Anaerobic ammonium oxidation (anammox) and denitrification in Peru margin sediments

Abstract

The upwelling system of coastal Peru supports very high primary production, contributing to an oxygen deficient zone (ODZ) in subsurface waters and high organic matter deposition rates to underlying sediments. Although anammox and denitrification have been relatively well studied in ODZ waters, few studies have investigated these processes in the underlying sediments. We sampled seven stations over a large geographic area along the Peru margin, spanning a water depth of 100–3240 m. At two of the central shelf stations (100 m and 325 m), we observed Thioploca , with a well-developed mat at the shallowest station (100 m). We measured sediment properties and conducted shipboard 15 N-incubations of homogenized sediments to determine potential rates of anammox and denitrification and potential controlling factors at each station. Diversity of anammox bacteria based on 16S rRNA and hydrazine oxidoreductase ( hzo ) sequences and hzo gene abundances were measured at each station. Overall, organic C content was high across the stations (3–12%), except for two of the deepest stations (~1.5%). Porewater ammonium fluxes and ammonium production rates in shipboard incubations, reflecting sediment organic carbon decomposition rates, were higher at the two central shelf stations compared to the other stations. The range in average potential rates was 2.1–80.4 nmol N cm −3 h −1 for denitrification and 1.8–44.2 nmol N cm −3 h −1 for anammox. The range in relative anammox ( ra ) across stations was 2.6–47.4%, with an average of 34.2%. The lowest ra was found at the shallowest shelf station with Thioploca mats and highest ammonium production rates. The ra jumped up to 45.9% at the station with the next highest ammonium production rates, corresponding to the deeper shelf station (325 m). At the other stations, ra was relatively high (39.6–47.4%), except at one station (16.3%), reflecting similar ammonium production rates due to decomposition across these stations. Anammox bacteria in the Candidatus Scalindua genus were the only anammox bacteria detected in Peru margin sediments based on 16S rRNA or hzo sequences. Copy number of hzo indicated abundant populations of anammox bacteria across the stations. However, hzo copy number did not correlate with anammox rates or ra . Overall, our results suggest that anammox contributes significantly to N 2 production in Peru margin sediments, except in shelf sediments with high decomposition rates and dense Thioploca mats. • Anammox and denitrification rates were generally high in Peru margin sediments. • Intracellular nitrate was a complicating factor in shelf sediments in the study. • Anammox rates were inhibited in the presence of a dense Thioploca mat. • Anammox rates jumped up in the presence of low density Thioploca . • Density and metabolic status of Thioploca may influence anammox rates.