Abstract
Nitrate reduction serves as a pivotal process in the global nitrogen cycle, playing a crucial role in natural ecosystems and industrial applications. Although the genus Alteromonas is not traditionally regarded as a nitrate reducer, several Alteromonas strains have recently been found to be capable of doing so. However, the evolutionary trajectory of this capability remains undiscovered. In this study, 32 bacterial strains were isolated and cultivated from the tidal flat sediment in Hangzhou Bay and classified into the classes Cytophagia (n = 2), Alphaproteobacteria (n = 2), Gammaproteobacteria (n = 17), Flavobacteriia (n = 5), and Bacilli (n = 6). One nitrate-reducing strain, designated as CYL-A6(T), was identified by polyphasic taxonomy and proposed as a novel Alteromonas species. Genomic analysis reveals that seven Alteromonas genomes encode the dissimilatory nitrate reduction genes narGHI. Evolutionary analysis showed that these three nitrate-reducing genes were present in the early common ancestor of the genus Alteromonas, while gene loss events occurred in the subsequent evolution. With the loss of nitrate-reducing genes in the ancestry nodes, a wide variety of genes related to energy production and conversion, as well as carbohydrate, nucleotide, coenzyme, and inorganic ion metabolism, were gained in those nodes, which enabled Alteromonas members to utilize diverse substrates for increased energy production. This study enhances the understanding of microbial diversity in marine tidal flat sediments, proposes a novel nitrate-reducing species of the genus Alteromonas, and highlights the ecological diversification and ecological niche breadth in the evolution of the microbial metabolic network.