A universal bacterial inoculum for dissolved organic carbon biodegradation experiments in freshwaters

Abstract

Dissolved organic carbon (DOC) is one of the largest biologically available sources of organic C in aquatic ecosystems, and its dynamics have implications for local and global C cycling. Usually, DOC biodegradation is assessed in laboratory incubations using native microbial communities as inoculum. The use of native inocula might hamper the comparability of biodegradation rates across systems and obstructs the identification of other controls on DOC biodegradation. Here, we propose the use of the Standardized Bacterial Inoculum (SBI) in experiments of freshwater DOC degradation that would allow for the assessment of the potential degradability of DOC and identification of the drivers of degradation without interferences from the diverse metabolic capabilities of native communities. The SBI is composed of six bacterial strains that grow easily under laboratory conditions and showed better DOC degradation performance than the single strains separately. The SBI performance was tested on simple C sources, humic acids, and natural organic matter from a range of freshwater ecosystems. The fraction and rates of C-substrates degradation by the SBI was consistently replicated under oxic and anoxic conditions. Moreover, DOC degradation rates by the SBI were equivalent to those of the native communities. Finally, the SBI consumed up to 31% of natural DOC from a variety of lakes, rivers, and peatlands over a 28-d period. Overall, the use of the SBI in future DOC degradation experiments will improve comparability among studies and will allow for the separation of effects caused by intrinsic DOC properties from those caused by heterogeneous microbial assemblages.