The present study is the first that simultaneously quantifies bacterial and fungal biomass and production on decaying plant litter in the littoral zone of a lake during one year. Additionally, bacterial biomass and production was assessed in the aerobic top sediment layer, in epiphyton on submerged reed culms and in the littoral water column. Because some of these compartments present challenges for the application of current methodologies for estimating bacterial biomass and production accurately, a series of methodological investigations were carried out.
Efficient detachment of bacterial cells is generally crucial for assessing bacterial abundance, biomass and community composition in both natural and technical systems (e.g., sewage plants). As there is no agreement on which procedure gives the best results with which type of system, the relative efficiency of four detachment instruments was tested for releasing bacteria associated with leaf litter, sediment and epiphytic biofilms from the littoral zone of a lake (Chapter 2). An ultrasonic probe method was the most efficient instrument for leaf litter and epiphytic biofilms, whereas a stomacher-type blender gave the best results for sediment samples. Neither the detachment instrument nor the treatment time affected the composition of bacterial morphotypes. These results indicate that the choice of the appropriate detachment device depends critically on the type of substratum examined.
The applicability of the [3H]leucine method for estimating bacterial production in the different habitats of a lake littoral (littoral water column, epiphyton, sediment and plant litter) was tested in Chapter 3. Saturation of leucine incorporation into protein occurred at 150 nmol/liter in littoral water samples, >960 nmol/liter in epiphyton and 50 micromol/liter in sediment and CPOM samples. Leucine incorporation was linear for up to 1 hour with all substrata. Specificity of leucine incorporation was tested in antibiotic experiments. Although eukaryotic organisms such as fungi are capable of taking up leucine at the concentrations used for incubation, experiments with antibiotics and analysis of bacterial and fungal leucine uptake kinetics suggest that the criterion of specificity for bacterial protein synthesis was still met.
Dynamics of bacteria and fungi associated with naturally decaying coarse particulate organic matter (CPOM) were studied in a fringing freshwater wetland dominated by the emergent macrophyte, Phragmites australis. Amounts of benthic CPOM, bacterial and fungal biomass, production, growth rate and microbial respiration were assessed on a monthly basis and annual microbial carbon demand was related to the net above-ground reed production. Microbial biomass was dominated by fungi, which made up >90% of total microbial biomass associated with CPOM. Conversely, bacterial production rates were 8 x higher than fungal production rates. Consequently, bacterial growth rates (range = 1.6 – 8.6 d^-1) exceeded fungal growth rates (range = 0.01 – 0.07 d^-1) by two orders of magnitude. Annual bacterial production per square meter of littoral reed stand was 710 g C and annual fungal production was 93 g C. The annual microbial community respiration was 874 g C m^-2. From respiration and production rates an average microbial growth efficiency of 48% was estimated. The net above-ground Phragmites could thus account for only 80% of the microbial carbon demand of litter associated bacteria and fungi, even if microbial carbon is recycled multiple times.
Bacterial biomass and production was assessed in four littoral compartments (littoral water column, epiphyton, top aerobic sediment layer and CPOM). On an areal basis annual bacterial production in the epiphyton (1.32 g C) and the littoral water column (4.7 g C) was more than 2 orders of magnitude lower than in the aerobic sediment (780 g C) and on CPOM (710 g C). A comparison with littoral primary production revealed that additional sources of carbon are required to support heterotrophic bacterial production in the littoral zone. Findings strongly suggest that in addition to the autochthonous organic primary production, organic carbon is also imported to the littoral zone where it is decomposed.
In conclusion, the results of the present study indicate an extremely high potential of microorganisms to mineralize organic carbon in the littoral zone of lakes dominated by emergent macrophytes such as Phragmites australis. These systems thus appear to be hotspots of carbon transformation with a P/R ratio <1 in spite of a high primary production.
email-address: nanna.buesing@eawag.ch