Tracing Carbon and Nitrogen Microbial Assimilation in Suspended Particles in Freshwaters

The dynamic interactions between dissolved organic matter (DOM) and particulate organic matter (POM) are central in nutrient cycling in freshwater ecosystems. However, the molecular-level mechanisms of such interactions are still poorly defined. Here, we study spatial differences in the chemical and molecular composition of suspended sediments in the River Chew, UK. We then applied a compound-specific stable isotope probing (SIP) approach to test the potential assimilation of 13C,15N-glutamate (Glu) and 15N-nitrate into proteinaceous biomass by particle-associated microbial communities over a 72-h period. Our results demonstrate that the composition of suspended sediments is strongly influenced by the effluent of sewage treatment works (STW). Fluxes and percentages of assimilation of both isotopically labelled substrates into individual proteinaceous amino acids (AAs) showed contrasting dynamics in processing at each site linked to primary biosynthetic metabolic pathways. Preferential assimilation of the organic molecule glutamate and evidence of its direct assimilation into newly synthesised biomass was obtained. Our approach provides quantitative molecular information on the mechanisms by which low molecular weight DOM is mineralised in the water column compared to an inorganic substrate. This is paramount for better understanding the processing and fate of organic matter in aquatic ecosystems.

Anoxygenic Photosynthesis in Photolithotrophic Sulfur Bacteria and Their Role in Detoxication of Hydrogen Sulfide

Hydrogen sulfide is a toxic compound that can affect various groups of water microorganisms. Photolithotrophic sulfur bacteria including Chromatiaceae and Chlorobiaceae are able to convert inorganic substrate (hydrogen sulfide and carbon dioxide) into organic matter deriving energy from photosynthesis. This process takes place in the absence of molecular oxygen and is referred to as anoxygenic photosynthesis, in which exogenous electron donors are needed. These donors may be reduced sulfur compounds such as hydrogen sulfide. This paper deals with the description of this metabolic process, representatives of the above-mentioned families, and discusses the possibility using anoxygenic phototrophic microorganisms for the detoxification of toxic hydrogen sulfide. Moreover, their general characteristics, morphology, metabolism, and taxonomy are described as well as the conditions for isolation and cultivation of these microorganisms will be presented.