Regulation of ammonium loss under contrasting upwelling conditions: sensitivity of Feammox to environmental drivers

Understanding the microbial processes involved in ammonium loss in highly productive marine systems is crucial for reconstructing the global nitrogen cycle. This study is the first to examine the anaerobic ammonium oxidation coupled with dissimilatory Fe(III) reduction (Feammox) and the abundance of iron-reducing bacteria (IRB) in a seagrass-dominated coastal lagoon exposed to two contrasting upwelling conditions. Potential Feammox rates varied from 6.0 to 39.2 mg N per m² per day and were positively correlated with the abundance of IRB (<i>Acidomicrobiaceae</i> A6 spp. and <i>Geobacteraceae</i> spp.), suggesting that IRB mediated the Feammox process. The limited impact of near-mouth productivity conditions on Feammox activity and IRB was largely inherent to sediment type (eelgrass or adjacent bare bottom) and station-specific, depending on the degree of confinement relative to the nearby ocean. The partial least squares structural equation modeling approach revealed that dissimilatory Fe(III) reduction exerted a direct effect on potential Feammox rates, while upwelling conditions indirectly influenced the process through sediment characteristics. The contribution of Feammox to total ammonium loss exceeded 60% and increased with the distance from the mouth of the lagoon. A minimum of 3.7 ± 0.5 mg N per m² per day was catalyzed by electron acceptors besides Fe(III), highlighting the co-occurrence of alternative chemoautotrophic pathways in ammonium removal. Furthermore, an average loss of 38.4 ± 6.7 t N per year was attributed to the anaerobic ammonium oxidation processes, accounting for 5.1 ± 1.6% of the annual oceanic N transported into the lagoon. These findings extend our current understanding of N and Fe cycles in coastal environments linked to eastern boundary upwelling systems.