Decaying cyanobacteria decrease N2O emissions related to diversity of intestinal denitrifiers of Chironomus plumosus

Nitrous oxide (N₂O) emission of fresh invertebrates has too long been neglected in eutrophic lakes, although the sediments these animals inhabit are presumably hot spots of N₂O emission. Thus, the experiment in this research was designed to gain insight into the influence of cyanobacterial degradation on the N₂O emission by fresh water invertebrates (Chironomus plumosus). The presence of decaying cyanobacteria in Lake Taihu decreased the N₂O emission rate of Chironomus plumosus larvae from the larvae body by almost 400% for the larvae as a whole. The N₂O emission rate decreased by 350% based on readings from studies of their gut, which was mostly due to stimulation of intestinal complete denitrification. The quantitative PCR results showed that intestinal gene abundance of nirK, nosZ (encoding the copper nitrite reductase and N₂O reductase, respectively) were significantly increased with the presence of decaying cyanobacteria. In contrast nirS (encoding the cytochrome cd₁ heme nitrite reductase) and the total bacteria decreased. In the gut of Chironomus plumosus, the diversity and richness of nosZ and nirK were lower with the cyanobacteria. Phylogenetic analysis of the intestinal function genes (nosZ and nirK) showed that the nosZ- and nirK-type denitrifying bacterial sequences were related to different phylotypes. Hence, additional cyanobacteria increased the abundance, but decreased the richness and diversity of intestinal nitrate-reducing bacteria, probably by providing more carbon source in the gut. The data obtained in this study elucidates that the decaying cyanobacteria decreased the emissions of N₂O by the aquatic invertebrates in freshwater sediment and could serve as a valuable resource for nitrogen removal affecting greenhouse gas emissions.