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Freshwater microbes' role in degrading humic matter

Biologists funded by the EU studied the contribution of microorganisms to the breakdown of humic matter in freshwater ecosystems in order to gain a better understanding of the species and mechanisms involved. The scientists used advanced sequencing technologies to investigate the genomic and metabolic diversity in two different lake environments.
Freshwater microbes' role in degrading humic matter
Humic substances (HSs) comprise a mixture of complex polymers formed in soil, sediments and natural waters as a result of bio- and physico-chemical transformation of plant biomass in a process known as humification. In freshwater ecosystems HSs are an important source of carbon and energy, and account for up to 80 % of the total dissolved organic carbon (DOC) pool.

The 'Structural and functional biodiversity of humic matter degrading freshwater microbial communities' (HUMADE) project investigated which enzymes are employed by microorganisms to breakdown HSs in the freshwater ecosystem. It was assumed that HSs cannot be broken down by simple reactions but instead require a large number of different enzymes.

Project partners studied the microbial communities in two different lakes located in the same area of north-east Germany. Lake Stechlin is large but poorly productive in terms of organic matter formed and it has a low DOC content. In contrast, Lake Grosse Fuchskuhle is a small and shallow body of water situated next to a peat bog and thus is characterized by high DOC concentrations.

Scientists extracted nucleic acids from samples taken from both lakes and studied them using next-generation sampling technology to enable a comparative analysis between aerobic and anaerobic habitats. Biologists also sequenced the genomes of bacterial isolates that were believed to be involved in freshwater HS turnover.

Bacteria samples included one genetic isolate of the genus Polynucleobacter from Lake Grosse Fuchskuhle and two genetic isolates of the genus Sphingomonas from Lake Stechlin. Polynucleobacter is extremely abundant in freshwater and can tolerate ultraviolet radiation and reactive oxygen species produced as a result of the photolytic reaction of organic matter, including HSs. Both Sphingomonas isolates showed active HS degradation in lab experiments and appeared to be capable of performing aerobic-anoxygenic photosynthesis, where light energy is captured and converted to adenosine triphosphate (ATP) without the production of oxygen, in the presence of oxygen. This energy source might be one solution for these organisms to gain additional energy for the breakdown of complex polymers.

Results from the HUMADE project provided a detail description of the microbial diversity and activity involved in freshwater HS cycling. The work will therefore help scientists to understanding biological mechanisms involved in the degradation of complex polymers in freshwater environments.

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