The largest pool of organic carbon in aquatic ecosystems is dissolved and can only be utilised by bacteria. Although most of the ambient DOC pool is refractory to bacterial decomposition it has recently been shown that semi-labile (turnover times in weeks) and labile (turnover times in hours or days) can accumulate over the growing season in the sea and is a major export term with implication for the global carbon budget. It is the purpose of the present project to identify/verify at which circumstances DOC can accumulate, to identify the chemical species involved in accumulation, and to characterize DOC with respect to microbial decomposition patterns. Theoretical considerations constraint the reasons for accumulation to either nutrient limitation of bacterial growth and/or low biodegradability of produced DOC.
The production and biodegradation of DOC is studied both in the field and in laboratory experiments, where different production scenarios with respect to zooplankton grazing and nutrient limitation are tested. Biodegradation is measured in batch incubations and with a bioreactor. Carbohydrates analyzed to molecular species are used as model molecules to characterize degradation dynamics.
In August 2002 we have access the EU Large Scale Facility in Bergen and will carry out a "full scale" experiment lasting for one month and with participation of about 18 scientists.
The study is supported by The Carlsberg Foundation, the Danish Natural Sciences Research Council, LSF in Bergen and EU. The study is carried out together with Theis Kragh, DMU and international partners.
Read more: Søndergaard, M., Williams, P.J. leB. et al. 2000. Net accumulation and the flux of DOC and DON in marine plankton communities. Limnol. Oceanogr. 45: 1097-1111.
This FW 5project has been concluded.
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It is hypothesized that DOC decomposition by bacteria is characterized by a succession of bacterial species/types specialized to utilize different component of the biodegradable pool. However, empirical verifications are meager. By DGGE-techniques and subsequent sequencing of the dominant bacterial genotypes we investigate the hypothesis in batch cultures of lake water and by the vertical position of specific bacteria in a bioreactor matured with the same water as used for the batch incubations. The function of the different communities developed over time and in the vertical gradient is tested by their ectoenzymatic profile. The purpose is to develop the understanding of functional and genotypic diversity in microbial communities.
The study is supported by The Carlsberg Foundation and the Danish Natural Sciences Research Council. Morten Søndergaard and Lasse Riemann.
We have empirical evidence that only between 5 and 20% of lake bacterioplankton counted by conventional DNA staining (e.g. DAPI) seem to be highly active. This is one explanation why many field studies have found apparently low growth rates; the abundance of the growing community is estimated too high. The above statement is somewhat controversial and currently discussed internationally. It is the purpose of this research to try to verify or discharge the hypothesis of low abundance of highly active bacteria in lakes by comparing several estimates of bacterial abundance and activity and to investigate environmental factors in control of the activity. Recently, we have found that substrate availability (measured as chlorophyll as a proxy) can explain from 40 to 50% of the variability in the distribution of active bacteria measured by the redox dye CTC. However, we still lack a convincing relationship between the abundance of “active” and independent measurements of activity.
Read more: Søndergaard, M. & Danielsen, M. 2001. Active bacteria (CTC+) in temperate lakes: Temporal and cross-system variations. J. Plankton Res. 23: 1195-1206.
Morten Søndergaard is leader of this initiative including participants from Danish Climate Centre, Danish Meteorological Institute (DMI), DHI-Water & Environment (DHI), National Environmental Research Institute, Dept. of Lakes and Estuaries (NERI), The Danish Institute for Fisheries Research (DFU), Department of Marine Ecology, Aarhus University (AU), Institute of Geography, University of Copenhagen (GI), Geographical Resource Analysis & Science A/S (GRAS), Department of maritime and regional history, Southern Denmark University (SDU).
Subcontractors: Prof. John Anderson and prof. Bent Odgaard.
Read more: www.conwoy.ku.dk.