Because of their small size, some microorganisms can come through the pores of bacterial filters. Such filtrable microorganisms are difficult to grow in lab conditions and therefore remain understudied. Scientists believe that filtrable microorganisms are widely spread in the biosphere and participate in many biogeochemical processes, such as the restoration of sulfur in deep-see regions. They also play an important role in the production and use of dissolved organic matter. This term refers to a group of compounds (such as amino acids, organic acids and monomeric sugars) that are easily utilizable sources of nutrients in freshwater systems.
These compounds occur in pristine lotic systems at very low concentrations mainly from primary producers (photosynthetic bacteria or algae) or from land runoffs, wastewater discharges, and are consumed by heterotrophic microorganisms (i.e. the microorganisms that are unable to synthesize organic matter from inorganic carbon). Therefore, it is important to understand the major microbial groups and environmental factors important for the transformation of organic compounds at natural, very low (nanomolar) concentrations.
A team of researchers from Immanuel Kant Baltic Federal University, the Kurchatov Institute, and the Winogradsky Institute of Microbiology together with their colleagues from the UK, and Spain, participated in the study led by Bangor University (UK) on the taxonomic composition and functional characteristics of filtrable freshwater organisms, as well as their contribution to the carbon cycle. To do so, they compared the samples of unfiltered and filtered water from the River Conwy (UK). In their experiment, the team used radioisotope tracers (radiolabelled organic substrates) and targeted metabolomics (mass-spectrometry) to measure substrates' uptake and respiration alongside the molecular analysis of microbial DNA to identify major microbial groups.
"Based on the results of the study, we concluded that filtrable fraction of microorganisms has a lower efficiency than the microbial community as a whole, and play a minor role in the processing of dissolved organic carbon. Moreover, their role in the system is further reduced because they have only a limited time to uptake dissolved organic matter in rivers and streams," explained Evgenii Lunev, a co-author of the work.
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