Elevated Temperature Simulating Heatwaves Restructures Active Nitrifying Communities and Associated Viruses in Tidal Flats and Agricultural Soils

Abstract Global heatwave intensification under climate change will impact the nitrogen cycle, yet its effect on active nitrifier groups or their interactions with viruses remains unclear. Using 13CO2-DNA-based stable-isotope probing coupled with metagenomics, we show that elevated temperatures under heatwave conditions fundamentally restructure active nitrifying communities and their associated viruses in Yangtze River estuary upper tidal flats and adjacent agricultural soils. In tidal flats, sustained high temperature constrained nitrification by reducing the abundance of active ammonia-oxidizing archaea and bacteria (AOA, AOB) and canonical nitrite-oxidizing bacteria (NOB). This was accompanied by a shift in the active community from marine to more thermotolerant but less salt-tolerant terrestrial ecotypes. Conversely, heatwave conditions in agricultural soils suppressed AOB but enhanced nitrification activity in thermotolerant terrestrial AOA ecotypes. Across both ecosystems, inferred virus-nitrifier interactions were temperature dependent. 13C-labeled nitrifier-infecting viruses exhibited coordinated shifts in virus-to-host abundance ratios and predicted lifestyles with their hosts, with sustained high temperatures reducing virus-to-host abundance ratios and favoring temperate infections, relative to higher abundance ratios and a greater proportion of predicted lytic cycles at lower temperatures. We identified AOA-infecting viruses that carry plastocyanin (pcy), encoding a key copper-dependent electron carrier in the AOA respiratory chain, with conserved active sites and a predicted protein fold that supports its capacity for electron transfer, potentially augmenting host energy metabolism. Together, our findings demonstrate that prolonged heatwaves drive coupled shifts in nitrifier community composition and virus–host interaction strategies in a land-use–dependent manner, with implications for nitrogen transformations and ecosystem feedbacks under climate extremes.