Decreased Stability in Ammonia-Oxidzing Archaea and Bacteria during Dry Conditions in a Salt Marsh

Abstract Frequency and severity of drought are predicted to increase as part of global climate change, yet there are significant gaps in our knowledge of how drought will impact microbial communities. We investigated the relationship between relative dryness and changes in microbial community abundances in a southeastern Connecticut salt marsh by QPCR of 16S rRNA genes (bacterial and archaeal) and selected functional genes (archaeal amoA , bacterial amoA , comammox amoA , nirS , and pmoA ) over multiple years (2006–2019). During this time, southeastern CT experienced a prolonged dry period from 2013 to 2018, as measured by the Palmer Drought Severity Index (PDSI). Average abundances of most genes ranged from 1.1 to 4.7 times higher in wet periods compared to dry periods, while abundance of archaeal amoA genes was 35 times higher in wet compared to dry periods. Furthermore, abundances of both ammonia oxidizing archaea and bacteria (AOA and AOB) varied by 30,000 and 9500-fold, respectively, during the study, with the highest variability occurring during dry conditions. Abundance of other genes varied by 7-38-fold over the course of the study. Additionally, AOA and AOB had the lowest stability values of all genes. Salinity was 39% higher and porewater ammonium was 100 times higher during dry years compared to wet years, but neither correlated with the abundances of AOA or AOB. Results from this study suggest that AOA and AOB, but not comammox, may be more susceptible to drought compared to other microbial communities, however, the specific cause of the decreased stability is uncertain.