Showing papers by "Roland Psenner published in 1997"

Temperature effects on the acidity of remote alpine lakes

Abstract: Climate variations and changes in sulphur and nitrogen deposition from the atmosphere influence the acid–base balance of sensitive lakes in a complex and site-specific way1–3. For example, although lakes in several regions have shown a decline in sulphate concentration following reductions in atmospheric sulphate deposition4–6, the expected recovery of pH and alkalinity has not always taken place, implicating an additional response to changes in the local climate. Here we report a study of 57 remote alpine lakes which shows that, between 1985 and 1995, lake pH and the concentration of sulphate, base cations and silica have increased, whereas inorganic nitrogen concentrations have decreased. This contrasts with atmospheric input trends, which have led to a decrease in sulphate and a slight increase in nitrogen deposition over the same period7,8. We propose that the changes in lake chemistry are therefore likely to be caused by enhanced weathering and increased biological activity resulting from an increase in air temperature of about 1 °C since 1985. Our analysis of an alpine lake core covering a 200-year period provides further evidence for a strong positive correlation between pH and mean air temperatures, and thus for the high sensitivity of lakes at high altitudes and high latitudes to climate warming. In these remote locations, temperature effects, rather than acid deposition, appear to dominate changes in lake acidity.

Morphological and compositional shifts in an experimental bacterial community influenced by protists with contrasting feeding modes.

Abstract: In a two-stage continuous-flow system, we studied the impacts of different protozoan feeding modes on the morphology and taxonomic structure of mixed bacterial consortia, which were utilizing organic carbon released by a pure culture of a Rhodomonas sp. grown on inorganic medium in the first stage of the system. Two of three second stages operated in parallel were inoculated by a bacterivorous flagellate, Bodo saltans, and an algivorous ciliate, Urotricha furcata, respectively. The third vessel served as a control. In two experiments, where algal and bacterial populations grew at rates and densities typical for eutrophic waters, we compared community changes of bacteria, algae, and protozoa under quasi-steady-state conditions and during the transient stage after the protozoan inoculation. In situ hybridization with fluorescent oligonucleotide probes and cultivation-based approaches were used to tentatively analyze the bacterial community composition. Initially the cell size distribution and community structure of all cultivation vessels showed similar patterns, with a dominance of 1- to 2.5-(mu)m-long rods from the beta subdivision of the phylum Proteobacteria ((beta)-Proteobacteria). Inoculation with the ciliate increased bacterial growth in this substrate-controlled variant, seemingly via a recycling of nutrients and substrate released by grazing on algae, but without any detectable effect on the composition of bacterial assemblage. In contrast, an inoculation with the bacterivore, B. saltans, resulted in a decreased proportion of the (beta)-Proteobacteria. One part of the assemblage (<4% of total bacterial numbers), moreover, produced large grazing-resistant threadlike cells. As B. saltans ingested only cells of <3 (mu)m, this strategy yielded a refuge for (symbl)70% of total bacterial biomass from being grazed. Another consequence of the heavy predation in this variant was a shift to the numerical dominance of the (alpha)-Proteobacteria. The enhanced physiological status of the heavily grazed-upon segment of bacterial community resulted in a much higher proportion of CFU (mean, 88% of total bacterial counts) than with other variants, where CFU accounted for (symbl)30%. However, significant cultivation-dependent shifts of the bacterial community were observed toward (gamma)-Proteobacteria and members of the Cytophaga/Flavobacterium group, which demonstrated the rather poor agreement between cultivation-based approaches and oligonucleotide probing.

Contrasting bacterial strategies to coexist with a flagellate predator in an experimental microbial assemblage.

Abstract: We studied predator-induced changes within a slowly growing mixed microbial assemblage that was sustained by algal exudates in a continuous cultivation system. In situ hybridization with fluorescent monolabeled oligonucleotide probes was used for a tentative community analysis. This method also allowed us to quantify the proportions of predators with ingested bacteria of different taxonomic groups. In addition, we determined grazing rates on bacteria with fluorescently labelled prey. Bacteria belonging to the alpha and beta subdivisions of the phylum Proteobacteria ((alpha)- and (beta)-Proteobacteria, respectively) showed very different responses to the addition of a bacterivorous flagellate, Bodo saltans. Within one day, filamentous protist-inedible bacteria developed; these belonged to the (beta)-Proteobacteria and constituted between 8.7 and 34% of bacteria from this subgroup. Total abundance of (beta)-Proteobacteria decreased from 3.05 x 10(sup6) to 0.23 x 10(sup6) cells ml(sup-1), and estimated cell division rates were low. Other morphologically inconspicuous protist-edible bacteria belonging to the (alpha)-Proteobacteria were found to respond to predation by an increase in growth rate. Although these bacteria were heavily grazed upon, as on average >85% of flagellate cells had ingested (alpha)-Proteobacteria, they numerically dominated after the addition of B. saltans (mean, 1.35 x 10(sup6) cells ml(sup-1)). It was thus mainly those fast-dividing strains of (alpha)-Proteobacteria that supported the growth of the flagellate population. We conclude that bacteria in mixed assemblages can adopt at least two distinct strategies as a reaction to intense flagellate predation: to outgrow predation pressure or to develop inedible, inactive filaments. Since these strategies occurred within 24 h after the addition of the flagellate, we hypothesize that chemical stimuli released by the predator may have triggered bacterial responses.

Inhibitory effect of solar radiation on thymidine and leucine incorporation by freshwater and marine bacterioplankton.

Abstract: We studied the effect of solar radiation on the incorporation of [(sup3)H]thymidine ([(sup3)H]TdR) and [(sup14)C]leucine ([(sup14)C]Leu) by bacterioplankton in a high mountain lake and the northern Adriatic Sea. After short-term exposure (3 to 4 h) of natural bacterial assemblages to sunlight just beneath the surface, the rates of incorporation of [(sup3)H]TdR and [(sup14)C]Leu were reduced at both sites by up to (symbl)70% compared to those for the dark control. Within the solar UV radiation (290 to 400 nm), the inhibition was caused exclusively by UV-A radiation (320 to 400 nm). However, photosynthetically active radiation (PAR) (400 to 700 nm) contributed almost equally to this effect. Experiments with samples from the high mountain lake showed that at a depth of 2.5 m, the inhibition was caused almost exclusively by UV-A radiation. At a depth of 8.5 m, where chlorophyll a concentrations were higher than those in the upper water column, the rates of incorporation of [(sup3)H]TdR were higher in those samples exposed to full sunlight or to UV-A plus PAR than in the dark control. In laboratory experiments with artificial UV light, the incorporation of [(sup3)H]TdR and [(sup14)C]Leu by mixed bacterial lake cultures was also inhibited mainly by UV-A. In contrast, in the presence of the green alga Chlamydomonas geitleri at a chlorophyll a concentration of 2.5 (mu)g liter(sup-1), inhibition by UV radiation was significantly reduced. These results suggest that there may be complex interactions among UV radiation, heterotrophic bacteria, and phytoplankton and their release of extracellular organic carbon. Our findings indicate that the wavelengths which caused the strongest inhibition of TdR and Leu incorporation by bacterioplankton in the water column were in the UV-A range. However, it may be premature to extrapolate this effect to estimates of bacterial production before more precise information on how solar radiation affects the transport of TdR and Leu into the cell is obtained.

Community structure, picoplankton grazing and zooplankton control of heterotrophic nanoflagellates in a eutrophic reservoir during the summer phytoplankton maximum

Abstract: An intensive 5 wk study was conducted to investigate the role of protists, especially heterotrophic nanoflagellates (HNF), in microbial food webs during the summer phytoplankton bloom in the epilimnion and metalimnion of the eutrophic Rimov reservoir (South Bohemia, Czech Republic). On average, protists consumed similar to 90% of bacterial production in both layers. The community composition of HNF and the relative importance of different HNF groups as picoplankton consumers were determined. Small HNF (<8 mu m), as chrysomonads, bodonids and choanoflagellates, usually accounted for <30% of total HNF biomass but numerically dominated the community in both layers. They consumed most of (similar to 70 to 85 %) the bacterioplankton as well as autotrophic picoplankton (APP, exclusively cyanobacteria) production in the reservoir, with the rest consumed by ciliates. Both ciliates and HNF had higher clearance rates on APP than on bacteria and their grazing was likely responsible for a sharp decrease in APP abundance (from 3-4 x 10(5) to <2 x 10(3) ml(-1)) and a very constant size structure of bacterioplankton in which short rods in the size class of 0.4 to 0.8 mu m constituted 55 to 80 % of the total bacterial biomass in both layers. The proportion of HNF to total picoplankton biomass in the epilimnion indicated that the picoplankton biomass was sufficiently high to support HNF growth for most of the study. Uptake of picoplankton by less numerous, but larger, HNF (kathablepharids, Goniomonas sp., and Streptomonas sp.) was negligible, while their biomass, especially in the metalimnion, exceeded that of small HNF and the total biomass of picoplankton. This suggested that food items other than picoplankton were consumed to meet their carbon requirements. Analyzing potential bottom-up and top-down factors controlling HNF numbers and biomass, we did not find a tight relationship between HNF and the concentration of bacteria and chlorophyll. Variability of HNF abundance and biomass in the epilimnion could largely be explained by cladocerans or by pooled abundances of all potential crustacean consumers of HNF. In the metalimnion, the mean cell volume of HNF was positively linked to chlorophyll but negatively to the abundance of Cyclopidae and to the pooled abundances of Ceriodaphnia quadrangula and Diaphanosoma brachyurum.

Cell-specific respiratory activity of aquatic bacteria studied with the tetrazolium reduction method, cyto-clear slides, and image analysis.

Abstract: We present an improvement of the INT [2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyltetrazolium chloride)] reduction method using Cyto-Clear slides, the fluorochrome DAPI (4(prm1),6(prm1)-diamidino-2 phenylindole), and an image analysis system. With this method we were able to simultaneously measure cell dimensions and formazan crystals as indicators of the respiratory activity of single bacteria. The method was tested on a natural bacterioplankton community of an oligotrophic high mountain lake (Gossenkollesee, Tyrolean Alps, Austria, 2,417 m above sea level) in midwinter ((symbl)1-m-thick ice and snow layer; dissolved organic carbon, 0.51 mg liter(sup-1); water temperature, 2(deg)C). About 25% of planktonic bacteria were respiratorily active, and a complex pattern of bacterial morphologies and specific respiratory activities was observed during a time series of INT incubation. Rod-shaped bacteria with cell lengths of between 1.6 and 4.8 (mu)m already showed visible activity after 0.5 h of INT incubation. Small cells (rods and cocci) in the size fraction <1.6 (mu)m and long filamentous bacteria (up to 120 (mu)m) were visibly active only after a 2-h incubation period. After 8 h of incubation, more than 90% of all cells between 3.2 and 6.4 (mu)m in cell length were respiratorily active, whereas only 5% of cells <1.6 (mu)m and 50% of filamentous bacteria contained formazan grains. We could distinguish five major bacterial phenotypes that showed distinct activity patterns with respect to incubation period and numbers and sizes of formazan crystals. There was no correlation between the total formazan volume per active cell and bacterial cell volume, and for any size class of active bacteria, total formazan volumes varied by about 2 orders of magnitude after 8 h of incubation. This indicates that cell-specific activity is extremely variable and is not related to size and that a small portion of all cells may account for the overall activity.

Ultraviolet Radiation in a High Mountain Lake of the Austrian Alps: Air and Underwater Measurements

Abstract: Global UV radiation was measured with a portable multichannel filter radiometer at the surface and underwater in a high mountain lake (2417 m above sea level) of the Austrian Alps during 16 days in summer 1995. During this period, total column ozone values that changed only by 34 Dobson units explained a significant part of the variability in UVB radiation at 305 nm as indicated by the negative correlation with the ratio 305:340 nm (rs= -0.810, P < 0.01). High radiation at the surface combined with high water transparency allowed substantial UVB radiation to reach the bottom of this lake. The diffuse attenuation coefficient for downward irradiance at 305 nm changed within 2 weeks from 0.24 m-1 (10% at 9.6 m depth) to 0.32 m-1 (10% at 7.2 m depth). This change in attenuation was related to the development of phytoplankton after the ice break-up as indicated by a six-fold increase in chlorophyll-a concentrations during this period. Our results suggest that phytoplankton and/ or phytoplankton-derived organic substances are important for the UV attenuation in this oligotrophic lake.

An in situ enclosure experiment to test the solar UVB impact on plankton in a high-altitude mountain lake. I. Lack of effect on phytoplankton species composition and growth

Abstract: The effect of solar UVB radiation on the growth and species composition of phytoplankton from a high-mountain lake (2417 m a.s.l.) was studied in situ for 16 days in two enclosures of 1 m-\ receiving either full sunlight or sunlight without UVB. A total of 20 species were identified in both enclosures, consisting mainly of dinoflagellates, chrysophytes and diatoms. During the experiment, there were no significant differences in phytoplankton species composition between the two enclos- ures. In both treatments, the abundance of phytoplankton increased continuously, and chlorophyll a changed by -5-fold. We observed high fluctuations in the abundance of several species. However, these fluctuations occurred in both enclosures, and hence they were not related to UVB radiation. Some species were affected by daily fluctuations of radiation (UVA + photosynthetically active radi- ation). Cyclotella aff. gordonensis showed a robust positive correlation, whereas species of Gymno- dimum were negatively correlated. For most species, the sensitivity to radiation found during the experiment was consistent with their diurnal vertical distribution in the lake. Our findings suggest that the phytoplankton from this very clear lake (10% of the surface UVB radiation at 305 nm reached 9.6 m depth) were well adapted to the high UVB radiation characteristic of high-elevation sites.

In situ classification and image cytometry of pelagic bacteria from a high mountain lake (gossenkollesee, austria).

Abstract: We describe a procedure to measure the cell sizes of pelagic bacteria after determinative hybridization with rRNA-targeted fluorescently labeled oligonucleotide probes. Our approach is based on established image analysis techniques modified for objects simultaneously stained with two fluorescent dyes. It allows the estimation of biomass and cell size distribution and the morphological characterization of different bacterial taxa in plankton samples. The protocol was tested in a study of the bacterioplankton community of a high mountain lake during and after the ice break period. Cells that hybridized with a probe for the domain Bacteria accounted for 70% of the bacterial abundance (range, 49 to 83%) as determined by 4(prm1),6(prm1)-diamidino-2-phenylindole staining (K. G. Porter and Y. S. Feig, Limnol. Oceanogr. 25:943-948, 1980), but for >85% of the total biomass (range, 78 to 99%). The size distribution for members of the beta subclass of the Proteobacteria shifted toward larger cells and clearly distinguished this group from the total bacterial assemblage. In the surface water layer beneath the winter cover, bacteria belonging to the beta 1 subgroup constituted about one-half of the beta subclass abundance. The mean cell volume of the beta 1 subgroup bacteria was significantly less than that of the beta subclass proteobacteria, and the beta 1 subgroup accounted for less than 30% of the total beta subclass biovolume. Two weeks later, the biovolume of the beta Proteobacteria had decreased to the level of the beta 1 subgroup, and both the biovolume size distributions and cell morphologies of the beta Proteobacteria and the beta 1 subgroup were very similar. We could thus quantify the disappearance of large, morphologically distinct beta subclass proteobacteria which were not members of the beta 1 subgroup during the ice break period. Our results demonstrate that changes in biovolumes and cell size distributions of different bacterial taxa, and eventually of individual populations, reveal hitherto unknown processes within aquatic bacterial assemblages and may open new perspectives for the study of microbial food webs.

Extracellular, low-affinity β-N-acetylglucosaminidases linked to the dynamics of diatoms and crustaceans in freshwater systems of different trophic degree

Abstract: Extracellular hydrolysis of 4-methylumbelliferyl β-N-acetylglucosaminide was measured in the oligomesotrophic Piburger See and the eutrophic Rimov reservoir during spring and summer phytoplankton blooms, respectively. Total enzymatic activity (TEA) ranged between 0.2 and 19.1 nmol 1 -1 h -1 in the reservoir and between 0.8 and 12.4 nmol 1 -1 h -1 in the lake. High-affinity (K m 100 μmol 1 -1 ) enzymes were kinetically identifiable in most samples from both localities. The low-affinity enzyme activity (LEA) usually accounted for >60% (mean: 80%) of TEA. LEA and diatom biomass significantly correlated over time in the reservoir epilimnion (r s = 0.578) and in the lake metalimnion (r s = 0.862). As diatoms possess chitin and take up its monomer, N-acetylglucosamine, two explanations of the observed relationships are suggested: extracellular β-N-acetylglucosaminidase activity partly originates either from ectoenzymes of chitinolytic bacteria attached to diatom cells or from ectoenzymes of diatoms, enabling them to take up N-acetylglucosamine from ambient amino sugars instead of synthesizing it de novo. A significant positive correlation of LEA with crustacean abundance was found in the lake epilimnion (r s = 0.850), apparently reflecting the growing spring populations of frequently moulting juvenile crustaceans. A possible contribution of chitinolytic bacteria, accompanying the crustacean populations, to LEA is discussed.