflagellates and ciliates) in the aerobic layer of a silty littoral freshwater sediment. On average, bacterial secondary production was stimulated between 11 and 29% by all macrofaunal groups compared to control experiments without macrofauna addition. Bacterial standing stock increased significantly by 8 and 13% in case of chironomids and cladocerans, respectively. Oligochaetes and chironomids produced significant negative effects on viral abundance while the results with cladocerans were inconsistent. The addition of oligochaetes and chironomids resulted in a significant decrease by on average 68 and 32% of viral decay rates, respectively, used as a measure of viral production. The calculated contribution of virus-induced lysis to benthic bacterial mortality was low, with 2.8 to 11.8% of bacterial secondary production, and decreased by 39 to 81% after the addition of macrofauna compared to the control. The abundances of heterotrophic nanoflagellates were significantly reduced by 20% by all tested macrofauna groups, while ciliates showed inconsistent results. The importance of heterotrophic nanoflagellate grazing on benthic bacteria was very low (<1% of bacterial secondary production) and was further reduced by elevated numbers of macrofauna. Thus, the selected deposit feeding macrofauna groups seem to have several direct and indirect and partly antagonistic effects on the benthic bacterial compartment through the enhancement of bacterial production and the reduction of virus-induced cell lysis and protozoan grazing.