Marine plastisphere expands the ecological niche and evolutionary dynamics of nrfA-dependent nitrite ammonifying...

The marine plastisphere affects nitrogen cycling processes, but its role in nrfA-dependent nitrite ammonification, a critical phase of dissimilatory nitrate reduction to ammonium (DNRA) with important implications for nitrogen retention and greenhouse gas dynamics, remains unexplored. In this study, we analyzed 269 plastisphere metagenomes and eight metatranscriptomes from global public datasets. The plastisphere contained elevated nrfA levels compared to seawater, and nrfA transcripts were consistently detected. A total of 285 putative nrfA-dependent nitrite ammonifying bacteria were identified, including 156 novel genera. Most plastisphere MAGs overlapped with other examined marine biofilms, whereas 109 MAGs were uniquely detected in plastisphere samples within the analyzed comparative datasets. Functional studies revealed diverse electron-donor utilization strategies supporting DNRA in plastisphere microorganisms. Evolutionary analyses showed that nrfA genes were distributed across different phyla through horizontal gene transfer, whereas purifying selection limited sequence divergence. These findings highlight a previously underappreciated genetic and transcriptional potential for DNRA in plastic-associated biofilms at the particle scale, with implications for nitrogen retention within plastisphere microhabitats.