For food web analysis it is of great value to get an idea of how bacteria interact with other components of the aquatic food chain. A multitude of studies shows that a major fraction of the carbon flow is guided through the microbial loop but until now the importance of the microbial food web is not taken into consideration in most aquatic ecosystem studies. From comparative studies in various freshwater and marine systems, e.g. Lake Constance, Lake Kinneret, Lake Stechlin, Red Sea, Caribbean, Eastern Pacific, and German Waddensea. we hope to get a broader understanding of the role heterotrophic bacteria in nutrient and carbon fluxes on both the micro and global scale.

Our main interests focus on the physical, chemical, and biological mechanisms of aggregation, the spatial and temporal distribution of macroscopic organic aggregates (marine snow), and their degradation and colonization by marine microorganisms. Major study areas are lakes of the Mecklenburg Lake District and the River Spree which is characterized by pronounced changes in limnological features during passage of the river-lakes system. Therefore it is interesting to study the abundance, spatial distribution, and composition of particles and aggregates (including resuspended sediments) in these environments with a high temporal and spatial resolution. The size structure, as well as the biochemical and microscopical composition of the aggregates can be used to characterize formation processes.

Changes of the bacterial community structure in various aquatic ecosystems (aggregates, the surrounding water, and the sediment) are analysed by using a multitude of techniques such as denaturing gel electrophoresis (DGGE), fluorescent in situ hybridisation (FISH) with 16S rRNA probes, and the more classical methods of isolation and cultivation.

To address the question of ecological function of specific bacterial communities molecular, physiological, and analytical methods will be combined. Increased knowledge of the bacterial community structure, their physiological activity, and the turnover of important microbial substrates (e.g. amino acids and carbohydrates) will lead to a better understanding of the ecological role of free-living and particle-associated bacteria.

Research goals

Diversity and Dynamics of Heterotrophic Bacterial Communities in Lake Stechlin, Grosse Fuchskuhle, Tiefwaren, and Breiter Luzin

• Seasonal Changes in Bacterial Community Composition 
• Role of Physico-chemical and Biological Parameters  
• Differences Between Bacterial Communities of Selected Lakes   
• Ecological Function of Specific Bacterial Groups or Species

Scientific Research by Dipl. Biol. Martin Allgaier

Studied Lakes

Parameter	Lake Stechlin	Lake Große Fuchskuhle	Lake Teifwaren	Lake Breiter Luzin
area [km2]	4,30	0,02	1,40	3,57
max. depth [m]	68,00	5,60	24,00	58,50
average depth [m]	22,80	3,50	8,20	25,20
volume [x 106 m3	96,90	0,05	12,90	67,50
shore line [km]	16,10	0,50	-	13,20
visibility [m]	8 - 12	0,5 - 4,8	1,6 - 4,0	1,8 - 4,6
trophie	oligotrophic	dystrophic	eutrophic	mesotrophic
pH	7,2 - 8,5	4,2 - 5,2	8,3 - 8,5	7,4 - 7,9

modified after: Babenzien, 1991; Gonsiorczyk et al., 2002

Lake Stechlin

Lake Stechlin is a clear water lake and was formed by melting of dead ice blocks and erosion of melt water channels after the last ice age (Weichselian stage). Formations of gravel and sand deposits reach up to 84.5 m above sea level and are located in the north-eastern part of the lake.

Location: 53:10N, 13:02E; 59.7 m above sea level, close to the village Neuglobsow

Lake Große Fuchskuhle

Lake Große Fuchskuhle is a naturally acidic bog lake that was artificially divided into four basins by large plastic curtains for biomanipulation experiments already in 1990. The Southwest (SW) basin is the most acidic part (pH down to 4.2) since it is most influenced by water run off from the adjacent bog area. In contrast, the Northeast (NE) basin is less influenced by humic acids (pH of ca. 5.8). Both basins are significantly different in their chemical and hence biological parameters. For example, humic substances account for up to 58% of dissolved organic matter (DOC) in the SW basin whereas they only account for <35% of DOC in the NE basin (Sachse et al. 2001, Biochem. 54:279-296).

Location: 53:10N, 13:02E, 59 m above sea level, close to the village Menz

Lake Breiter Luzin

Lake Breiter Luzin is one of the largest lakes of the "upper" Feldberg lake district. It is has a max. depth of 58,5 m, an area of 3,57 km2, and a volume of 67.5 Mio. m3. Lake Breiter Luzin is situated in the transition zone between ground and end moraine of the last ice age (Weichselian stage). The lake is mesotrophic since it is conected with eutrophic Lake Haussee via a small channel which allows the import of nutrients.

Location: 53:20N, 13:28E, 84.2 m above sea level, close to the town Feldberg

Lake Tiefenwaren

Lake Tiefwaren is a dimictic hard water lake and has a max. depth of 24 m. Lake Tiefwaren was formed by melting of ice blocks and erosion of melt water channels after the last ice age (Weichselian stage). It is an eutrophic lake under current restoration by a combination of aluminate and lime precipitation (TIBEAN, "Tiefenwasserbe-lüftungsanlage", deep water aeration facility). It is located north of Lake Müritz at the outskirts of the town Waren.

Location: 53:31N, 12:42E, ca. 63.5 m above sea level close to the town Waren (Müritz)

Microorganisms in the pelagial of an acidic bog lake

The photo below shows microorganisms growing in the pelagial of a naturally acidic bog lake (Lake Grosse Fuchskuhle) in the North East of Germany. The lake is rain fed and without surface inlet or outlet, has a dissolved organic carbon (DOC) concentration of 11 to 13 mg/L, a low pH of 4.2-5.2, and a high amount of recalcitrant humic substances (HS) reaching to 58% of the DOC (sea above). Bacterial counts using the DAPI method reveal 1 to 4 million bacterial cells ml-1 (Sachse et al. 2001). The identity of these microorganisms is so far unknown, although there are several molecular hints that members of β-Proteobacteria and Actinobacteria are growing favorably in this lake.

The aim of our study is to provide phylogenetic information of the microbial groups found in the lake and to test their possible dependency on certain substrates. Isolation of these bacteria will be done by mimicking natural conditions in the lake, including natural substrates such as phenolic compounds and humic matter (RO-Isolates from the lake). Physiological characterization of the specific groups will be done through Fluorescence In-Situ Hybridization (FISH) coupled with Microautoradiography (MAR-FISH) and through classical microbiological procedures. Finally, information regarding the ecological function of these bacterial groups will be acquired through substrate-amended and unamended Lake Grosse Fuchskuhle water samples.

Sachse, A., D. Babenzien, G. Ginzel, J. Gelbrecht, & C.E.W. Steinberg. 2001. Biogeochemistry. 54: 279-296

Kristine Michelle Hutalle M. Sc.

Aggregation has been shown to be an important process affecting vertical and horizontal transport of organic matter and nutrients in aquatic systems. Previous studies at the IGB led to develop a sediment transport model (SEDIFLOW) which is solely based on physical parameters. However, it does not allow for reliable prediction of particulate matter (PM) transport on a seasonal time scale.

In addition to better understand temporal dynamics of riverine sediment transport several chemical and biological parameters such as aggregate abundance and size, particulate organic carbon (POC), bacterial abundances, and transparent exopolymeric particles (TEP) will be measured. Samples will be taken at two different sampling locations downstream of Lake Neuendorf and Lake Schwieloch ca. 70 km southeast of Berlin, Germany. Individual sampling points are located at different distances from the outflow of the lakes to follow changes in particle dynamics during horizontal transport.

Figure: river-lake system of Spree and view along the river reach near Beeskow

For reliable documentation of changes in particle size and abundance a new camera system with laser light illumination will be developed.
Dipl. Biol. Sebastian Eixler

These studies are part of the DFG-Project: "Studies on aggregation in rivers and its impact on the sinking flux of suspended sediments" (2004-2007, collaboration with Dr. Heinz Bungartz, Department of Ecohydrology)

For a fundamental understanding of aquatic ecosystems it is necessary to gain knowledge on diversity, distribution and function of their bacterial communities. The important role of aquatic microorganisms is underestimated, in particular for the phosphate cycle in lakes and sediments. Phosphate is a production-limiting factor in lakes and a starting point for lake-restoration concepts. The accurate notice of the biotic and abiotic phosphate turnover is indispensable for the design of useful redevelopment and restoration strategies. However there are still a lot of gaps in our present understanding, especially in the interaction of geochemical and biological mechanisms.

Hence, we will study the role of bacteria in phosphate diagenesis. Indirect effects of microbial induced changes in pH and redox potential on abiotic reactions of phosphate are undoubted. But the direct role of microorganisms in fixation and mobilization of phosphate is often neglected. For example, studies of activated sludge show that some bacteria are able to store hugh amounts of polyphosphate in their cells. This stored polyphosphate is released under specific environmental conditions. In sediments polyphosphate has been frequently detected (Hupfer et al., 1995; Carman et al., 2000; Hupfer et al., 2004).

Figure: 31P-NMR-spectrum of the sediment surface (biofilm) of lake Scharmützelsee. Sampling-date: 06.04.2004. a: orthophosphate, b: phosphomonoester, c: phosphodiester, d: pyrophosphate und polyphosphate (end-groups), e: polyphosphate (middle-groups).
There is also evidence for the presence of polyphosphate accumulating bacteria (Uhlmann & Bauer, 1988; Hupfer et al., 1995). Until now there is no knowledge on their phylogenetic classification, living conditions, and their contribution in the aquatic phosphate cycle. The present analyses of polyphosphate accumulating bacteria in activated sludge point out that they may have the potential to influence the phosphate dynamic in lakes.

Coupling of microbiological and molecular methods (e.g. PCR-DGGE, FISH, microscopy) with chemical-analytical techniques (31P-NMR, sequential extraction) is a concept for analyzing the presence and importance of these and other microorganisms in the phosphate diagenesis in lakes.
Dipl. Biol. Stefanie Glöss

Bacteria-alga interactions vary from symbiotic to parasitic relationships. Whether this relationship tends to the one or the other site mainly depends on environmental parameters, such as the availability of inorganic nutrients and organic matter. In addition, algae and bacteria produce a multitude of organic compounds (AVOC’s = Algal Volatile Organic Compounds) which can act as potential resources or even inhibitors. Already, from 1930 on scientists discovered inhibitory substances produced by microorganisms (e.g. sulphonamide) and at the beginning of 1940 those produced by algae (chlorellin). However, until today their ecological role still remains unknown for aquatic systems.

In our project we will study the interplay between bacteria and algae by using specific model systems (clone-cultures and natural populations). Physiological methods will be combined with techniques to identify phylogeny and to measure activities of both bacteria and algae at the same time. By using this approach we will be able to study the interplay between specific groups of bacteria and algae and to determine its possible role in aquatic energy and nutrient cycling.

The studies are part of the DFG-Project: "Dynamics of bacteria-alga interactions: a key for understanding organic matter cycling and aggregation processes" (2005-2008, collaboration with Dr. Lothar Krienitz, Department of Limnology of Stratified Lakes).

It is of great advantage to use artificial agar beats as a model system for bacterial colonization studies. Aggregates are of great importance for marine ecosytems since particle-associated bacteria are characterized by high metabolic activities. By using individual and well defined aggregates it is possible
to determine respiration and production of attached microbes simultaneously and, thus, to determine their growth efficiency.
Monitoring changes in bacterial community structure, their activity, and changes of important nutrients and carbon on natural as well as on artificial aggregates will increase our knowledge on ecology of attached microbes.

Individual aggregates of defined age and composition were formed after incubation of a mixture of marine diatoms (Chaetoceros sp., Skeletonema costatum, and Thalassiosira weissflogii) in rolling tanks. Average bacterial growth efficiency was 0.49 + 0.12 on 1.5 and 2.5 days old aggregates and it was independent of the growth rate (µ). The ratio of aminopeptidase and ß-glucosidase activity peaked on 2.5 day old aggregates and decreased thereafter. On 3.5 days old aggregates respiration was significantly increased leading to reduced growth efficiencies (0.28 + 0.10). Respiration was then correlated with POC and PON content of the same aggregate. The bacterial community on aggregates was dominated by members of the Cytophaga/Flavobacteria cluster and of the g-Proteobacteria. These results demonstrate that bacterial growth efficiencies are dependent on the age as well as the composition of aggregates and that a rapid and efficient transfer of organic matter into bacterial biomass takes place on aggregates.

We use dark-field microscopy to examine motility in natural assemblages of marine bacteria. Five to 70% of all bacteria off Scripps Pier (August 1997 to June 1998) were motile during a 1 minute observation period. Bacterial motility was highest during summer (50-70%), decreased during fall (50=>20%), stayed low in winter (<5-25%), and increased in spring (<5=>60%). Motile % showed a distinct diurnal pattern at the end of August, when >40% of all bacteria were motile.
Increase of % motile throughout mesocosm experiments followed the development of a phytoplankton bloom by 2-3 d and increased in parallel to increased detrital POC. Removal of particles (>1 µm) by filtration led to lower % motile as compared to unfiltered seawater suggesting that particulate organic matter may control % motile in the sea.

The speed of individual bacteria was highly variable (<10 to >500 µm s-1) and included bacteria showing reversals of direction. In natural samples, the majority of motile bacteria were running only a small fraction of time (>40% were running <20% during 5 min) including frequent bursts of motility.

Low concentrations of inorganic nutrients lead to enhanced stress for algae due to increasing competition with bacteria. Motility and chemotactic behaviour enable marine bacteria to rapidly colonize and subsequently lyse the algae. Thus, the initial symbiotic relation between bacteria and algae becomes a parasitic one. This scenario has not only profound implications for the structure of the microbial food web but also for global carbon cycling.

Lake Tiefwaren

Lake Tiefwaren is a dimictic hard water lake with a maximum depth of 24 m and it is located in the Mecklenburg Lake District north of Lake Müritz close to the town Waren. In former times it was a mesotrophic lake, but with anthropogenic influence it became eutrophic. Between 2001 and 2006 there was a restoration of the lake by a combination of aluminate and lime precipitation and an aeration of the hypolimnic zone (TIBEAN, "Tiefenwasserbelüftungsanlage", deep water aeration facility).


Restoration

With the restoration of lake Tiefwaren (Addition of NaAl(OH)4 and Ca(OH)2 for the precipitation of phosphate and the aeration of the hypolimnion) by the TIBEAN the concentration of phosphorus decreases in the water body.

The hypolimnetic Al- and CaCO3-treatments, combined with deep-water aeration and trophic state oriented fishery management, and the formation of a new sediment boundary layer with a high P-binding capacity, the trophic conditions shifted from a highly eutrophic to mesotrophic state.

TIBEAN at Lake Schmaler Luzin (filler plug) and at Lake Tiefwaren (transportation pontoon for chemicals) [pictures: S. Appel]

Actinos

The phylum Actinobacteria contains the Gram-positive bacteriawith a high content of Guanine plus Cytosine in their genome (former HGC-group). Actinobacteria have a characteristic cell wall structure with lots of murein and peptidoglycan layers.

For details of the different composition of the cell wall and a comparision of Gram positive and Gram negative cell wall strucutre have a look on the picture at http://www.cehs.siu.edu/fix/medmicro/pix/walls.gif.

The group Actinobacteriaceae is highly divers and heterogene and includes different clusters of microorganisms (e.g. Actinomycets, Streptomycets, Microbacteriaceae).
Actinobacteria are one of the dominant groups of pelagic bacteria in freshwater lakes with abundances of 30% up to 58% [Allgaier & Grossart, 2006] and they are ubiquitous.

The project

The project is targeted on the effects of the restoration procedure on the bacterial community and on the phylogenetic diversity of pelagical microorganisms.
We want to study and analyse the changes and variances of the Lake Tiefwaren over the period of restoration in relation to the epilimnic microbial diversity and the composition of the bacterial community.
We suggest that the restoration procedure causes a significant shift in the structure and compositionof the heterotrophic bacterial community in the epilimnic zone of the lake. We conjecture an increase of the abundance of Actinobacteria with the variation of the biotic and abiotic environmental factors (alkalinity, concentration of phosphorus and phosphate, oxygen content, trophic state, etc.).
Presumably there is a correlation between alkalinity and the occurence of Actinibacteria, Proteobacteria and Bacteroidetes. However, the function of Actinobacteria in the ecosystem still has to be explored.
Another major topic of this project is a cultivation experiment to enrich and isolate not-yet-cultured Actinobacteria of the most abundant and dominant subclusters, e.g. the acI group, but also rare and uncommon species. The cultivation combines dilution series and high-throughput methods, detection will be carried out by molecular methods.

Dipl. Biol. Stefan Rösel