Roland Psenner

Bacterial growth in supercooled cloud droplets

It is well known that the atmosphere is a conveyor of microorganisms, and that bacteria can act as ice or cloud condensation nuclei, but clouds have not been considered as a site where organisms can live and reproduce. Here we show that bacteria in cloud droplets collected at high altitudes are actively growing and reproducing at temperatures at or below 0°C. Since ∼60% of the earth surface is covered by clouds, cloud water should be considered as a microbial habitat.

Phylogenetic and functional heterogeneity of sediment biofilms along environmental gradients in a glacial stream.

ABSTRACT We used in situ hybridization with fluorescently labeled rRNA-targeted oligonucleotide probes concurrently with measurements of bacterial carbon production, biomass, and extracellular polymeric substances (EPS) to describe the bacterial community in sediments along a glacial stream. The abundance of sediment-associatedArchaea, as detected with the ARCH915 probe, decreased downstream of the glacier snout, and a major storm increased their relative abundance by a factor of 5.5 to 7.9. Bacteria of theCytophaga-Flavobacterium group were also sixfold to eightfold more abundant in the storm aftermath. Furthermore, elevated numbers of Archaea and members of theCytophaga-Flavobacterium group characterized the phylogenetic composition of the supraglacial ice community. We postulate that glacial meltwaters constitute a possible source of allochthonous bacteria to the stream biofilms. Although stream water temperature increased dramatically from the glacier snout along the stream (3.5 km), sediment chlorophyll a was the best predictor for bacterial carbon production and specific growth rates along the stream. Concomitant with an increase in sediment chlorophylla, the EPS carbohydrate-to-bacterial-cell ratio declined 11- to 15-fold along the stream prior to the storm, which is indicative of a larger biofilm matrix in upstream reaches. We assume that a larger biofilm matrix is required to assure prolonged transient storage and enzymatic processing of allochthonous macromolecules, which are likely the major substrate for microbial heterotrophs. Bacteria of theCytophaga-Flavobacterium cluster, which are well known to degrade complex macromolecules, were most abundant in these stream reaches. Downstream, higher algal biomass continuously supplies heterotrophs with easily available exudates, therefore making a larger matrix unnecessary. As a result, bacterial carbon production and specific growth rates were higher in downstream reaches.

Groundwater samples do not reflect bacterial densities and activity in subsurface systems

Total cell numbers, abundance of respiring bacteria and [3H]thymidine and [14C]leucine incorporation rates were investigated in four groundwater wells of low nutrient content. Total cell numbers in the pumped groundwater were low (14 × 103 to 279 × 103 cells mL−1), and [3Hthymidine and [14C]leucine incorporation rates were, with one exception, below the detection limit. Therefore we exposed sediments in situ for 2 months which allowed us to determine bacterial numbers and incorporation rates of labeled substrates by bacteria attached to sediment particles. The two habitats differed considerably in all bacterial parameters both in magnitude and seasonal trends. Total bacterial numbers of sandy sediments (52.1 ± 21.3 × 106 cells cm−3) corresponded in average to 663 cm3 of pumped groundwater (78.5 ± 61.5 × 106 cells L−1). For the fraction of respiring bacteria this ratio was on average 3032 cm3 (sediments: 10.3 ± 5.4 × 106 respiring cells cm−3; groundwater: 3.39 ± 6.01 × 106 respiring cells L−1). The percentage of respiring bacteria in sandy sediments was between 6.0 and 41.4% (average 19.8) compared to 1.0 to 24.9% (average 5.23) in the pumped groundwater. Our results stress the importance of studying the microbial communities attached to sediment, as pumped groundwater samples may not be representative for the real structure and dynamics of microbial assemblages in subsurface environments.

Predator-Specific Enrichment of Actinobacteria from a Cosmopolitan Freshwater Clade in Mixed Continuous Culture

ABSTRACT We investigated whether individual populations of freshwater bacteria in mixed experimental communities may exhibit specific responses to the presence of different bacterivorous protists. In two successive experiments, a two-stage continuous cultivation system was inoculated with nonaxenic batch cultures of the cryptophyteCryptomonas sp. Algal exudates provided the sole source of organic carbon for growth of the accompanying microflora. The dynamics of several 16S rRNA-defined bacterial populations were followed in the experimental communities. Although the composition and stability of the two microbial communities differed, numerous members of the first assemblage could again be detected during the second experiment. The introduction of a size-selectively feeding mixotrophic nanoflagellate (Ochromonas sp.) always resulted in an immediate bloom of a single phylotype population of members of the classActinobacteria (Ac1). These bacteria were phylogenetically affiliated with an uncultured lineage of gram-positive bacteria that have been found in freshwater habitats only. The Ac1 cells were close to the average size of freshwater bacterioplankton and significantly smaller than any of the other experimental community members. In contrast, no increase of the Ac1 population was observed in vessels exposed to the bacterivorous ciliate Cyclidium glaucoma. However, when the Ochromonas sp. was added after the establishment of C. glaucoma, the proportion of population Ac1 within the microbial community rapidly increased. Populations of a beta proteobacterial phylotype related to an Aquabacteriumsp. decreased relative to the total bacterial communities following the addition of either predator, albeit to different extents. The community structure of pelagic microbial assemblages can therefore be influenced by the taxonomic composition of the predator community.

Microorganisms in the atmosphere over Antarctica

Antarctic microbial biodiversity is the result of a balance between evolution, extinction and colonization, and so it is not possible to gain a full understanding of the microbial biodiversity of a location, its biogeography, stability or evolutionary relationships without some understanding of the input of new biodiversity from the aerial environment. In addition, it is important to know whether the microorganisms already present are transient or resident - this is particularly true for the Antarctic environment, as selective pressures for survival in the air are similar to those that make microorganisms suitable for Antarctic colonization. The source of potential airborne colonists is widespread, as they may originate from plant surfaces, animals, water surfaces or soils and even from bacteria replicating within the clouds. On a global scale, transport of air masses from the well-mixed boundary layer to high-altitude sites has frequently been observed, particularly in the warm season, and these air masses contain microorganisms. Indeed, it has become evident that much of the microbial life within remote environments is transported by air currents. In this review, we examine the behaviour of microorganisms in the Antarctic aerial environment and the extent to which these microorganisms might influence Antarctic microbial biodiversity.

Living in a dusty world : Airborne dust as a key factor for alpine lakes

For a long time studies on precipitation and lake-water chemistry have focussed on the deposition of strong acids. Dust in snow and rain was analyzed much less intensively, but recent findings suggest that it may be the missing link which explains why many low-alkalinity lakes in the Alps and the Pyrenees did not become acidic and nutrient levels are seasonally high, whereas lakes in areas which are rarely influenced by dust depositions, for instance in Scandinavia, have acidified. Beside being a dominant factor for nutrient inputs to oceans, rain forests and remote lakes, dust can also play a major role in global warming and cooling, and it may significantly contribute to soil formation at mountain sites in the Mediterranean. I suggest that future changes in dust deposition and warming will be key factors for the development of alpine lakes.

Global change revealed by palaeolimnological records from remote lakes: a review

Over recent decades, palaeolimnological records from remote sites have provided convincing evidence for the onset and development of several facets of global environmental change. Remote lakes, defined here as those occurring in high latitude or high altitude regions, have the advantage of not being overprinted by local anthropogenic processes. As such, many of these sites record broad-scale environmental changes, frequently driven by regime shifts in the Earth system. Here, we review a selection of studies from North America and Europe and discuss their broader implications. The history of investigation has evolved synchronously with the scope and awareness of environmental problems. An initial focus on acid deposition switched to metal and other types of pollutants, then climate change and eventually to atmospheric deposition-fertilising effects. However, none of these topics is independent of the other, and all of them affect ecosystem function and biodiversity in profound ways. Currently, remote lake palaeolimnology is developing unique datasets for each region investigated that benchmark current trends with respect to past, purely natural variability in lake systems. Fostering conceptual and methodological bridges with other environmental disciplines will upturn contribution of remote lake palaeolimnology in solving existing and emerging questions in global change science and planetary stewardship.

Spatio-temporal niche separation of planktonic Betaproteobacteria in an oligo-mesotrophic lake.

We investigated the diversity of planktonic Betaproteobacteria and the seasonal population changes of betaproteobacterial taxa in an oligo-mesotrophic lake (Piburger See, Austria). Focus was put on the vertical distribution of the investigated populations and on differences between their respective cell fractions with apparent amino acid incorporation. On average, 66% of betaproteobacterial cells and 73% of their diversity could be attributed to four clades within three lineages that were further analysed by fluorescence in situ hybridization. The numbers of bacteria from the R-BT subclade of the beta I lineage and from the PnecB subgroup of the beta II lineage were rather constant throughout the water column. In contrast, members of another subgroup of beta II (PnecC) and bacteria related to Methylophilus (beta IV) were particularly numerous in the oxygen-depleted zone. In general, only moderate seasonal changes in abundance were observed in the upper water layers, whereas there was a clear relationship between decreasing oxygen levels and the rise of bacteria from the PnecC and beta IV clades in deeper strata. On average, almost 80% of beta I bacteria, but < 15% of cells from the beta IV clade, showed amino acid incorporation. Our results suggest that the studied populations occupy distinct vertical and ecophysiological niches in Piburger See.

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

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.

Environmental changes in an alpine lake (Gossenköllesee, austria) over the last two centuries – the influence of air temperature on biological parameters

Changes in microfossils (diatoms, chrysophytes, chironomids and cladocera remains), geochemistry and deposition of atmospheric pollutants have been investigated in the sediment records of the alpine lake Gossenköllesee (Tyrol, Austria) spanning the last two centuries. The sediment records were compared with seasonal and annual air temperature trends calculated for the elevation (2417 m a.s.l.) and the geographical position (47° 13′46′′N, 11° 00′51′′E) of the lake, and with precipitation records available since 1866 from Innsbruck. Temperature trends followed a 20–30 year oscillation between cold and warm periods. Regarding long-term changes, temperature trends showed a U-shaped trend between 1780 and 1950, followed by a steep increase since 1975.Physical, geochemical, and organic parameters were not controlled by air temperature. Among the biological records only diatoms and chrysophytes reacted to air temperature changes: the relative abundance of planktonic diatoms increased during warm periods and changes in mean annual alpine air temperature explained 36.5% of their variation. The relation between abundance of seasonal stomatocyst types and air temperature varied on two different time scales: while summer stomatocysts were influenced by short term temperature fluctuations, the autumn stomatocysts were affected only by the long term changes. Other biological parameters exhibited a constant species composition (chironomids, pigments) or changes were small and independent of temperature (cladocera). Spheroidal carbonaceous fly-ash particles, and trends in Pb and Cr indicated increasing deposition of atmospheric pollutants but had no detectable effects on the biological parameters either. In respect to temperature variations over the last 200 years, this alpine lake is much less sensitive than expected and has thus to be regarded as a well buffered site. However, temperature alone is not sufficient to understand changes in species composition and other biogeochemical processes with unknown historical patterns might have affected species composition more strongly.