Tradeoffs of increasing temperatures for the spread of antimicrobial resistance in river biofilms

River microbial communities regularly act as the first defense barrier against the spread of antimicrobial resistance genes (ARG) that enter environmental microbiomes through wastewaters. However, how the invasion dynamics of wastewater-born ARGs into river biofilm communities will shift due to increasing average and peak temperatures worldwide through climate change remains unknown. Here we aimed at elucidating the effects of increasing temperatures on both, the natural river biofilm resistome, as well as the river biofilms invadability by foreign, wastewater-born ARGs. To achieve this, natural biofilms were grown in a pristine German river and transferred to artificial laboratory recirculation flume systems at three different temperatures (20{degrees}C, 25{degrees}C, 30{degrees}C). Already after one week of acclimatization to the temperatures, significant increases in the abundance of most naturally occurring ARGs were detected in the biofilms exposed to the highest temperature. Thereafter, biofilms were exposed to a single pulse of wastewater and the invasion dynamics of wastewater-born ARGs were analyzed over a period of two weeks. While initially after one day ARGs were able to invade all biofilms successfully and in equal proportions, the foreign invading ARGs were lost at a far increased rate at 30{degrees}C over time. ARG levels dropped to the initial natural levels at 30{degrees}C after 14 days. Contrary at the lower temperatures ARGs remained far elevated and certain ARGs were able to establish themselves in the biofilms. Overall, we here demonstrate tradeoffs of increasing temperature between increases in naturally occurring and faster loss dynamics of invading ARGs.