Trophic status and local conditions affect microbial potential for denitrification versus internal nitrogen cycling in lake sediments

Abstract

The nitrogen (N) cycle is of global importance as N is an essential element and a limiting nutrient in terrestrial and aquatic ecosystems. Excessive anthropogenic N fertilizer usage threatens sensitive downstream aquatic ecosystems. Although freshwater lake sediments remove N through various microbial transformation processes, few studies have investigated the microbial communities involved. In an integrated biogeochemical and microbiological study on a eutrophic and oligotrophic lake, we estimated N removal rates in the sediments from porewater concentration gradients. Simultaneously, the abundance of different microbial N transformation genes was investigated using metagenomics on a seasonal and spatial scale. We observed that contrasting nutrient concentrations in the sediments were reflected in distinct microbial community compositions and significant differences in the abundance of various N transformation genes. Within each lake, we observed a more pronounced spatial than seasonal variability. The eutrophic Lake Baldegg showed a higher denitrification potential with higher nosZ gene (N2O reductase) abundance and higher nirS:nirK (nitrite reductase) ratio, indicating a greater capacity for complete denitrification. Correspondingly, this lake had a higher N removal efficiency. The oligotrophic Lake Sarnen, in contrast, had a higher potential for DNRA and nitrification, and specifically a high abundance of Nitrospirae, including some capable of comammox. In general, the oligotrophic lake ecosystems had a higher microbial diversity, thus acting as an important habitat for oligotrophic microbes. Our results demonstrate that knowledge of the genomic N transformation potential is important for interpreting N process rates and understanding the limitations of the N cycle response to environmental drivers.

High-throughput Sequencing-based Analysis of Microbial Diversity in Daihai Lake

“…At present, the input of nitrogen into lake waters mainly originates from human activities, and the environmental problems caused by the rapid increase in nitrogen sources are attracting increasing attention. The increase in nitrogen has a wide range of impacts on ecosystems, including biogeochemical cycling, greenhouse gas emissions and eutrophication of lakes (Li et al 2015, Baumann et al 2021, Qian 2015, and microorganisms play an important role in these processes. From the analysis of the microbial community structure and physicochemical properties of the water column, the community structure of holobacteria and denitrifying bacteria were influenced by different indicators, but the ammonia concentration had a strong influence on both microbial groups.…”